This page intentionally left blank

Transcription

1

2 This page intentionally left blank

3 Vibration of Mechanical Systems This is a textbook for a first course in mechanical vibrations. There are many books in this area that try to include everything, thus they have become exhaustive compendiums that are overwhelming for an undergraduate. In this book, all the basic concepts in mechanical vibrations are clearly identified and presented in a concise and simple manner with illustrative and practical examples. Vibration concepts include a review of selected topics in mechanics; a description of single-degree-of-freedom (SDOF) systems in terms of equivalent mass, equivalent stiffness, and equivalent damping; a unified treatment of various forced response problems (base excitation and rotating balance); an introduction to systems thinking, highlighting the fact that SDOF analysis is a building block for multi-degree-of-freedom (MDOF) and continuous system analyses via modal analysis; and a simple introduction to finite element analysis to connect continuous system and MDOF analyses. There are more than 6 exercise problems and a complete solutions manual. The use of MATLAB R software is emphasized. Alok Sinha is a Professor of Mechanical Engineering at The Pennsylvania State University (PSU), University Park. He received his PhD degree in mechanical engineering from Carnegie Mellon University. He has been a PSU faculty member since August His areas of teaching and research are vibration, control systems, jet engines, robotics, neural networks, and nanotechnology. He is the author of Linear Systems: Optimal and Robust Control. He has served as a Visiting Associate Professor of Aeronautics and Astronautics at MIT, Cambridge, MA, and as a researcher at Pratt & Whitney, East Hartford, CT. He has also been an associate editor of ASME Journal of Dynamic Systems, Measurement, and Control. At present, he serves as an associate editor of ASME Journal of Turbomachinery and AIAA Journal. Alok Sinha is a Fellow of ASME. He has received the NASA certificate of recognition for significant contributions to the Space Shuttle Microgravity Mission.

4

5 VIBRATION OF MECHANICAL SYSTEMS Alok Sinha The Pennsylvania State University

6 CAMBRIDGE UNIVERSITY PRESS Cambridge, New York, Melbourne, Madrid, Cape Town, Singapore, São Paulo, Delhi, Dubai, Tokyo Cambridge University Press The Edinburgh Building, Cambridge CB2 8RU, UK Published in the United States of America by Cambridge University Press, New York Information on this title: Alok Sinha 21 This publication is in copyright. Subject to statutory exception and to the provision of relevant collective licensing agreements, no reproduction of any part may take place without the written permission of Cambridge University Press. First published in print format ISBN ISBN ebook (EBL) Hardback Cambridge University Press has no responsibility for the persistence or accuracy of urls for external or third-party internet websites referred to in this publication, and does not guarantee that any content on such websites is, or will remain, accurate or appropriate.

7 To My Wife Hansa and My Daughters Divya and Swarna

8

9 CONTENTS Preface page xiii 1 Equivalent Single-Degree-of-Freedom System and Free Vibration Degrees of Freedom Elements of a Vibratory System Mass and/or Mass-Moment of Inertia 5 Pure Translational Motion 5 Pure Rotational Motion 6 Planar Motion (Combined Rotation and Translation) of a Rigid Body 6 Special Case: Pure Rotation about a Fixed Point Spring 8 Pure Translational Motion 8 Pure Rotational Motion Damper 1 Pure Translational Motion 1 Pure Rotational Motion Equivalent Mass, Equivalent Stiffness, and Equivalent Damping Constant for an SDOF System A Rotor Shaft System Equivalent Mass of a Spring Springs in Series and Parallel 16 Springs in Series 16 Springs in Parallel An SDOF System with Two Springs and Combined Rotational and Translational Motion Viscous Dampers in Series and Parallel 22 vii

10 viii Contents Dampers in Series 22 Dampers in Parallel Free Vibration of an Undamped SDOF System Differential Equation of Motion 25 Energy Approach Solution of the Differential Equation of Motion Governing Free Vibration of an Undamped Spring Mass System Free Vibration of a Viscously Damped SDOF System Differential Equation of Motion Solution of the Differential Equation of Motion Governing Free Vibration of a Damped Spring Mass System 41 Case I: Underdamped ( <ξ<1or< c eq < c c ) 42 Case II: Critically Damped (ξ = 1orc eq = c c ) 45 Case III: Overdamped (ξ >1orc eq > c c ) Logarithmic Decrement: Identification of Damping Ratio from Free Response of an Underdamped System ( <ξ<1) 51 Solution Stability of an SDOF Spring Mass Damper System 58 Exercise Problems 63 2 Vibration of a Single-Degree-of-Freedom System Under ConstantandPurelyHarmonicExcitation Responses of Undamped and Damped SDOF Systems to a Constant Force 72 Case I: Undamped (ξ = ) and Underdamped ( <ξ<1) 74 Case II: Critically Damped (ξ = 1orc eq = c c ) 75 Case III: Overdamped (ξ >1orc eq > c c ) Response of an Undamped SDOF System to a Harmonic Excitation 82 Case I: ω ω n 83 Case II: ω = ω n (Resonance) 84 Case I: ω ω n 87 Case II: ω = ω n Response of a Damped SDOF System to a Harmonic Excitation 88 Particular Solution 89 Case I: Underdamped ( <ξ<1or< c eq < c c ) 92

11 Contents ix Case II: Critically Damped (ξ = 1orc eq = c c ) 92 Case III: Overdamped (ξ >1orc eq > c c ) Steady State Response Force Transmissibility Quality Factor and Bandwidth 16 Quality Factor 16 Bandwidth Rotating Unbalance Base Excitation Vibration Measuring Instruments Vibrometer Accelerometer Equivalent Viscous Damping for Nonviscous Energy Dissipation 128 Exercise Problems Responses of an SDOF Spring Mass Damper System toperiodicandarbitraryforces Response of an SDOF System to a Periodic Force Periodic Function and its Fourier Series Expansion Even and Odd Periodic Functions 142 Fourier Coefficients for Even Periodic Functions 143 Fourier Coefficients for Odd Periodic Functions Fourier Series Expansion of a Function with a Finite Duration Particular Integral (Steady-State Response with Damping) Under Periodic Excitation Response to an Excitation with Arbitrary Nature Unit Impulse Function δ(t a) Unit Impulse Response of an SDOF System with Zero Initial Conditions 156 Case I: Undamped and Underdamped System ( ξ<1) 158 Case II: Critically Damped (ξ = 1orc eq = c c ) 158 Case III: Overdamped (ξ>1orc eq >c c ) Convolution Integral: Response to an Arbitrary Excitation with Zero Initial Conditions Convolution Integral: Response to an Arbitrary Excitation with Nonzero Initial Conditions 165 Case I: Undamped and Underdamped ( ξ<1or c eq <c c ) 166

12 x Contents Case II: Critically Damped (ξ = 1orc eq = c c ) 166 Case III: Overdamped (ξ >1orc eq > c c ) Laplace Transformation Properties of Laplace Transformation Response of an SDOF System via Laplace Transformation Transfer Function and Frequency Response Function 173 Significance of Transfer Function 175 Poles and Zeros of Transfer Function 175 Frequency Response Function 176 Exercise Problems VibrationofTwo-Degree-of-Freedom-Systems Mass, Stiffness, and Damping Matrices Natural Frequencies and Mode Shapes Eigenvalue/Eigenvector Interpretation Free Response of an Undamped 2DOF System 198 Solution Forced Response of an Undamped 2DOF System Under Sinusoidal Excitation Free Vibration of a Damped 2DOF System Steady-State Response of a Damped 2DOF System Under Sinusoidal Excitation Vibration Absorber Undamped Vibration Absorber Damped Vibration Absorber 22 Case I: Tuned Case ( f = 1orω 22 = ω 11 ) 224 Case II: No restriction on f (Absorber not tuned to main system) Modal Decomposition of Response 227 Case I: Undamped System (C = ) 228 Case II: Damped System (C ) 228 Exercise Problems Finite and Infinite (Continuous) Dimensional Systems Multi-Degree-of-Freedom Systems Natural Frequencies and Modal Vectors (Mode Shapes) Orthogonality of Eigenvectors for Symmetric Mass and Symmetric Stiffness Matrices 242

13 Contents xi Modal Decomposition 245 Case I: Undamped System (C = ) 246 Case II: Proportional or Rayleigh Damping Continuous Systems Governed by Wave Equations Transverse Vibration of a String 25 Natural Frequencies and Mode Shapes 251 Computation of Response Longitudinal Vibration of a Bar Torsional Vibration of a Circular Shaft Continuous Systems: Transverse Vibration of a Beam Governing Partial Differential Equation of Motion Natural Frequencies and Mode Shapes 267 Simply Supported Beam 269 Cantilever Beam Computation of Response Finite Element Analysis Longitudinal Vibration of a Bar 279 Total Kinetic and Potential Energies of the Bar Transverse Vibration of a Beam 286 Total Kinetic and Potential Energies of the Beam 291 Exercise Problems 295 APPENDIX A: EQUIVALENT STIFFNESSES (SPRING CONSTANTS) OF BEAMS, TORSIONAL SHAFT, AND LONGITUDINALBAR APPENDIX B: SOME MATHEMATICAL FORMULAE APPENDIX C: LAPLACE TRANSFORM TABLE References 35 Index 37

14

15 PREFACE This book is intended for a vibration course in an undergraduate Mechanical Engineering curriculum. It is based on my lecture notes of a course (ME37) that I have been teaching for many years at The Pennsylvania State University (PSU), University Park. This vibration course is a required core course in the PSU mechanical engineering curriculum and is taken by junior-level or third-year students. Textbooks that have been used at PSU are as follows: Hutton (1981) and Rao (1995, First Edition 1986). In addition, I have used the book by Thomson and Dahleh (1993, First Edition 1972) as an important reference book while teaching this course. It will be a valid question if one asks why I am writing another book when there are already a large number of excellent textbooks on vibration since Den Hartog wrote the classic book in One reason is that most of the books are intended for senior-level undergraduate and graduate students. As a result, our faculties have not found any book that can be called ideal for our junior-level course. Another motivation for writing this book is that I have developed certain unique ways of presenting vibration concepts in response to my understanding of the background of a typical undergraduate student in our department and the available time during a semester. Some of the examples are as follows: review of selected topics in mechanics; the description of the chapter on single-degreeof-freedom (SDOF) systems in terms of equivalent mass, equivalent stiffness, and equivalent damping; unified treatment of various forced xiii

16 xiv Preface response problems such as base excitation and rotating balance; introduction of system thinking, highlighting the fact that SDOF analysis is a building block for multi-degree-of-freedom (MDOF) and continuous system analyses via modal analysis; and a simple introduction of finite element analysis to connect continuous system and MDOF analyses. As mentioned before, there are a large number of excellent books on vibration. But, because of a desire to include everything, many of these books often become difficult for undergraduate students. In this book, all the basic concepts in mechanical vibration are clearly identified and presented in a simple manner with illustrative and practical examples. I have also attempted to make this book self-contained as much as possible; for example, materials needed from previous courses, such as differential equation and engineering mechanics, are presented. At the end of each chapter, exercise problems are included. The use of MATLAB software is also included. ORGANIZATION OF THE BOOK In Chapter 1, the degrees of freedom and the basic elements of a vibratory mechanical system are presented. Then the concepts of equivalent mass, equivalent stiffness, and equivalent damping are introduced to construct an equivalent single-degree-of-freedom model. Next, the differential equation of motion of an undamped SDOF spring mass system is derived along with its solution. Then the solution of the differential equation of motion of an SDOF spring mass damper system is obtained. Three cases of damping levels underdamped, critically damped, and overdamped are treated in detail. Last, the concept of stability of an SDOF spring mass damper system is presented. In Chapter 2, the responses of undamped and damped SDOF spring mass systems are presented. An important example of input shaping is shown. Next, the complete solutions of both undamped and

17 Preface xv damped spring mass systems under sinusoidal excitation are derived. Amplitudes and phases of steady-state responses are examined along with force transmissibility, quality factor, and bandwidth. Then the solutions to rotating unbalance and base excitation problems are provided. Next, the basic principles behind the designs of a vibrometer and an accelerometer are presented. Last, the concept of equivalent viscous damping is presented for nonviscous energy dissipation. In Chapter 3, the techniques to compute the response of an SDOF system to a periodic excitation are presented via the Fourier series expansion. Then it is shown how the response to an arbitrary excitation is obtained via the convolution integral and the unit impulse response. Last, the Laplace transform technique is presented. The concepts of transfer function, poles, zeros, and frequency response function are also introduced. In Chapter 4, mass matrix, stiffness matrix, damping matrix, and forcing vector are defined. Then the method to compute the natural frequencies and the mode shapes is provided. Next, free and forced vibration of both undamped and damped two-degree-of-freedom systems are analyzed. Last, the techniques to design undamped and damped vibration absorbers are presented. In Chapter 5, the computation of the natural frequencies and the mode shapes of discrete multi-degree-of-freedom and continuous systems is illustrated. Then the orthogonality of the mode shapes is shown. The method of modal decomposition is presented for the computation of both free and forced responses. The following cases of continuous systems are considered: transverse vibration of a string, longitudinal vibration of a bar, torsional vibration of a circular shaft, and transverse vibration of a beam. Last, the finite element method is introduced via examples of the longitudinal vibration of a bar and the transverse vibration of a beam.

18

19 1 EQUIVALENT SINGLE-DEGREE-OF-FREEDOM SYSTEM AND FREE VIBRATION The course on Mechanical Vibration is an important part of the Mechanical Engineering undergraduate curriculum. It is necessary for the development and the performance of many modern engineering products: automobiles, jet engines, rockets, bridges, electric motors, electric generators, and so on. Whenever a mechanical system contains storage elements for kinetic and potential energies, there will be vibration. The vibration of a mechanical system is a continual exchange between kinetic and potential energies. The vibration level is reduced by the presence of energy dissipation elements in the system. The problem of vibration is further accentuated because of the presence of time-varying external excitations, for example, the problem of resonance in a rotating machine, which is caused by the inevitable presence of rotor unbalance. There are many situations where the vibration is caused by internal excitation, which is dependent on the level of vibration. This type of vibration is known as selfexcited oscillations, for example, the failure of the Tacoma suspension bridge (Billah and Scanlan, 1991) and the fluttering of an aircraft wing. This course deals with the characterization and the computation of the response of a mechanical system caused by time-varying excitations, which can be independent of or dependent on vibratory response. In general, the vibration level of a component of a machine has to be decreased to increase its useful life. As a result, the course also 1

20 2 Vibration of Mechanical Systems examines the methods used to reduce vibratory response. Further, this course also develops an input/output description of a dynamic system, which is useful for the design of a feedback control system in a future course in the curriculum. The book starts with the definition of basic vibration elements and the vibration analysis of a single-degree-of-freedom (SDOF) system, which is the simplest lumped parameter mechanical system and contains one independent kinetic energy storage element (mass), one independent potential energy storage element (spring), and one independent energy dissipation element (damper). The analysis deals with natural vibration (without any external excitation) and forced response as well. The following types of external excitations are considered: constant, sinusoidal, periodic, and impulsive. In addition, an arbitrary nature of excitation is considered. Then, these analyses are presented for a complex lumped parameter mechanical system with multiple degrees of freedom (MDOF). The design of vibration absorbers is presented. Next, the vibration of a system with continuous distributions of mass, such as strings, longitudinal bars, torsional shafts, and beams, is presented. It is emphasized that the previous analyses of lumped parameter systems serve as building blocks for computation of the response of a continuous system that is governed by a partial differential equation. Last, the fundamentals of finite element analysis (FEA), which is widely used for vibration analysis of a real structure with a complex shape, are presented. This presentation again shows the application of concepts developed in the context of SDOF and MDOF systems to FEA. In this chapter, we begin with a discussion of degrees of freedom and the basic elements of a vibratory mechanical system that are a kinetic energy storage element (mass), a potential energy storage element (spring), and an energy dissipation element (damper). Then, an SDOF system with many energy storage and dissipation elements, which are not independent, is considered. It is shown how an equivalent SDOF model with one equivalent mass, one equivalent spring,

21 Equivalent Single-Degree-of-Freedom System and Free Vibration 3 and one equivalent damper is constructed to facilitate the derivation of the differential equation of motion. Next, the differential equation of motion of an undamped SDOF spring mass system is derived along with its solution to characterize its vibratory behavior. Then, the solution of the differential equation of motion of an SDOF spring mass damper system is obtained and the nature of the response is examined as a function of damping values. Three cases of damping levels, underdamped, critically damped, and overdamped, are treated in detail. Last, the concept of stability of an SDOF spring mass damper system is presented along with examples of self-excited oscillations found in practice. 1.1 DEGREES OF FREEDOM Degrees of freedom (DOF) are the number of independent coordinates that describe the position of a mechanical system at any instant of time. For example, the system shown in Figure has one degree of freedom x, which is the displacement of the mass m 1. In spite of the two masses m 1 and m 2 in Figure 1.1.2, this system has only one degree of freedom x because both masses are connected by a rigid link, and the displacements of both masses are not independent. The system shown in Figure has two degrees of freedom x 1 and x 2 because both masses m 1 and m 2 are connected by a flexible link or a spring, and the displacements of both masses are independent. Next, consider rigid and flexible continuous cantilever beams as shown in Figures and The numbers of degrees of freedom for rigid and flexible beams are and, respectively. Each continuous beam can be visualized to contain an infinite number of point masses. These point masses are connected by rigid links for a rigid beam as shown in Figure 1.1.2, whereas they are connected by flexible links for a flexible beam as shown in Figure Consequently, there is one degree of freedom associated with each of the point masses in a flexible beam.

22 4 Vibration of Mechanical Systems Massless spring m 1 x DOF = 1 Figure An SDOF system with a single mass Massless spring x Rigid link x DOF = 1 m 1 m 2 Figure An SDOF system with two masses Massless spring m 1 x 1 Massless spring m 2 x 2 DOF = 2 Figure Two DOF systems with two masses Point masses connected by rigid links DOF = Rigid beam Figure A rigid beam fixed at one end Point masses connected by flexible links DOF = Flexible beam Figure A flexible beam fixed at one end

23 Equivalent Single-Degree-of-Freedom System and Free Vibration 5 x(t) f (t) m Figure A mass in pure translation 1.2 ELEMENTS OF A VIBRATORY SYSTEM There are three basic elements of a vibratory system: a kinetic energy storage element (mass), a potential energy storage element (spring), and an energy dissipation element (damper). The description of each of these three basic elements is as follows Mass and/or Mass-Moment of Inertia Newton s second law of motion and the expression of kinetic energy are presented for three types of motion: pure translational motion, pure rotational motion, and planar (combined translational and rotational) motion. Pure Translational Motion Consider the simple mass m (Figure 1.2.1) which is acted upon by a force f (t). Applying Newton s second law of motion, mẍ = f (t) (1.2.1) where ẋ = dx dt and ẍ = d2 x dt 2 (1.2.2a, b) The energy of the mass is stored in the form of kinetic energy (KE): KE = 1 2 mẋ2 (1.2.3)

24 6 Vibration of Mechanical Systems σ eq O f eq l r r cosθ θ v c mg C θ θ & = ω Figure A mass in pure rotation Pure Rotational Motion Consider the mass m (Figure 1.2.2) which is pinned at the point O, and acted upon by an equivalent external force f eq and an equivalent external moment σ eq. This mass is undergoing a pure rotation about the point O, and Newton s second law of motion leads to I o θ = mgr sin θ + f eq l + σ eq (1.2.4) where I o is the mass-moment of inertia about the center of rotation O, θ is the angular displacement, and l is the length of the perpendicular from the point O to the line of force. The KE of the rigid body is KE = 1 2 I o θ 2 (1.2.5) The potential energy (PE) of the rigid body is PE = mg(r r cos θ) (1.2.6) Planar Motion (Combined Rotation and Translation) of a Rigid Body Consider the planar motion of a rigid body with mass m and the massmoment of inertia I c about the axis perpendicular to the plane of motion and passing through the center of mass C (Figure 1.2.3). Forces f i, i = 1, 2,...,n f, and moments σ i, i = 1, 2,...,n t, are acting on this

25 Equivalent Single-Degree-of-Freedom System and Free Vibration 7 f 3 σ 1 v c f 1 C ω y σ σ 2 3 f 2 x Figure Planar motion of a rigid body rigid body. Let x c and y c be x- and y- coordinates of the center of mass C with respect to the fixed x y frame. Then, Newton s second law of motion for the translational part of motion is given by mẍ c = i f xi (t) (1.2.7) mÿ c = i f yi (t) (1.2.8) where f xi and f yi are x- and y- components of the force f i. Newton s second law of motion for the rotational part of motion is given by I c θ = I c ω = i σ i (t) + i σ c f i (1.2.9) where σf c i is the moment of the force f i about the center of mass C. And, θ and ω are the angular position and the angular velocity of the rigid body, respectively. The KE of a rigid body in planar motion is given by KE = 1 2 mv2 c I cω 2 (1.2.1) where v c is the magnitude of the linear velocity of the center of mass, that is, v 2 c = ẋ2 c + ẏ2 c (1.2.11)

26 8 Vibration of Mechanical Systems f (t) Massless f (t) x 1 k x 2 Figure A massless spring in translation Special Case: Pure Rotation about a Fixed Point Note that the pure rotation of the rigid body (Figure 1.2.2) is a special planar motion for which v c = rω (1.2.12) and Equation leads to KE = 1 2 (mr 2 + I c )ω 2 (1.2.13) Using the parallel-axis theorem, I o = I c + mr 2 (1.2.14) Therefore, Equation is obtained for the case of a pure rotation about a fixed point Spring The spring constant or stiffness and the expression of PE are presented for two types of motion: pure translational motion and pure rotational motion. Pure Translational Motion Consider a massless spring, subjected to a force f (t) on one end (Figure 1.2.4). Because the mass of the spring is assumed to be zero, the net force on the spring must be zero. As a result, there will be an equal and opposite force on the other end. The spring deflection is the difference between the displacements of both ends, that is, spring deflection = x 2 x 1 (1.2.15)

27 Equivalent Single-Degree-of-Freedom System and Free Vibration 9 Torque, σ Massless Torque, σ θ 1 θ 2 Figure A massless spring in rotation and the force is directly proportional to the spring deflection: f (t) = k(x 2 x 1 ) (1.2.16) where the proportionality constant k is known as the spring constant or stiffness. The PE of the spring is given by PE = 1 2 k(x 2 x 1 ) 2 (1.2.17) It should be noted that the PE is independent of the sign (extension or compression) of the spring deflection, x 2 x 1. Pure Rotational Motion Consider a massless torsional spring, subjected to a torque σ (t) on one end (Figure 1.2.5). Because the mass of the spring is assumed to be zero, the net torque on the spring must be zero. As a result, there will be an equal and opposite torque on the other end. The spring deflection is the difference between angular displacements of both ends, that is, spring deflection = θ 2 θ 1 (1.2.18) and the torque is directly proportional to the spring deflection: σ (t) = k t (θ 2 θ 1 ) (1.2.19) where the proportionality constant k t is known as the torsional spring constant or torsional stiffness. The PE of the torsional spring is given by PE = 1 2 k t(θ 2 θ 1 ) 2 (1.2.2)

28 1 Vibration of Mechanical Systems Massless f (t) f (t) f (t) x1 c x 2 Mass (a) (b) x 2 Figure (a) A massless damper in translation; (b) A mass attached to the right end of the damper It should be noted that the PE is independent of the sign of the spring deflection, θ 2 θ Damper The damping constant and the expression of energy dissipation are presented for two types of motion: pure translational motion and pure rotational motion. Pure Translational Motion Consider a massless damper, subjected to force f (t) on one end (Figure 1.2.6a). Because the mass of the damper is assumed to be zero, the net force on the damper must be zero. As a result, there will be an equal and opposite force on the other end, and the damper force is directly proportional to the difference of the velocities of both ends: f (t) = c(ẋ 2 (t) ẋ 1 (t)) (1.2.21) where the proportionality constant c is known as the damping constant. The damper defined by Equation is also known as the linear viscous damper. If there is a mass attached to the damper at the right end (Figure 1.2.6b) with the displacement x 2, the work done on the mass

29 Equivalent Single-Degree-of-Freedom System and Free Vibration 11 against the damping force for an infinitesimal displacement dx 2 is f (t)dx 2 = f (t) dx 2 dt dt = f (t)ẋ 2dt (1.2.22) The energy dissipated by the damper equals the work done on mass against the damping force, that is, from Equation , energy dissipated by the damper = f (t)ẋ 2 dt = c(ẋ 2 ẋ 1 )ẋ 2 dt (1.2.23) As an example, consider x 1 (t) = and x 2 (t) = Asin(ωt φ), where A and φ are constants, and ω is the frequency of oscillation. In this case, from Equation , energy dissipated by the damper = cẋ 2 2 dt = ca2 ω 2 cos 2 (ωt φ)dt (1.2.24) Substituting υ = ωt φ into Equation , energy dissipated by the damper = ca 2 ω cos 2 υdυ (1.2.25) As a result, the energy dissipated by the damper per cycle of oscillation is 2π ca 2 ω cos 2 υdυ = πcωa 2 (1.2.26) It should be noted that the energy dissipated by the viscous damper per cycle of oscillation is proportional to the square of the vibration amplitude. Pure Rotational Motion Consider a massless torsional damper, subjected to a torque σ (t) on one end (Figure 1.2.7). Because the mass of the damper is assumed to be zero, the net torque on the damper must be zero. As a result, there will be an equal and opposite torque on the other end, and the damper torque is directly proportional to the difference of the angular velocities of both ends: σ (t) = c t ( θ 2 (t) θ 1 (t)) (1.2.27)

30 12 Vibration of Mechanical Systems Massless θ 1 Torque, σ Torque, σ θ 2 c t Figure A massless torsional damper where the proportionality constant c t is known as the damping constant. The damper defined by Equation is also known as the linear viscous damper. If there is a mass attached to the damper at the right end with the angular displacement θ 2, the work done against the damping torque for an infinitesimal displacement dθ 2 is σ (t)dθ 2 = σ (t) dθ 2 dt dt = σ (t) θ 2 dt (1.2.28) The energy dissipated by the damper equals the work done on mass against the damping torque, that is, from Equation , energy dissipated by the damper = σ (t) θ 2 dt = c t ( θ 2 θ 1 ) θ 2 dt (1.2.29) The expression similar to Equation can be derived for the energy dissipated by the torsional viscous damper per cycle of oscillation. 1.3 EQUIVALENT MASS, EQUIVALENT STIFFNESS, AND EQUIVALENT DAMPING CONSTANT FOR AN SDOF SYSTEM In this section, equivalent mass, equivalent stiffness, and equivalent damping constant are derived for a rotor shaft system, spring with

31 Equivalent Single-Degree-of-Freedom System and Free Vibration 13 l 2 l 2 Simply supported shaft Rotor with mass = m Figure A rotor shaft system nonnegligible mass, parallel and series combinations of springs and dampers, and a combined rotational and translational system A Rotor Shaft System Consider a rotor with mass m which is supported at the mid-span of a simply supported shaft of length l (Figure 1.3.1). The mass of the shaft is negligible in comparison with the mass of the rotor. For the purpose of transverse vibration modeling, the shaft is considered as a simply supported beam (Crandall et al., 1999). For a simply supported beam, when a load P is applied at the mid-span (Figure 1.3.2), the deflection δ of the shaft at the mid-span is obtained from the results provided in Appendix A (Equation A.2) by δ = Pl3 (1.3.1) 48EI where E and I are the Young s modulus of elasticity and the area moment of inertia, respectively. l 2 l 2 P Simply supported shaft δ Deflection Figure Deflection of a simply supported shaft

32 14 Vibration of Mechanical Systems Massless 48EI k eq = 3 l x(t) m Figure Equivalent SDOF system for a rotor shaft system Therefore, the equivalent stiffness of the shaft is defined as k eq = P δ = 48EI l 3 (1.3.2) And, an equivalent SDOF system can be constructed as shown in Figure 1.3.3, where x(t) is the displacement of the rotor mass Equivalent Mass of a Spring Consider an SDOF system (Figure 1.3.4) in which the mass of the spring m s is not negligible with respect to the main mass m. Spring-mass = m s y x& l y dy v y k eq x(t) m l y x& Figure A spring mass system with nonnegligible mass of spring

33 Equivalent Single-Degree-of-Freedom System and Free Vibration 15 Then, an equivalent system with a massless spring can be obtained on the basis of the total KE. Let the length of the spring be l, and assume that the mass of the spring is uniformly distributed over its length. Then, the mass of a spring strip of the length dy will be dm y = m s dy (1.3.3) l To determine the KE, the velocity v y of the spring strip at a distance y from the base of the spring must be known. But, we only have the following =, v y = (1.3.4) = l, v y = ẋv y = ẋ (1.3.5) The velocity at an intermediate point ( < y <l) is not known. Therefore, it is assumed that the velocity profile over the length of the spring is linear as shown in Figure 1.3.4, that is, v y = ẋ l y (1.3.6) The KE of the spring strip of length dy at a distance y from the base of the spring is dke s = 1 2 dm y(v y ) 2 (1.3.7) Using Equations (1.3.3) and (1.3.7), dke s = 1 ( ) m s ẋ 2 2 l l y dy (1.3.8) Therefore, the total KE of the spring is given by KE s = l ( ) 1 m s ẋ 2 2 l l y dy = 1 2 m s ẋ 2 l 3 l y 2 dy = 1 m s 2 3 ẋ2 (1.3.9)

34 16 Vibration of Mechanical Systems Massless spring k eq x(t) m m + s 3 Figure Equivalent SDOF system with a massless spring Hence, the total KE of the system with a nonnegligible spring mass is given by KE = 1 2 mẋ2 + 1 m s 2 3 ẋ2 = 1 2 m eqẋ 2 (1.3.1) where the equivalent mass m eq is given by m eq = m + m s 3 (1.3.11) And, the equivalent SDOF system can be created with a massless spring and a mass m eq as shown in Figure The systems shown in Figures and will have the same amount of KE under the assumption of a linear velocity profile for the spring Springs in Series and Parallel Springs in Series Consider a series combination of massless springs with stiffnesses k 1 and k 2 (Figure 1.3.6). k k 1 2 f (t) Massless x1 Massless x Figure Springs in series

35 Equivalent Single-Degree-of-Freedom System and Free Vibration 17 f (t) k f (t) f (t) 1 k f (t) 2 x 1 x 1 x Figure Free body diagrams for springs in series The free body diagram of each spring is shown in Figure for which the following relationships can be written: and f (t) = k 1 x 1 (1.3.12) f (t) = k 2 (x x 1 ) (1.3.13) It is important to note that both springs, which are in series, carry the same amount of force. From Equations and , x 1 = k 2 x (1.3.14) k 1 + k 2 Substituting Equation into Equation , where f (t) = k eq x (1.3.15) k eq = k 1k 2 (1.3.16) k 1 + k 2 Here, k eq is the equivalent stiffness and the system in Figure can be replaced by a system with only one spring with the stiffness k eq as shown in Figure Equation can also be written as 1 k eq = 1 k k 2 (1.3.17) Springs in Parallel Consider a parallel combination of massless springs with stiffnesses k 1 and k 2 (Figure 1.3.9).

36 18 Vibration of Mechanical Systems k k 1 2 Massless x1 Massless f (t) x = + k eq k1 k2 k eq f (t) Massless x Figure Equivalent system with only one spring for a series combination The free body diagram of each spring is shown in Figure for which the following relationships can be written: f 1 (t) = k 1 x (1.3.18) f 2 (t) = k 2 x (1.3.19) and f (t) = f 1 (t) + f 2 (t) (1.3.2) It is important to note that both springs, which are in parallel, undergo the same amount of deflection. Substituting Equations and into Equation 1.3.2, f (t) = k eq x (1.3.21) where k eq = k 1 + k 2 (1.3.22) k 1 Massless Massless k 2 x f (t) Figure A parallel combination of springs

37 Equivalent Single-Degree-of-Freedom System and Free Vibration 19 f 1( t ) k f ( ) 1 1 t x f 1( t ) f (t) f 2( t ) f 2( t ) k f ( ) 2 2 t x Figure Free body diagrams for springs in parallel Here, k eq is the equivalent stiffness and the system in Figure can be replaced by a system with only one spring with the stiffness k eq as shown in Figure An SDOF System with Two Springs and Combined Rotational and Translational Motion Consider the system shown in Figure , in which a cylinder rolls without slipping. The displacement of the cylinder s center of mass, marked as C, is denoted by the symbol x(t). Because of the displacement x(t), the stepped pulley rotates by an amount θ(t). For a small displacement x(t), from Figure , θ(t) = x(t) (1.3.23) r 2 Massless k 1 k eq = k 1 + k 2 Massless f (t) k eq f (t) k 2 x Massless x Figure Equivalent system with only one spring for a parallel combination

38 2 Vibration of Mechanical Systems Radius = r Cylinder with mass = m k 1 C x Stepped pulley Rigid link o x x 1 k 2 Roll without slip Radius = r2 θ Radius = r 3 Mass-moment of inertia = Figure An SDOF System with rotational and translational motion I o Using Equation 1.2.1, the KE of the cylinder is given as KE cyl = 1 2 mẋ I cω 2 (1.3.24) where the mass-moment of inertia of the cylinder about C is I c = 1 2 mr 2 (1.3.25) and the angular velocity ω of the cylinder for rolling without slipping is ω = ẋ (1.3.26) r Substituting Equations and into Equation , The KE of the stepped pulley is KE cyl = mẋ2 (1.3.27) KE sp = 1 2 I oω 2 sp (1.3.28) where ω sp is the angular velocity of the stepped pulley. From Equation , ω sp = θ = ẋ r 2 (1.3.29)

39 Equivalent Single-Degree-of-Freedom System and Free Vibration 21 x r 2 θ r 3 x 1 θ Figure Displacement and equivalent rotation Substituting Equation into Equation , KE sp = 1 2 I o r 2 2 ẋ 2 (1.3.3) From Equations and 1.3.3, the total KE in the system is KE tot = KE cyl + KE sp = 1 2 m eqẋ 2 (1.3.31) where m eq = 1.5m + I r 2 2 (1.3.32) From Figure , the total PE in the system is where from Figure , PE tot = 1 2 k 1x k 2x 2 1 (1.3.33) x 1 = r 3 θ (1.3.34) Substituting Equation into Equation and using Equation , PE tot = 1 2 k eqx 2 (1.3.35) where ( ) 2 r3 k eq = k 1 + k 2 (1.3.36) r 2 Here, m eq (Equation ) and k eq (Equation ) are the equivalent mass and the equivalent stiffness of the SDOF system

40 22 Vibration of Mechanical Systems k eq m eq x 2 r3 k eq = k1 + k2 r1 I m 1.5 eq = m + 2 r2 Figure An SDOF system with a single spring and a single mass equivalent to system in Figure shown in Figure And, the system shown in Figure can also be described as the equivalent SDOF (Figure ) Viscous Dampers in Series and Parallel Dampers in Series Consider a series combination of massless viscous dampers with damping coefficients c 1 and c 2 (Figure ). The free body diagram of each damper is shown in Figure for which the following relationships can be written: f (t) = c 1 ẋ 1 (1.3.37) f (t) = c 2 (ẋ ẋ 1 ) (1.3.38) An important point to note here is that both dampers, which are in series, carry the same amount of force. From Equations and , ẋ 1 = c 2 ẋ (1.3.39) c 1 + c 2 Substituting Equation into Equation , f (t) = c eq ẋ (1.3.4) Massless Massless f (t) c x 1 1 c2 Figure Dampers in series x

41 Equivalent Single-Degree-of-Freedom System and Free Vibration 23 Massless Massless f (t) f (t) f (t) f (t) c 1 x 1 x 1 c 2 x Figure Free body diagrams for dampers in series where c eq = c 1c 2 c 1 + c 2 (1.3.41) Here, c eq is the equivalent damping coefficient and the system in Figure can be replaced by a system with only one damper with the coefficient c eq as shown in Figure Equation can also be written as 1 c eq = 1 c c 2 (1.3.42) Dampers in Parallel Consider a parallel combination of massless dampers with coefficients c 1 and c 2 (Figure ). The free body diagram of each damper is shown in Figure for which the following relationships can be written: f 1 (t) = c 1 ẋ (1.3.43) f 2 (t) = c 2 ẋ (1.3.44) Massless Massless Massless f (t) f (t) c x 1 1 c2 x c1c2 c eq = c1 + c2 x Figure Equivalent system with only one damper for a series combination

42 24 Vibration of Mechanical Systems Massless Massless c 1 f (t) c 2 Massless x Figure A parallel combination of dampers and f (t) = f 1 (t) + f 2 (t) (1.3.45) An important point to note here is that both dampers, which are in parallel, have the same velocity. Substituting Equations and into Equation , f (t) = c eq ẋ (1.3.46) where c eq = c 1 + c 2 (1.3.47) Massless f 1( t ) f 1( t ) c 1 x Massless Massless f 1( t ) f 2( t ) f 2( t ) f (t) c 2 x f 2( t ) x Figure Free body diagrams for dampers in parallel

43 Equivalent Single-Degree-of-Freedom System and Free Vibration 25 Massless Massless c 1 f (t) Massless c 2 Massless x c eq = c 1 + c 2 f (t) x Figure Equivalent system with only one damper for a parallel combination Here, c eq is the equivalent damping constant and the system in Figure can be replaced by a system with only one damper with the coefficient c eq as shown in Figure FREE VIBRATION OF AN UNDAMPED SDOF SYSTEM This section deals with the derivation and the solution of the differential equation of motion of an undamped SDOF system. The solution of the differential equation of motion is used to characterize the nature of free vibration Differential Equation of Motion Consider an SDOF spring mass system with equivalent spring stiffness k eq and equivalent mass m eq (Figure 1.4.1). First, consider the system with the unstretched spring (Figure 1.4.1). As we let the mass be under gravity, the spring will deflect due to the weight. There will be a static equilibrium configuration where the net force on the mass will be zero. The free body diagram is shown in Figure 1.4.2, where is the static deflection or the deflection of the spring in the static equilibrium configuration. For the static equilibrium condition, k eq = m eq g (1.4.1)

44 26 Vibration of Mechanical Systems P.E. = Potential energy k eq Zero P.E. k eq k eq m eq Unstretched spring Δ m eq Static equilibrium x(t) m eq Perturbed from static equilibrium Figure An undamped SDOF spring mass system Let the displacement x(t) be a perturbation from the static equilibrium (Figure 1.4.1). From the free body diagram in Figure (Perturbed from Static Equilibrium), net force in x-direction = k eq (x + ) + m eq g (1.4.2) Newton s second law of motion states that Net force in x-direction = mass acceleration (1.4.3) Therefore, k eq (x + ) + m eq g = m eq ẍ (1.4.4) k eq Δ k eq ( Δ + x( t)) meq m eq g Static equilibrium meq m eq g Perturbed from static equilibrium Figure Free body diagrams

45 Equivalent Single-Degree-of-Freedom System and Free Vibration 27 k eq x(t) m eq Perturbed from static equilibrium Figure Equivalent free body diagram after canceling k eq and m eq g Using Equation 1.4.1, m eq ẍ + k eq x = (1.4.5) The same differential equation is obtained by neglecting the weight m eq g and the spring force k eq as well. The resulting free body diagram is shown in Figure 1.4.3, and net force in x-direction = k eq x (1.4.6) Newton s second law of motion yields k eq x = m eq ẍ (1.4.7) Energy Approach The KE of the system (perturbed from static equilibrium in Figure is given by and the PE is given by The total energy is U = 1 2 m eqẋ 2 (1.4.8) P = 1 2 k eq(x + ) 2 m eq g(x + ) (1.4.9) T = U + P = 1 2 m eqẋ k eq(x + ) 2 m eq g(x + ) (1.4.1) Since there is no sink (damping) or source (external force) of energy, the total energy T is a constant. Therefore, dt = (1.4.11) dt

46 28 Vibration of Mechanical Systems From Equations and , or 1 2 m eq2ẋẍ k eq2(x + )ẋ m eq gẋ = (1.4.12) (m eq ẍ + k eq x + k eq m eq g)ẋ = (1.4.13) Because of the static equilibrium condition in Equation 1.4.1, (m eq ẍ + k eq x)ẋ = (1.4.14) As ẋ(t) is not zero for all t, m eq ẍ + k eq x = (1.4.15) The same differential equation of motion can be obtained by writing the PE as P = 1 2 k eqx 2 (1.4.16) Note that the contributions of the weight mg and the corresponding static deflection have been simultaneously neglected. The total energy is given by T = U + P = 1 2 m eqẋ k eqx 2 (1.4.17) The condition in Equation yields (m eq ẍ + k eq x)ẋ = (1.4.18) Therefore, m eq ẍ + k eq x = (1.4.19) Example 1.4.1: A Horizontal Rigid Bar Consider the system shown in Figure 1.4.4a, in which a rigid bar, pinned at point A, and is supported by springs with stiffnesses k 1 and k 2 which are located at distances a and b from the pin A, respectively. Under the gravity, the bar will rotate to be in the static equilibrium

47 Equivalent Single-Degree-of-Freedom System and Free Vibration 29 k 2 k 2 A a b g I A A Static equilibrium θ k 1 k 1 (a) (b) Figure (a) A horizontal bar with an unstretched spring; (b) Perturbation from static equilibrium configuration configuration. Let θ(t) bethesmall angular displacement of the bar from the static equilibrium position (Figure 1.4.4b). The free body diagram of the bar is shown in Figure Note that the weight of the bar and spring forces due to static deflections (in static equilibrium configuration) have not been included as they cancel out. It should also be noted that the spring deflections with respect to static equilibrium configurations are calculated for a small angular displacement and are found (Figure 1.4.6) to be bθ and aθ for springs with stiffnesses k 2 and k 1, respectively. Net torque about point A in θ direction = k 1 aθa k 2 bθb (1.4.2) Applying Newton s second law of motion, net torque about point A in θ direction = I A θ (1.4.21) k 2 bθ Static equilibrium R A k 2 aθ θ Figure Free body diagram for a horizontal bar

48 3 Vibration of Mechanical Systems b a θ bθ θ aθ Figure Spring displacements for a horizontal bar From Equations and , the differential equation of motion is I A θ + (k 1 a 2 + k 2 b 2 )θ = (1.4.22) Energy Method KE,U = 1 2 I A θ 2 (1.4.23) The total energy is PE,P = 1 2 k 1(aθ) k 2(bθ) 2 (1.4.24) T = U + P = 1 2 I A θ k 1(aθ) k 2(bθ) 2 (1.4.25) The condition of a constant value of the total energy yields dt = I A θ + (k 1 a 2 + k 2 b 2 )θ = (1.4.26) dt Example 1.4.2: A Vertical Rigid Bar Consider the rigid bar of mass m in the vertical configuration (Figure 1.4.7a). This bar is pinned at the point A, and is connected to a spring with the stiffness k at a distance b from the point A. The center of the gravity of the bar C, is located at a distance a from the pin A. Let θ(t) be the small angular displacement of the bar (Figure 1.4.7b). The free body diagram of the bar is shown in Figure 1.4.8a, where R A is the reaction force at the point A. The geometry associated with the angular displacement is shown in Figure 1.4.8b, where the angle θ(t) has been magnified for the sake of clarity. The spring

49 Equivalent Single-Degree-of-Freedom System and Free Vibration 31 A A b a b a k C(cg) k θ m I A (a) mg (b) Figure (a) Vertical bar in static equilibrium; (b) Vertical bar perturbed from static equilibrium deflection is b sin θ, which is approximated as bθ for a small θ. Taking moment about the point A, kbθb cos θ mga sin θ = I A θ (1.4.27) where I A is the mass-moment of inertia about A.Forasmallθ,sinθ θ and cos θ 1. Therefore, I A θ + (mga + kb 2 )θ = (1.4.28) kbθ R A A a a θ a a cosθ θ C a( 1 cosθ ) a sin θ Magnified θ b mg bsin θ (a) (b) Figure (a) Free body diagram for vertical rigid bar; (b) Geometry associated with Figure 1.4.8a

50 32 Vibration of Mechanical Systems Note that the weight of the bar mg, is included here because it does not cause any spring deflection in the vertical static equilibrium configuration. Energy Method KE,U = 1 2 I A θ 2 (1.4.29) PE,P = mga(1 cos θ) k(bθ)2 (1.4.3) In Equation 1.4.3, 1 cos θ is almost equal to zero as cos θ 1. However, 1 cos θ is not negligible in comparison with θ 2, which is also quite small. Therefore, cos θ 1 should not be used in Equation The total energy is given by T = 1 2 I Aθ 2 + mga(1 cos θ) k(bθ)2 (1.4.31) Because the total energy is a constant, dt dt Using sin θ θ, Equation yields = [I A θ + mga sin θ + kb 2 θ] θ = (1.4.32) I A θ + (mga + kb 2 )θ = (1.4.33) Example 1.4.3: Inclined Spring Consider a mass with an inclined massless spring with the stiffness k (Figure 1.4.9). The inclination of the spring is represented by the angle α. When the displacement of the mass m is x, the length of the spring changes from l to l + δ (Figure 1.4.1). From the cosine law, cos(π α) = l2 + x 2 (l + δ) 2 2lx (1.4.34)

51 Equivalent Single-Degree-of-Freedom System and Free Vibration 33 k x α m Figure A mass connected with an inclined spring After some algebra, [ (l + δ) 2 = l x ] x2 cos α + l l 2 (1.4.35) Assume that x/l 1. In this case, the term (x/l) 2 can be neglected and [ l + δ = l x ].5 l cos α (1.4.36) Using the binomial expansion (Appendix B) and neglecting higherorder terms, l + δ = l [1 + x ] l cos α (1.4.37) Therefore, δ = x cos α (1.4.38) The spring force kδ will be directed at an angle β (Figure 1.4.1). Applying Newton s second law in the x-direction, kδ cos β = mẍ (1.4.39) l α l + δ β kδ x m l α l + δ x Figure Free body diagram for system in Figure 1.4.9

52 34 Vibration of Mechanical Systems Because x/l 1, cos β = cos α. Hence Equations and lead to mẍ + (k cos 2 α)x = (1.4.4) Therefore, the equivalent stiffness of the spring is k eq = k cos 2 α (1.4.41) Solution of the Differential Equation of Motion Governing Free Vibration of an Undamped Spring Mass System Assume that (Boyce and DiPrima, 25) x(t) = De st (1.4.42) where D and s are to be determined. Substituting Equation into the differential equation of motion in Equation , (m eq s 2 + k eq )De st = (1.4.43) Here, D is not zero for a nontrivial solution. Therefore, for Equation to be true for all time t, m eq s 2 + k eq = (1.4.44) This is called the characteristic equation. The roots of this equation are s 1 = jω n and s 2 = jω n (1.4.45a, b) where j = 1 is the imaginary number and k eq ω n = (1.4.46) m eq The general solution of the differential Equation is expressed as x(t) = D 1 e jω nt + D 2 e jω nt (1.4.47)

53 Equivalent Single-Degree-of-Freedom System and Free Vibration 35 where D 1 and D 2 are constants. Recall the well-known trigonometric identity, e ± jω nt = cos ω n t ± j sin ω n t (1.4.48) Using Equation , Equation leads to x(t) = (D 1 + D 2 ) cos ω n t + j(d 1 D 2 )sinω n t (1.4.49) It can be shown that D 1 and D 2 are complex conjugates. Therefore, both (D 1 + D 2 ) and j(d 1 D 2 ) will be real numbers. Denote Equation is written as A 1 = D 1 + D 2 and B 1 = j(d 1 D 2 ) (1.4.5a, b) x(t) = A 1 cos ω n t + B 1 sin ω n t (1.4.51) The coefficients A 1 and B 1 depend on initial conditions x() and ẋ(). It is easily seen that Differentiating Equation , A 1 = x() (1.4.52) ẋ(t) = ω n A 1 sin ω n t + ω n B 1 cos ω n t (1.4.53) Substituting t = in Equation , Equation is written as B 1 = ẋ() ω n (1.4.54) x(t) = x() cos ω n t + ẋ() sin ω n t (1.4.55) ω n Alternatively, Equation can also be expressed as where A and ψ are determined as follows: x(t) = Asin(ω n t + ψ) (1.4.56) x(t) = Asin(ω n t + ψ) = Asin ψ cos ω n t + Acos ψ sin ω n t (1.4.57)

54 36 Vibration of Mechanical Systems Comparing Equations and , Asin ψ cos ω n t + Acos ψ sin ω n t = x() cos ω n t + ẋ() ω n Equating coefficients of cos ω n t and sin ω n t on both sides, sin ω n t (1.4.58) Asin ψ = x() (1.4.59) Acos ψ = ẋ() ω n (1.4.6) Squaring Equations and and then adding them, ( ) ẋ() 2 A 2 sin 2 ψ + A 2 cos 2 ψ = (x()) 2 + (1.4.61) Using the fact that sin 2 ψ + cos 2 ψ = 1, ω n ( ) ẋ() 2 A = (x()) 2 + (1.4.62) ω n Dividing Equation by Equation 1.4.6, [ ] ψ = tan 1 ωn x() ẋ() (1.4.63) It should be noted that the value of A is taken to be positive. Furthermore, there is a single solution for the angle ψ. The quadrant in which the angle ψ lies is determined by the signs of the numerator ω n x() and the denominator ẋ(). For example, if ω n x() = 1 and ẋ() = 1, [ ] +1 ψ = tan 1 = 3π rad (1.4.64) 1 4 In summary, the free vibration of an undamped spring mass system is given by x(t) = Asin(ω n t + ψ); A > (1.4.65)

55 Equivalent Single-Degree-of-Freedom System and Free Vibration A x(t) = A sin(ω n t + ψ) 1 Displacement, x(t) πlω n Time, t (sec) Figure Free response of an undamped SDOF spring mass system where ( ) k eq ẋ() 2 [ ] ω n = ; A =+ (x()) m 2 + ; and ψ = tan 1 ωn x() eq ω n ẋ() (1.4.66a, b, c) The free vibration of an undamped SDOF system is purely sinusoidal with the amplitude A and the frequency ω n (Equation and Figure ). This frequency (ω n ) is called the natural frequency, which is an intrinsic property of the SDOF spring mass system. Denoting Newton as N, meter as m, kilogram as kg, the unit of ω n is derived as follows: unit of k eq Nm 1 kg m sec unit of ω n = = = 2 m 1 unit of m eq kg kg = sec 1 = rad/sec (1.4.67)

56 38 Vibration of Mechanical Systems l m p A I A a g Static equilibrium k Figure A diver on a spring board Note that the radian (rad) has been inserted in Equation as it is a dimensionless quantity. The time-period of oscillation (T p ), which is the time required to complete one full cycle (= 2π rad) of oscillation is shown in Figure and expressed as T p = 2π ω n (1.4.68) Using the fact that one cycle = 2π rad, the frequency of oscillation (f ) can also be expressed in the units of cycles/sec as follows: The unit of cycles/sec is called Hertz (Hz). f = ω n 2π = 1 T p cycles/sec (1.4.69) Example 1.4.4: A Diving Board Consider a springboard, which is pinned at the point A and is supported by a spring with stiffness k (Figure ). Let the massmoment of inertia of the board about the point A be I A. A person of mass m p is standing at the edge of the board in static equilibrium. Suddenly, this person jumps from the board and the board starts vibrating.

57 Equivalent Single-Degree-of-Freedom System and Free Vibration 39 New static equilibrium θ (t) A aθ θ a g I A k Figure New static equilibrium for the diving board When the person leaves the board, there will be a new static equilibrium configuration. Let the new static equilibrium configuration be at an angle θ from the original static equilibrium position (Figure ). Then, kaθ a = m p gl (1.4.7) or, θ = m pgl (1.4.71) ka 2 From the results in Example 1.4.1, the differential equation of motion (after the person jumps) will be I A θ + ka 2 θ = (1.4.72) Therefore, the undamped natural frequency will be ka ω n = 2 (1.4.73) I A The initial conditions will be Hence, from Equation , θ() = θ and θ() = (1.4.74) θ(t) = θ sin ω n t (1.4.75)

58 4 Vibration of Mechanical Systems ceq k eq ceq ceq k eq k eq m eq Δ Unstretched spring m eq Static equilibrium x(t) m eq Perturbed from static equilibrium Figure Spring mass damper system 1.5 FREE VIBRATION OF A VISCOUSLY DAMPED SDOF SYSTEM This section deals with the derivation and the solution of the differential equation of motion of a viscously damped SDOF system. The solution of the differential equation of motion is used to characterize the nature of free vibration for different values of damping Differential Equation of Motion A viscously damped SDOF system is shown in Figure At the static equilibrium, the velocity of the mass is zero; therefore, the damper does not provide any force, and Equation 1.4.1, m eq g = k eq, still holds. From the free body diagram in Figure (Perturbed from Static Equilibrium), net force in x-direction = k eq x c eq ẋ (1.5.1) From Newton s second law of motion, k eq x c eq ẋ = m eq ẍ (1.5.2)

59 Equivalent Single-Degree-of-Freedom System and Free Vibration 41 k eq Δ c eq x& k eq ( Δ + x( t)) c eq x& k eq x(t) m eq m eq g Static equilibrium m eq m eq m eq g Perturbed from static equilibrium Figure Free body diagram of the spring mass damper system or m eq ẍ + c eq ẋ + k eq x = (1.5.3) Equation is the governing differential equation of motion Solution of the Differential Equation of Motion Governing Free Vibration of a Damped Spring Mass System Assume that (Boyce and DiPrima, 25) x(t) = De st (1.5.4) where D and s are to be determined. Substituting Equation into Equation 1.5.3, (m eq s 2 + c eq s + k eq )De st = (1.5.5) Here, D is not zero for a nontrivial solution. Therefore, for Equation to be true for all time t, m eq s 2 + c eq s + k eq = (1.5.6) Roots of the quadratic Equation are given by c eq ± ceq 2 4k eqm eq s 1,2 = (1.5.7) 2m eq

60 42 Vibration of Mechanical Systems The value of ceq 2 4k eqm eq has a direct influence on the nature of the solution or the response. Roots are complex conjugates when ceq 2 4k eqm eq <, whereas they are real numbers for ceq 2 4k eqm eq. The critical damping c c is defined as the value of c eq for which c 2 eq 4k eqm eq = (1.5.8) From Equation 1.5.8, the expression for the critical damping coefficient c c is obtained as Then, the damping ratio ξ is defined as c c = 2 k eq m eq = 2m eq ω n (1.5.9) ξ = c eq (1.5.1) c c Now, Equation is written as where s 1,2 = c eq 2m eq ± ( ceq 2m eq ) 2 k eq m eq (1.5.11) c eq = c eq c c = ξω n (1.5.12) 2m eq c c 2m eq Equation is written as s 1,2 = ξω n ± ξ 2 ωn 2 ω2 n = ξω n ± ω n ξ 2 1 (1.5.13) Three cases of damping are defined as follows: a. Underdamped ( <ξ<1or< c eq < c c ) b. Critically damped (ξ = 1orc eq = c c ) c. Overdamped (ξ >1orc eq > c c ) Case I: Underdamped ( <ξ<1 or < c eq < c c ) In this case, the roots (s 1 and s 2 ) are complex conjugates. From Equation , s 1,2 = ξω n ± jω n 1 ξ 2 (1.5.14)

61 Equivalent Single-Degree-of-Freedom System and Free Vibration 43 or, s 1 = ξω n + jω d and s 2 = ξω n jω d (1.5.15a, b) where The general solution is expressed as ω d = ω n 1 ξ 2 (1.5.16) x(t) = D 1 e s 1t + D 2 e s 2t = e ξω nt (D 1 e jω dt + D 2 e jω dt ) (1.5.17) where D 1 and D 2 are constants. Similar to the solution procedure for an undamped spring mass system, the following well-known trigonometric identity is again used: Therefore, Equation leads to e ± jω dt = cos ω d t ± j sin ω d t (1.5.18) x(t) = e ξω nt (A 1 cos ω d t + B 1 sin ω d t) (1.5.19) where A 1 and B 1 are real numbers defined by Equations 1.4.5a,b. To determine A 1 and B 1, initial conditions x() and ẋ() are used. First, Differentiating Equation , At t =, Using Equation 1.5.2, x() = A 1 (1.5.2) ẋ(t) = e ξωnt ( ω d A 1 sin ω d t + ω d B 1 cos ω d t) ξω n e ξωnt (A 1 cos ω d t + B 1 sin ω d t) (1.5.21) ẋ() = ω d B 1 ξω n A 1 (1.5.22) B 1 = ẋ() + ξω nx() ω d (1.5.23)

62 44 Vibration of Mechanical Systems Alternatively, Equation can be written as x(t) = e ξωnt Asin(ω d t + ψ) (1.5.24) To determine the amplitude A and the phase ψ in terms of A 1 and B 1, Equations and are compared to yield Asin(ω d t + ψ) = A 1 cos ω d t + B 1 sin ω d t (1.5.25) or Asin ψ cos ω d t + Acos ψ sin ω d t = A 1 cos ω d t + B 1 sin ω d t (1.5.26) Equating the coefficients of cos ω d t and sin ω d t on both sides, Asin ψ = A 1 (1.5.27) Acos ψ = B 1 (1.5.28) Therefore, following derivations in Section 1.4, [ ] ẋ() + ξωn x() 2 A = [x()] 2 + (1.5.29) ω d [ ] ψ = tan 1 ω d x() ẋ() + ξω n x() (1.5.3) Summary: The free vibration of an underdamped ( <ξ<1or < c eq < c c ) spring mass damper system is given by x(t) = Ae ξωnt sin(ω d t + ψ); A > (1.5.31) where ω d, A, and ψ are given by Equations , , and The free response of an underdamped system is shown in Figure Compared to free vibration of an undamped system in Figure , the following observations are made: a. Amplitude is exponentially decaying and lim x(t) ast. b. The natural frequency of the damped system is ω d, which is smaller than the undamped natural frequency ω n. The time period of the free response of an underdamped system is 2π/ω d, which will be larger than that of the undamped system.

63 Equivalent Single-Degree-of-Freedom System and Free Vibration Aexp( ξ*t) < ξ < Displacement, x.5 Aexp( ξ*t ) 1 α β α not equal to β α + β = 2π/ω d Time, t (sec) Figure Free response of an underdamped SDOF system c. Unlike a pure sinusoidal function, the half-way point in a period of underdamped free response does not correspond to zero velocity, when the starting point of a period is taken to be zero velocity condition. This point is expressed in Figure by the following relationship: α β. Case II: Critically Damped (ξ = 1 or c eq = c c ) In this case, the roots (s 1 and s 2 ) are negative real numbers and equal. From Equation , s 1 = s 2 = ω n (1.5.32) Therefore, the general solution (Boyce and DiPrima, 25) of the differential Equation will be x(t) = A 1 e s 1t + B 1 te s 1t = A 1 e ω nt + B 1 te ω nt (1.5.33)

64 46 Vibration of Mechanical Systems where A 1 and B 1 are constants. At t =, Differentiating Equation , x() = A 1 (1.5.34) ẋ(t) = ω n A 1 e ω nt ω n B 1 te ω nt + B 1 e ω nt (1.5.35) At t =, ẋ() = ω n A 1 + B 1 (1.5.36) Using Equation , Therefore, from Equation , B 1 = ẋ() + ω n x() (1.5.37) x(t) = x()e ω nt + [ẋ() + ω n x()]te ω nt (1.5.38) The response of a critically damped system is nonoscillatory as shown in Figure Furthermore, lim x(t) ast. Case III: Overdamped (ξ >1 or c eq > c c ) From Equation , both roots s 1 and s 2 are negative real numbers. s 1 = ξω n + ω n ξ 2 1 < (1.5.39) s 2 = ξω n ω n ξ 2 1 < (1.5.4) And, the general solution of the differential Equation will be x(t) = A 1 e s 1t + B 1 e s 2t (1.5.41) where A 1 and B 1 are constants. At t =, Differentiating Equation , x() = A 1 + B 1 (1.5.42) ẋ(t) = A 1 s 1 e s 1t + B 1 s 2 e s 2t (1.5.43)

65 Equivalent Single-Degree-of-Freedom System and Free Vibration Overdamped (ξ > 1) Underdamped ( < ξ < 1) Displacement, x.5.5 Critically damped (ξ = 1) Time, t (sec) Figure Free responses of underdamped, critically damped, and overdamped SDOF systems At t =, Solving Equations and , and ẋ() = A 1 s 1 + B 1 s 2 (1.5.44) A 1 = s 2x() ẋ() s 2 s 1 (1.5.45) B 1 = s 1x() + ẋ() (1.5.46) s 2 s 1 The free response of an overdamped system is also nonoscillatory as shown in Figure Furthermore, lim x(t) ast. Comparing responses of underdamped, critically damped, and overdamped systems, it is seen in Figure that the free response of a critically damped system decays at the fastest rate. In other

66 48 Vibration of Mechanical Systems Im ag < ξ < 1 jω n ξ = ξ ξ = 1 ξ = Shield ξ > 1 ω n ξ > 1 ξ = Re al < ξ < 1 jω n Figure Locations of characteristic roots in the complex plane as ξ is increased from to ( Shield indicates that the root going toward the origin never crosses the imaginary axis) words, the rate of decay of free response of a critically damped system is higher than that of an overdamped system. This result is counterintuitive as a higher value of damping is expected to be associated with a higher rate of energy loss and therefore a higher value of the decay rate of free response. To understand this result, locations of characteristic roots s 1 and s 2 are plotted in Figure as ξ is increased from to. The following information is used for the construction of Figure 1.5.5: a. For ξ =, s 1 =+jω n and s 2 = jω n (1.5.47) b. For <ξ<1, s 1 = ξω n + jω n 1 ξ 2 and s 2 = ξω n jω n 1 ξ 2 (1.5.48) The real part (RP) and the imaginary part (IP) ofs 1 and s 2 are as follows: RP = ξω n and IP =±ω n 1 ξ 2 (1.5.49)

67 Equivalent Single-Degree-of-Freedom System and Free Vibration 49 Therefore, RP 2 + IP 2 = ξ 2 ωn 2 + ω2 n (1 ξ 2 ) = ωn 2 (1.5.5) This is the equation of a circle with the radius ω n and the center at the origin of the complex plane. Since RP is negative, the roots move along the semicircle in the left half of the complex plane as ξ varies from to 1 (Figure 1.5.5). c. For ξ = 1, s 1 = s 2 = ω n (1.5.51) d. For ξ>1, s 1 = ξω n + ω n ξ 2 1 < (1.5.52) s 2 = ξω n ω n ξ 2 1 < (1.5.53) Note that s 1 <ω n, s 2 >ω n (1.5.54) Equation is rewritten as x(t) = A 1 e s 1 t + B 1 e s 2 t (1.5.55) Because s 2 > s 1 for an overdamped system, the term with e s 2 t dies at a rate faster than the term with e s1 t. As a result, the dominant term in Equation is the one with e s1 t. The critically damped system decays at the rate of e ωnt. Since s 1 <ω n,the decay rate of an overdamped system is slower than that of a critically damped system. Example 1.5.1: A Rigid Bar Supported by a Spring and a Damper Consider a rigid bar of length l, which is pinned at the point A and is supported by a spring with the stiffness k and a damper with the coefficient c (Figure 1.5.6). The mass of the bar is m and is concentrated at its right end as shown in Figure

68 5 Vibration of Mechanical Systems l 1 c m A a l 2 k Massless and rigid bar Figure Rigid bar with a spring and a damper The free body diagram is shown in Figure for a small angular displacement θ from its static equilibrium position, where R A is the unknown reaction force at the point A. Taking moment about the point A, kaθa cl 1 θl 1 = I A θ (1.5.56) where I A is the mass-moment of inertia of the bar about A. Here, I A = ml 2 (1.5.57) Substituting Equation into Equation , ml 2 θ + cl 2 θ 1 + ka 2 θ = (1.5.58) Therefore, m eq = ml 2 ; k eq = ka 2 ; and c eq = cl 2 1 (1.5.59) l 1 cl 1 θ A R A a kaθ l θ m small θ l = l 1 + l 2 Figure Free body diagram of a rigid bar in Figure 1.5.6

69 Equivalent Single-Degree-of-Freedom System and Free Vibration 51 The undamped natural frequency is k eq ω n = = a k m eq l m (1.5.6) The critical damping for this system is c c = 2 k eq m eq = 2la km (1.5.61) Consider the following numerical values: m = 1kg,c = 2 N sec/m, a =.4m,l 1 =.5 m, and l = 1m Case I: If the stiffness k = 1 N/m, c c = 2la km = 8N sec/m As a result, c > c c and the system is overdamped. The damping ratio ξ = 2.5 and the damped natural frequency is not defined. Case II: If the stiffness k = 9 N/m, c c = 2la km = 24 N sec/m As a result, c < c c and the system is underdamped. The damping ratio ξ =.833 and the damped natural frequency ω d = ω n 1 ξ 2 = rad/sec Logarithmic Decrement: Identification of Damping Ratio from Free Response of an Underdamped System ( <ξ<1) Let us assume that two successive peak displacements in the response of an underdamped system are known. If the first known peak displacement occurs at t = t 1 (Figure 1.5.8), Equation yields x(t 1 ) = x 1 = Ae ξω nt 1 sin(ω d t 1 + ψ) (1.5.62)

70 52 Vibration of Mechanical Systems x 1 x 2 Displacement, x t 1 2π/ω d t Time, t (sec) Figure Free vibration of an underdamped system (illustration of log decrement) Let the second peak displacement be at t = t 2 (Figure 1.5.8). From Equation , x(t 2 ) = x 2 = Ae ξω nt 2 sin(ω d t 2 + ψ) (1.5.63) From the characteristic of free underdamped response, As a result, t 2 = t 1 + 2π ω d (1.5.64) sin(ω d t 2 + ψ) = sin(ω d t 1 + ψ + 2π) = sin(ω d t 1 + ψ) (1.5.65) Dividing Equation by Equation and using Equations , x 1 x 2 = e ξω n(t 2 t 1 ) = e 2πξωn ω d (1.5.66)

71 Equivalent Single-Degree-of-Freedom System and Free Vibration 53 Using the fact that ω d = ω n 1 ξ 2 (Equation ), x 1 2πξ = e 1 ξ 2 (1.5.67) x 2 The ratio of the two successive amplitudes only depends on the damping ratio ξ, that is, it is independent of the undamped natural frequency ω n. Taking natural logarithm of Equation with respect to the base e, ln x 1 = 2πξ x 2 1 ξ 2 (1.5.68) The natural logarithm of two successive amplitudes or peak displacements is known as the logarithmic decrement δ, that is, Therefore, from Equations and , δ = ln x 1 x 2 (1.5.69) δ = 2πξ 1 ξ 2 (1.5.7) For a small ξ, 1 ξ 2 1 and from Equation 1.5.7, ξ = δ 2π (1.5.71) In general, Equation is directly solved to yield ξ = δ (2π)2 + δ 2 (1.5.72) The approximation Equation is found to be valid for ξ.2or equivalently δ.4π. Since ratio of two successive amplitudes only depends on the damping ratio, x 1 x 2 = x 2 x 3 = = x m 1 x m = x m x m+1 (1.5.73)

72 54 Vibration of Mechanical Systems 1.5 x 1 x 2 x 3 x 4 x 5 x 6 x 7 x Displacement, x Time, t (sec) Figure Free response of an underdamped system for a small damping ratio where x i+1 is the amplitude after i cycles of oscillation, i = 1, 2,...,m 1, m (Figure 1.5.9). Therefore, x 1 x m+1 = x 1 x 2 x 2 x 3 xm 1 x m x m x m+1 (1.5.74) Using Equation , ( ) x m 1 x1 = (1.5.75) x m+1 x 2 Taking natural logarithms of both sides of Equation and using the definition of logarithmic decrement Equation , ln x ( ) m 1 x1 = ln = mln x 1 = mδ (1.5.76) x m+1 x 2 x 2 Therefore, the logarithmic decrement δ can also be computed by the following relationship: δ = 1 m ln x 1 x m+1 (1.5.77)

73 Equivalent Single-Degree-of-Freedom System and Free Vibration 55 A A x x = Racket tip displacement Figure A tennis racket and a ball where x m+1 is the amplitude after m cycles of oscillation. The expression in Equation is useful for low values of the damping ratio ξ for which the difference between the two successive amplitudes can be so small that they may not be accurately differentiable by a measuring instrument, that is, the expression in Equation may not accurately predict the value of the logarithmic decrement. But, if one considers the amplitude after a certain number of cycles of oscillation, for example, m = 7 in Figure 1.5.9, the difference between x 1 and x m+1 can be quite significant. As a result, the ratio of x 1 and x m+1 can be estimated quite accurately via a measuring instrument and Equation can lead to an accurate value of δ and the damping ratio ξ using Equation Example 1.5.2: Vibration of a Tennis Racket A tennis ball hits the tennis racket (Figure 1.5.1) and imparts a velocity of 1.5 m/sec to the racket tip. The natural frequency and the damping ratio of the tennis racket (Oh and Yum, 1986) are given to be Hz and.297, respectively. Determine the maximum displacement of the racket tip. Solution Given: ω n = Hz = rad/sec, ξ =.297, x() =, and ẋ() = 1.5m/sec Therefore, ω d = ω n 1 ξ 2 = rad/sec

74 56 Vibration of Mechanical Systems For an underdamped system, the free response is described by Equations : x(t) = Ae ξω nt sin(ω d t + ψ) where and [ ] ẋ() + ξωn x() 2 A = [x()] 2 + = ẋ() =.76 m ω d ω d [ ] ψ = tan 1 ω d x() = ẋ() + ξω n x() Therefore, For the maximum displacement, x(t) = Ae ξω nt sin(ω d t) ẋ(t) = Ae ξω nt ω d cos(ω d t) + A( ξω n )e ξω nt sin(ω d t) = Let t be the time corresponding to the maximum displacement. Then, tan(ω d t ) = ω d 1 ξ 2 = = ξω n ξ Therefore, ω d t = rad or t = =.78 sec ω d The maximum displacement is computed as x(t ) =.76 e sin(1.541) =.73 m

75 Equivalent Single-Degree-of-Freedom System and Free Vibration Displacement, x Time, t (sec) Figure Measured free vibration of a damped spring mass system (displacement in mm) Example 1.5.3: Damping Ratio and Undamped Natural Frequency from Free Response For the free vibration of an SDOF system (Figure ), the amplitudes 1.43 mm and mm are measured at sec and sec, respectively. Find the undamped natural frequency ω n and the damping ratio ξ. The number of cycles of oscillation between two measured amplitudes is 9. Hence, the time period of damped oscillation is T d = = sec 9 Therefore, ω d = 2π = rad/sec T d From Equation , the logarithmic decrement δ is δ = 1 9 ln =.63

76 58 Vibration of Mechanical Systems From Equation , ξ = δ (2π)2 + δ 2 =.1 Here, (2π) 2 + δ 2 = (2π) 2. Therefore, the approximation in Equation can also be used. 1.6 STABILITY OF AN SDOF SPRING MASS DAMPER SYSTEM The stability of a linear SDOF spring mass damper system refers to the nature of the free vibration caused by nonzero initial conditions in the absence of any external excitation. There are three possible situations: a. Stable: The free vibration response dies out as time goes to infinity, that is, (x(t) ast ) b. Marginally Stable/Unstable: The response remains bounded but nonzero as time goes to infinity. c. Unstable: The response becomes unbounded as the time goes to infinity, that is, (x(t) as t ) As shown in Section 1.5.2, the characteristic of the free response is governed by e st term, where s is a root of the characteristic Equation 1.5.6: m eq s 2 + c eq s + k eq s = (1.6.1) In general, a root s is represented as a complex number: s = s R + js I ; j = 1 (1.6.2) where s R and s I are real and imaginary parts of s. For a purely real root, s I =. Similarly, for a purely imaginary root, s R =. From Equation 1.6.2, e st = e (s R+ js I )t = e srt (cos(s I t) + j sin(s I t)) (1.6.3)

77 Equivalent Single-Degree-of-Freedom System and Free Vibration 59 Im ag. s I Left Half Right Half s R Re al Figure Complex s plane Because cosine and sine terms are bounded between 1 and +1, the sign of the real part s R will determine whether the response will die out, remain nonzero and bounded, or grow to be unbounded as t. The system will be stable, marginally stable/unstable, and unstable when s R < (left half of the complex plane, Figure 1.6.1), s R = (imaginary axis of the complex plane, Figure 1.6.1), and s R > (right half of the complex plane, Figure 1.6.1), respectively. This fact can be summarized as follows: a. The spring mass damper system will be stable, provided both roots are located in the left half (excluding imaginary axis) of the complex plane. b. The spring mass damper system will be marginally stable/ unstable, provided at least one root is on the imaginary axis, and no root is in the right half of the complex plane. c. The spring mass damper system will be unstable, provided one root is in the right half of the complex plane. Equation is a second-order polynomial or a quadratic equation. The roots of this equation can be easily calculated, and therefore their locations in the complex plane can be easily determined to evaluate the stability of the system. The necessary and sufficient conditions for the stability in the context of the quadratic Equation are as follows: a. None of the coefficients vanishes. b. All coefficients must have the same sign.

78 6 Vibration of Mechanical Systems Static equilibrium k c kaθ caθ & a A cg b R A θ b mg A small θ R A : Reaction force Figure Inverted pendulum and free body diagram Since m eq >, necessary and sufficient conditions for stability are c eq > (1.6.4) and k eq > (1.6.5) In other words, a spring mass damper system is guaranteed to be stable, provided both equivalent damping and equivalent stiffness are positive. Example 1.6.1: Inverted Pendulum (Negative Stiffness) Consider an inverted pendulum which is supported by a spring with the stiffness k, located at a distance a from the pivot point A (Figure 1.6.2). The mass of the pendulum is m and the center of gravity is located at a distance b from the pivot point A. Let θ be the clockwise small rotation of the bar from its static equilibrium configuration. Taking moment about the point A, kaθa + mgbθ ca θa = I A θ (1.6.6) where I A is the mass-moment of inertia of bar about the point A. Rearranging Equation 1.6.6, I A θ + ca 2 θ + (ka 2 mgb)θ = (1.6.7)

79 Equivalent Single-Degree-of-Freedom System and Free Vibration 61 v r : Relative wind velocity f a : Aerodynamic force v a f a v r v c Figure Wind across a cable with a circular cross section The equivalent stiffness k eq for the system is The equivalent stiffness k eq is negative if k eq = ka 2 mgb (1.6.8) k < mgb (1.6.9) a 2 When the condition in Equation is satisfied, the vertical static equilibrium configuration is unstable. Example 1.6.2: Galloping of Transmission Lines During Winter (Negative Damping) There are long electric transmission cables (Den Hartog, 1956), which have circular cross sections. Assume that there is also a cross wind with the velocity v a and the cable is undergoing a small transverse oscillation. Let v c be the instantaneous cable velocity, which is directed downwards as shown in Figure The cable will experience the relative wind velocity v r, and the effective aerodynamic force f a on the cable will be along the direction of the relative velocity v r. The vertical component of this aerodynamic force is in the direction opposite to the downward velocity of the cable. In other words, the aerodynamic force opposes the cable motion, and as a result, aerodynamic forces dissipate energy and the equivalent damping c eq is positive. During the winter season in certain regions, there is an ice formation and effective cross section of the cable becomes noncircular

80 62 Vibration of Mechanical Systems v r : Relative wind velocity f a : Aerodynamic force Ice v a vr v c fa Figure Wind across an ice-coated cable as shown in Figure In this case, the aerodynamic force can also be downwards when the velocity of the cable is downward. In other words, the wind will further strengthen the downward motion of the cable, and as a result, aerodynamic forces add energy to the system and the equivalent damping c eq is negative. Example 1.6.3: Galloping of a Square Prism (Negative Damping) Consider a square prism with the mass m, which is subjected to a cross wind with the velocity v a (Figure 1.6.5). The width and the height of the prism are w and h, respectively. The displacement of the prism with respect to the static equilibrium position is denoted by x(t). For a small velocity ẋ, the vertical component of the aerodynamic force f av (t) is given by (Thompson, 1982) f av (t) = 1 2 ρv awhβẋ (1.6.1) where ρ is the air density. The constant β has been experimentally found to be a positive number. The differential equation of motion is easily derived to be mẍ + cẋ + kx = f av (t) (1.6.11) Substituting Equation into Equation , mẍ + (c c n )ẋ + kx = (1.6.12)

81 Equivalent Single-Degree-of-Freedom System and Free Vibration 63 f av f av : Vertical component of aerodynamic force v r : Relative wind velocity w x& α v a m h x Static equilibrium v r k c Figure Wind across a square prism where c n = 1 2 ρv awhβ (1.6.13) When c < c n, the equivalent damping is negative and system becomes unstable. EXERCISE PROBLEMS P1.1 Consider the SDOF system shown in Figure P1.1. All the shafts and the connections among them are massless. The material of the shaft is steel. Also, m 1 = 1.1kg, m 2 = 1.4kg, m b = 2.1kg The stiffness k is half of the cantilever beam stiffness.

82 64 Vibration of Mechanical Systems Find the equivalent mass and the equivalent stiffness with respect to the displacement x..4 m k m b m 2.8 m.4 m dia. = 1 cm Midspan m 1 x dia. = 2 cm dia. = 3 cm.9 m Figure P1.1 A combination of beams and a rigid bar P1.2 Find the equivalent mass of a spring under the assumption that the velocity distribution along the length of the spring is parabolic. P1.3 Consider the cantilever beam with mass m and length l (Figure P1.3). Obtain the equivalent mass of the cantilever beam under the assumption that the beam deflection is y(z) = x πz [1 cos( 2 l )]. z = z z = l x(t) Figure P1.3 A cantilever beam P1.4 An object with mass m and rectangular cross section A is floating in a liquid with mass density ρ (Figure P1.4). Derive the governing

83 Equivalent Single-Degree-of-Freedom System and Free Vibration 65 differential equation of motion, and obtain the natural frequency of the system. Floating object x Liquid level Figure P1.4 A floating object P1.5 An L-shaped bracket hinged at point A is supported by two springs with stiffnesses k 1 and k 2 (Figure P1.5). The mass of the bracket is m and is uniformly distributed. k 2 a 2 l 2 a 1 Hinge A l 1 k 1 Figure P1.5 L-shaped bracket supported by springs Find the equivalent mass, the equivalent stiffness and the natural frequency of the system. P1.6 The mass of a complex-shaped object is 3 kg. When this object is suspended like a pendulum (Figure P1.6), its frequency of oscillation is 3 cycles/min. The center of mass is at a distance of.2 m from the pivot point A. Find the mass-moment of inertia of the object about its center of mass.

84 66 Vibration of Mechanical Systems A.2 m Center of mass Figure P1.6 Complex-shaped object as a pendulum P1.7 An uniform rigid bar of length l = 5 cm and mass m = 7kg is hinged at one end (Figure P1.7). At the other end, this bar is suddenly attached to a massless spring with stiffness k = 1, N/m. Derive an expression for the angular oscillation of the bar. g A l k Figure P1.7 A uniform rigid bar P1.8 A cylinder of mass m 2 rolls without slipping inside the box with mass m 1 (Figure P1.8). Derive the equivalent mass and the stiffness of the system.

85 Equivalent Single-Degree-of-Freedom System and Free Vibration 67 x(t) Box with mass m 1 m 2 Cylinder with mass k Figure P1.8 A cylinder inside a box P1.9 Consider the gear shaft system in Figure P1.9. The length and the diameter of shaft A are 5 cm and 4 cm, respectively. Similarly, the length and the diameter of shaft B are 6 cm and 3 cm, respectively. Masses of gears A and B are 1.5 kg and.5 kg, respectively. The gear ratio is 2. Gear B Shaft B Shaft A Gear A Figure P1.9 Gear shaft system Assuming that the shaft material is steel, determine the equivalent mass and the equivalent stiffness with respect to the angular displacement of gear A. What is the natural frequency? P1.1 An object with mass = 5 kg is attached to a table with four steel legs of diameter =.15 m and length =.1 m (Figure P1.1). a. Derive the differential equation for vibration of the object in vertical direction. b. Determine the natural frequency of vibration. c. What is the maximum possible amplitude of vibration so that the maximum vibratory stress in each table leg is less than 7% of the

86 68 Vibration of Mechanical Systems Figure P1.1 An object on the table yield point stress? For this maximum amplitude, plot the allowable region of initial displacement and the initial velocity of the object. d. Develop a MATLAB program to solve the governing differential equation. Compare the solution from your program to that obtained analytically for an allowable initial displacement and the initial velocity determined in part (c). P1.11 A tank (Figure P1.11) with mass m 1 = 2, kg fires a cannon with mass m 2 = 2 kg and velocity = 1 m/sec. The recoil mechanism consists of a spring with stiffness = 11, N/m, and a damper. There are three possible values of the damping coefficient:.2c c, c c, and 1.5c c where c c is the critical damping. Cannon c k Tank Figure P1.11 A tank with recoil mechanism a. For each value of the damping constant, determine the time required to come back to the original firing position. Validate your analytical results via comparison with results from numerical integration of the solution of the differential equation via MATLAB.

87 Equivalent Single-Degree-of-Freedom System and Free Vibration 69 A l 1 l c Massless and rigid bar k k m (a) Displacement of mass (cm) Time, t (sec) Figure P1.12 (a) Mass supported via a spring and a massless rigid bar; (b) Free vibration of mass m (b)

88 7 Vibration of Mechanical Systems b. What will be your recommendation regarding the choice of the damping coefficient? Explain your answer. P1.12 Consider the system shown in Figure P1.12a, where m = 1 kg, l 1 = 35 cm, and l = 5 cm. A record of free vibration is shown in Figure 1.12b. Find the values of the spring constant k and the damping coefficient c. P1.13 Consider a spring mass system (Figure P1.13) where the mass is on a surface with the coefficient of friction μ. Assuming that the initial displacement of the mass is x(), determine the expression for the displacement x(t). k m x Coulomb friction Figure P1.13 Frictionally damped spring mass system P1.14 Consider a simple electromagnetic suspension system shown in Figure P1.14. I h f m x(t) mg Figure P1.14 An electromagnetic suspension system The electromagnetic force f m is given by f m = α I2 h 2

89 Equivalent Single-Degree-of-Freedom System and Free Vibration 71 where I and h are the coil current and the air gap, respectively. The constant α = μ N 2 A p where μ, N, and A p are the air permeability, the number of coil turns, and the face area per single pole of the magnet, respectively. Let h be the desired air gap. Then, the current I is calculated from the following static equilibrium condition: α I2 = mg h 2 Let x(t) be the dynamic displacement of the mass with respect to the static equilibrium position. Derive the differential equation of motion and determine the stability of system. Show that the dynamic characteristic of this system is equivalent to that of an inverted pendulum. P1.15 Consider the system in Figure P1.15. Determine the natural frequencies when the mass m is constrained to move along x and y directions, respectively. k 1 y k 2 α m β x Figure P1.15 Mass supported by two inclined springs

90 2 VIBRATION OF A SINGLE-DEGREE-OF-FREEDOM SYSTEM UNDER CONSTANT AND PURELY HARMONIC EXCITATION First, responses of undamped and damped single-degree-of-freedom (SDOF) spring mass systems are presented in the presence of a constant external force. An important example of input shaping is shown. Using the input shaping procedure, the system settles to a steady state in a finite time in spite of a low level of damping. Next, complete solutions of both undamped and damped spring mass systems under sinusoidal excitation are derived. Amplitudes and phases of the steady-state responses are derived along with force transmissibility, quality factor, and bandwidth. These results are fundamental tools for machine design. Then, solutions to rotating unbalance and base excitation problems are provided. Next, the basic principles behind the designs of vibration measuring instruments (vibrometer and accelerometer) are presented. Last, the concept of equivalent viscous damping is presented for nonviscous energy dissipation. 2.1 RESPONSES OF UNDAMPED AND DAMPED SDOF SYSTEMS TO A CONSTANT FORCE Consider a damped SDOF system subjected to a force f (t) (Figure 2.1.1). Using the free body diagram in Figure 2.1.1, net force in x-direction = k eq x c eq ẋ + f (t) (2.1.1) 72

91 Vibration of an SDOF System under Constant and Purely Harmonic Excitation 73 c eq k eq c eq x& k eq x(t) m eq x(t) m eq f (t) f (t) Figure An SDOF spring mass damper system subjected to external excitation Applying Newton s second law of motion, k eq x c eq ẋ + f (t) = m eq ẍ (2.1.2) Therefore, the differential equation of motion is m eq ẍ + c eq ẋ + k eq x = f (t) (2.1.3) Let the force f (t) be a step function as shown in Figure From Equation 2.1.3, the differential equation of motion for a step forcing function is m eq ẍ + c eq ẋ + k eq x = f ; t (2.1.4) f (t) f t Figure A step forcing function

92 74 Vibration of Mechanical Systems The solution of Equation is composed of two parts: x(t) = x h (t) + x p (t) (2.1.5) where x h (t) is the homogeneous solution satisfying and x p (t) is a particular solution satisfying m eq ẍ h + c eq ẋ h + k eq x h = (2.1.6) m eq ẍ p + c eq ẋ p + k eq x p = f (2.1.7) To determine the particular solution x p (t), it is assumed that x p (t) = x, a constant (2.1.8) Substituting Equation into Equation 2.1.7, m eq ẍ + c eq ẋ + k eq x = f (2.1.9) Since x is a constant, ẋ = and ẍ =. Therefore, x = f (2.1.1) k eq Therefore, from Equation 2.1.5, x(t) = x h (t) + f k eq (2.1.11) where the homogeneous part x h (t) depends on the amount of damping as discussed in Sections 1.4 and 1.5. Case I: Undamped (ξ = ) and Underdamped ( <ξ<1) From Equation , From Equation , x h (t) = e ξω nt (A 1 cos ω d t + B 1 sin ω d t) (2.1.12) x(t) = e ξω nt (A 1 cos ω d t + B 1 sin ω d t) + f k eq (2.1.13)

93 Vibration of an SDOF System under Constant and Purely Harmonic Excitation 75 At t =, Differentiating Equation , At t =, Using Equation , x() = A 1 + f k eq A 1 = x() f k eq (2.1.14) ẋ(t) = e ξωnt ( ω d A 1 sin ω d t + ω d B 1 cos ω d t) ξω n e ξωnt (A 1 cos ω d t + B 1 sin ω d t) (2.1.15) ẋ() = ω d B 1 ξω n A 1 (2.1.16) B 1 = ẋ() + ξω n(x() f /k eq ) (2.1.17) ω d With zero initial conditions (x() = and ẋ() = ), Equations and yield where A 1 = f k eq (2.1.18) B 1 = ξω n ω d f k eq = χ f k eq (2.1.19) χ = ξ 1 ξ 2 (2.1.2) For zero initial conditions, x(t) = f k eq [1 e ξω nt cos ω d t e ξω nt χ sin ω d t]; t (2.1.21) Case II: Critically Damped (ξ = 1 or c eq = c c ) From Equation , x h (t) = A 1 e ω nt + B 1 te ω nt (2.1.22)

94 76 Vibration of Mechanical Systems From Equation , At t =, x(t) = A 1 e ω nt + B 1 te ω nt + f k eq (2.1.23) Differentiating Equation , x() = A 1 + f k eq A 1 = x() f k eq (2.1.24) ẋ(t) = ω n A 1 e ω nt ω n B 1 te ω nt + B 1 e ω nt (2.1.25) At t =, ẋ() = ω n A 1 + B 1 (2.1.26) Using Equation , ( B 1 = ẋ() + ω n x() f ) k eq (2.1.27) For zero initial conditions (x() = and ẋ() = ), substitution of Equations and into Equation yields x(t) = f k eq [1 e ω nt ω n te ω nt ]; t (2.1.28) Case III: Overdamped (ξ >1 or c eq > c c ) From Equations and 1.5.4, both characteristic roots s 1 and s 2 are negative real numbers, s 1 = ξω n + ω n ξ 2 1 < (2.1.29) s 2 = ξω n ω n ξ 2 1 < (2.1.3) From Equation , x h (t) = A 1 e s 1t + B 1 e s 2t (2.1.31)

95 Vibration of an SDOF System under Constant and Purely Harmonic Excitation 77 From Equation , At t =, x(t) = A 1 e s 1t + B 1 e s 2t + f k eq (2.1.32) x() = A 1 + B 1 + f (2.1.33) k eq Differentiating Equation , ẋ(t) = A 1 s 1 e s 1t + B 1 s 2 e s 2t (2.1.34) At t =, Solving Equations and , and ẋ() = A 1 s 1 + B 1 s 2 (2.1.35) A 1 = s 2(x() f /k eq ) ẋ() s 2 s 1 (2.1.36) B 1 = s 1(x() f /k eq ) + ẋ() (2.1.37) s 2 s 1 For zero initial conditions (x() = and ẋ() = ), substitution of Equations and into Equation yields x(t) = f k eq [ 1 s 2 s 2 s 1 e s 1t + s 1 s 2 s 1 e s 2t where s 1 and s 2 are given by Equations and Finally, it should be noted that ] ; t (2.1.38) x h (t) as t (2.1.39) for any damping c eq > orξ>. Therefore, in the steady state (t ), from Equation x ss = f k eq (2.1.4)

96 78 Vibration of Mechanical Systems Underdamped ( < ξ < 1) Critical y Damped (ξ = 1) x() = x() & = Displacement, x/(t /k eq ) Overdamped (ξ > 1) Nondimensional time, ω n t Figure Response to the unit step forcing function with zero initial conditions The step responses (Equations , , and ) are plotted in Figure for zero initial conditions. Example 2.1.1: Robot Vibration Consider a single link robot manipulator with a rigid link but with a flexible revolute joint (Figure 2.1.4a). The length of the link and the torsional stiffness of the joint are l and k t, respectively. Static equilibrium without payload k t θ m A θ l Torque h(t) mgl t (a) (b) Figure (a) A robot with a payload; and (b) Torque due to sudden payload

97 Vibration of an SDOF System under Constant and Purely Harmonic Excitation θ(t)/θ Nondimensional time, ω n t Figure Angular oscillation of a robot arm When the gripper suddenly gets a payload of mass m, the robot link undergoes a torque that is a step function h(t) (Figure 2.1.4). The differential equation of motion is I A θ + k t θ = h(t) (2.1.41) where θ(t) is measured from the static equilibrium configuration without the payload, and I A is the mass-moment of inertia of the link with the payload about the joint axis A. Initial conditions are θ() = and θ() =. Using Equation with ξ =, where θ(t) = θ (1 cos ω n t); t (2.1.42) θ = mgl (2.1.43) k t It should be noted that θ represents the new static equilibrium configuration. The response (Figure 2.1.5) clearly indicates that the robot arm will sustain a nondecaying oscillation of magnitude θ about

98 8 Vibration of Mechanical Systems f (t) a a1 t1 t Figure A staircase forcing function the new static equilibrium configuration after it suddenly grips a payload of mass m. Example 2.1.2: Input Shaping (Singer and Seering, 199) Consider an underdamped spring mass damper SDOF system (Figure 2.1.1), which is subjected to the force f (t) shown in Figure Assume that the initial conditions are zeros. The differential equation of motion is m eq ẍ + c eq ẋ + k eq x = f (t) (2.1.44) To determine the response x(t), the force f (t) is expressed as f (t) = f 1 (t) + f 2 (t) (2.1.45) where f 1 (t) and f 2 (t) are shown in Figure Using Equation , the response due to f 1 (t) will be x 1 (t) = a 1 k eq [1 e ξω nt cos ω d t e ξω nt χ sin ω d t]; t (2.1.46) f 1( t ) f 2( t ) a2 a 1 a 1 t 1 t t 1 t Figure Components of staircase forcing function in Figure 2.1.6

99 Vibration of an SDOF System under Constant and Purely Harmonic Excitation 81 Similarly, using Equation , the response due to f 2 (t) will be x 2 (t) = a 2 a 1 k eq [1 e ξω n(t t 1 ) cos ω d (t t 1 ) e ξω n(t t 1 ) χ sin ω d (t t 1 )]; t t 1 (2.1.47) It is obvious that x 2 (t) = fort t 1. Using the principle of superposition, x(t) = x 1 (t) + x 2 (t) (2.1.48) Substituting Equations and into Equation and after some algebra, where x(t) = a 2 k eq e ξωnt k eq (α sin ω d t + β cos ω d t); t t 1 (2.1.49) α = a 1 χ + e ξω nt 1 (a 2 a 1 )(sin ω d t 1 + χ cos ω d t 1 ) (2.1.5) β = a 1 + e ξω nt 1 (a 2 a 1 )(cos ω d t 1 χ sin ω d t 1 ) (2.1.51) It should be noted that α = and β = when where t 1 = π ω d and a 1 = a q q = e ξπ (2.1.52a,b) 1 ξ 2 (2.1.53) When t 1 and a 1 are chosen according to Equation , x(t) = a 2 k eq for t > t 1 (2.1.54) In other words, the system reaches its steady state in a finite time t 1 without any oscillation, even when the damping is zero or almost zero. Note that the input command a 2, t >, will yield a sustained oscillation as shown in Figure Hence, by shaping the input command as shown in Figure 2.1.6, the system reaches the steady state without any vibration.

100 82 Vibration of Mechanical Systems k eq x(t) k eq x(t) m eq Static equilibrium x(t) m eq f sin ωt Free body diagram f sin ωt Figure An undamped spring mass system subjected to sinusoidal excitation 2.2 RESPONSE OF AN UNDAMPED SDOF SYSTEM TO A HARMONIC EXCITATION Consider an undamped SDOF system subjected to a sinusoidal excitation with the amplitude f and the frequency ω. Using the free body diagram in Figure 2.2.1, net force in x-direction = k eq x + f sin ωt (2.2.1) Applying Newton s second law of motion, k eq x + f sin ωt = m eq ẍ (2.2.2) Therefore, the differential equation of motion is m eq ẍ + k eq x = f sin ωt (2.2.3) The solution of Equation is composed of two parts: x(t) = x h (t) + x p (t) (2.2.4) where x h (t) is the homogeneous solution satisfying m eq ẍ h + k eq x h = (2.2.5)

101 Vibration of an SDOF System under Constant and Purely Harmonic Excitation 83 and x p (t) is a particular solution satisfying m eq ẍ p + k eq x p = f sin ωt (2.2.6) The homogeneous solution in Equation is x h (t) = A 1 cos ω n t + B 1 sin ω n t (2.2.7) where A 1 and B 1 are constants. And, k eq ω n = (2.2.8) m eq The form of the particular solution is dependent on whether the excitation frequency ω equals the natural frequency ω n. Case I: ω ω n x p (t) = Asin ωt (2.2.9) where A can be positive or negative. To determine A, Equation is substituted into Equation 2.2.6, ( ω 2 m eq + k eq )Asin ωt = f sin ωt (2.2.1) Equating coefficients of sin ωt on both sides, ( ω 2 m eq + k eq )A = f (2.2.11) or or A = f k eq ω 2 m eq (2.2.12) A = 1 (2.2.13) f /k eq 1 r 2 where r is the frequency ratio defined as r = ω ω n (2.2.14)

102 84 Vibration of Mechanical Systems Note that f /k eq is the steady-state deflection when a constant (ω = or r = ) force f is applied. A constant force corresponds to ω = or r =. From Equations 2.2.4, 2.2.7, and 2.2.9, the total solution will be x(t) = A 1 cos ω n t + B 1 sin ω n t + Asin ωt (2.2.15) At t =, x() = A 1 (2.2.16) Differentiating Equation , ẋ(t) = A 1 ω n sin ω n t + B 1 ω n cos ω n t + Aω cos ωt (2.2.17) At t =, ẋ() = B 1 ω n + Aω (2.2.18) Solving Equation , B 1 = ẋ() Aω ω n (2.2.19) Substituting Equations and into Equation , ẋ() Aω x(t) = x() cos ω n t + sin ω n t + Asin ωt (2.2.2) ω n where the amplitude A is given by Equation Case II: ω = ω n (Resonance) In this case, the form of the particular integral Equation is not valid. Note that the expression in Equation is already represented by the homogeneous solution in Equation when ω = ω n. If the form given in Equation was valid, Equation would yield A = at ω = ω n ; that is, the displacement would become infinite from any finite initial displacement as soon as the external force is applied, which is physically impossible.

103 Vibration of an SDOF System under Constant and Purely Harmonic Excitation 85 The particular integral for ω = ω n is Differentiating Equation twice, x p (t) = A r t cos ωt (2.2.21) ẍ p (t) = 2A r ω sin ωt A r tω 2 cos ωt (2.2.22) Substituting Equations and into Equation 2.2.6, 2A r m eq ω sin ωt A r tm eq ω 2 cos ωt + A r tk eq cos ωt = f sin ωt (2.2.23) Because k eq = m eq ωn 2 = m eqω 2, 2A r m eq ω sin ωt = f sin ωt (2.2.24) Equating coefficients of sin ωt on both sides, or 2A r m eq ω n = f (2.2.25) A r = f 2m eq ω n = f ω n 2k eq (2.2.26) From, 2.2.7, and , the total solution will be At t =, x(t) = A 1 cos ω n t + B 1 sin ω n t + A r t cos ω n t (2.2.27) Differentiating Equation , x() = A 1 (2.2.28) ẋ(t) = A 1 ω n sin ω n t + B 1 ω n cos ω n t A r tω n sin ω n t + A r cos ω n t (2.2.29) At t =, ẋ() = B 1 ω n + A r (2.2.3)

104 86 Vibration of Mechanical Systems Displacement, x/(f /k eq ) Displacement, x/(f /k eq ) 2 6 ω =.5 ω n ω = ω n ω =.9 ω n ω = 1.5 ω n Nondimensional time, ω n t Nondimensional time, ω n t Figure Responses at excitation frequencies ω =.5ω n,ω n, 1.5ω n, and.9ω n with zero initial conditions or B 1 = ẋ() A r ω n (2.2.31) Substituting Equations and into Equation , x(t) = x() cos ω n t + ẋ() A r ω n sin ω n t + A r t cos ωt (2.2.32) where A r is given by Equation Example 2.2.1: Responses at Different Excitation Frequencies with Zero Initial Conditions In Figure 2.2.2, responses are plotted for different values of excitation frequencies ω with zero initial conditions (x() = and ẋ() = ).

105 Vibration of an SDOF System under Constant and Purely Harmonic Excitation 87 Case I: ω ω n With zero initial conditions, Equation yields x(t) = Aω sin ω n t + Asin ωt (2.2.33) ω n where A is given by Equation When ω =.5ω n and ω = 1.5ω n, responses contain both the frequencies ω and ω n (Figure 2.2.2). When ω is close to ω n, for example, ω =.9ω n, the response exhibits a beating phenomenon (Figure 2.2.2) which can be explained by expressing Equation as or x(t) = A(sin ωt sin ω n t) A ω ω n ω n sin ω n t (2.2.34) x(t) = 2Asin(.5(ω ω n )t) cos(.5(ω + ω n )t) A ω ω n sin ω n t ω n (2.2.35) The second term on the right-hand side of Equation is small. Ignoring this term, x(t) = 2Asin(.5(ω ω n )t) cos(.5(ω + ω n )t) (2.2.36) Equation can be interpreted as the response with the frequency.5(ω + ω n ) with the time-varying amplitude. The frequency of the amplitude variation is.5(ω ω n ), as found in Figure 2.2.2, and is much less than.5(ω + ω n ). Case II: ω = ω n With zero initial conditions, Equation yields x(t) = A r sin ω n t + A r t cos ωt (2.2.37) ω n Using Equation , x(t) = f sin ω n t f ω n t cos ωt (2.2.38) 2k eq 2k eq It is obvious that the second term on the right-hand side of Equation will be dominant after some time t. This will be true for a

106 88 Vibration of Mechanical Systems c eq c eq x& k eq x(t) k eq x(t) m eq f sin ωt Static equilibrium x(t) m eq f sin ωt Free body diagram Figure A damped SDOF spring mass system subjected to sinusoidal excitation large ω n even for a small time t. In other words, for a large ω n and/or after some time t, x(t) f ω n t cos ωt (2.2.39) 2k eq The response in Figure is described by Equation , in which the amplitude of vibration grows to infinity in a linear manner with respect to time. From the structural integrity point of view, the bad news is that the amplitude of vibration grows to infinity. But, the good news is that it takes time for the amplitude to build up to infinity, and there is time to save the structure from catastrophic failure. This fact is used in determining the rate at which a rotor shaft must be accelerated past its resonant speed (or critical speed ) when the desired rotor speed is greater than the critical speed. 2.3 RESPONSE OF A DAMPED SDOF SYSTEM TO A HARMONIC EXCITATION Consider a damped SDOF system subjected to a sinusoidal excitation with the amplitude f and the frequency ω (Figure 2.3.1). Using the

107 Vibration of an SDOF System under Constant and Purely Harmonic Excitation 89 free body diagram in Figure 2.3.1, net force in x-direction = k eq x c eq ẋ + f sin ωt (2.3.1) Applying Newton s second law of motion, k eq x c eq ẋ + f sin ωt = m eq ẍ (2.3.2) Therefore, the differential equation of motion is m eq ẍ + c eq ẋ + k eq x = f sin ωt (2.3.3) The solution of Equation is composed of two parts: x(t) = x h (t) + x p (t) (2.3.4) where x h (t) is the homogeneous solution satisfying m eq ẍ h + c eq ẋ h + k eq x h = (2.3.5) and x p (t) is a particular solution satisfying m eq ẍ p + c eq ẋ p + k eq x p = f sin ωt (2.3.6) Particular Solution Assume that x p (t) = Asin(ωt φ) (2.3.7) Substituting Equation into Equation 2.3.6, m eq ω 2 Asin(ωt φ) + c eq ωacos(ωt φ) + k eq Asin(ωt φ) = f sin ωt (2.3.8) or (k eq m eq ω 2 )Asin(ωt φ) + c eq ωacos(ωt φ) = f sin ωt (2.3.9) or (k eq m eq ω 2 )A[sin ωt cos φ cos ωt sin φ] + c eq ωa[cos ωt cos φ + sin ωt sin φ] = f sin ωt (2.3.1)

108 9 Vibration of Mechanical Systems or [(k eq m eq ω 2 )Acos φ + c eq ωasin φ]sinωt + [c eq ωacos φ (k eq m eq ω 2 )Asin φ] cos ωt = f sin ωt (2.3.11) Equating coefficients of sin ωt and cos ωt on both sides, (k eq m eq ω 2 )Acos φ + c eq ωasin φ = f (2.3.12a) and c eq ωacos φ (k eq m eq ω 2 )Asin φ = ( b) Representing Equations a and b in matrix form, [ ][ ] [ ] k eq ω 2 m eq c eq ω Acos φ f = c eq ω (k eq ω 2 m eq ) Asin φ (2.3.13) or [ ] Acos φ = Asin φ [ k eq ω 2 m eq c eq ω c eq ω (k eq ω 2 m eq ) ] 1 [ ] f (2.3.14) or [ ] Acos φ = 1 Asin φ [ (k eq ω 2 m eq ) c eq ω c eq ω +(k eq ω 2 m eq ) ][ ] f (2.3.15) where From Equation , and = (k eq ω 2 m eq ) 2 (c eq ω) 2 (2.3.16) Acos φ = Asin φ = (k eq ω 2 m eq )f (k eq ω 2 m eq ) 2 + (c eq ω) 2 (2.3.17) c eq ωf (k eq ω 2 m eq ) 2 + (c eq ω) 2 (2.3.18)

109 Vibration of an SDOF System under Constant and Purely Harmonic Excitation 91 Using Equations and , f A = (Acos φ) 2 + (Asin φ) 2 =+ (keq ω 2 m eq ) 2 + (c eq ω) 2 (2.3.19) and tan φ = Asin φ Acos φ = c eq ω (2.3.2) (k eq ω 2 m eq ) Dividing the numerators and denominators of Equations and by k eq, A = f k eq ( 1 ω 2 m eq k eq ) 2 + ( ceq ω k eq ) 2 (2.3.21) and Now, tan φ = c eq ω k eq 1 ω 2 m eq k eq (2.3.22) c eq ω k eq = c cξω k eq ω 2 m eq k eq = ω2 = r 2 (2.3.23) ωn 2 = 2m eqω n ξω k eq = 2ξ ω ω n = 2ξr (2.3.24) Substituting Equations and into Equations and , A 1 = (2.3.25) f /k eq (1 r 2 ) 2 + (2ξr) 2 and tan φ = 2ξr (2.3.26) 1 r 2 where r is the frequency ratio defined as r = excitation frequency natural frequency = ω ω n (2.3.27)

110 92 Vibration of Mechanical Systems Case I: Underdamped ( <ξ<1 or < c eq < c c ) From Equation , x h (t) = e ξω nt (A 1 cos ω d t + B 1 sin ω d t) (2.3.28) From Equations 2.3.4, , and 2.3.7, At t =, x(t) = e ξω nt (A 1 cos ω d t + B 1 sin ω d t) + Asin(ωt φ) (2.3.29) x() = A 1 Asin φ A 1 = x() + Asin φ (2.3.3) Differentiating Equation , ẋ(t) = e ξωnt ( ω d A 1 sin ω d t + ω d B 1 cos ω d t) ξω n e ξωnt (A 1 cos ω d t + B 1 sin ω d t) + ωacos(ωt φ) (2.3.31) At t =, Using Equation 2.3.3, ẋ() = ω d B 1 ξω n A 1 + ωacos φ (2.3.32) B 1 = ẋ() + ξω n(x() + Asin φ) ωacos φ (2.3.33) ω d The total response x(t) = x h (t) + x p (t) is shown in Figure for ξ =.4 and ξ =.5. Case II: Critically Damped (ξ = 1 or c eq = c c ) From Equation , x h (t) = A 1 e ω nt + B 1 te ω nt (2.3.34) From Equations 2.3.4, , and 2.3.7, x(t) = A 1 e ωnt + B 1 te ωnt + Asin(ωt φ) (2.3.35) At t =, x() = A 1 Asin φ A 1 = x() + Asin φ (2.3.36)

111 Vibration of an SDOF System under Constant and Purely Harmonic Excitation 93 Displacement, x/(f /k eq ) Displacement, x/(f /k eq ) 15 ξ = ξ = Nondimensional time, ω n t 1 ξ = ξ = Nondimensional time, ω n t Figure Response (x() = f /k eq and ẋ() = ) for ξ =.4,.5, 1, and 1.5 Differentiating Equation , ẋ(t) = ω n A 1 e ωnt ω n B 1 te ωnt + B 1 e ωnt + ωacos(ωt φ) (2.3.37) At t =, ẋ() = ω n A 1 + B 1 + ωacos φ (2.3.38) Using Equation , B 1 = ẋ() + ω n (x() + Asin φ) ωacos φ (2.3.39) The total response x(t) = x h (t) + x p (t) is shown in Figure for ξ = 1.

112 94 Vibration of Mechanical Systems Case III: Overdamped (ξ >1 or c eq > c c ) From Equation , x h (t) = A 1 e s 1t + B 1 e s 2t (2.3.4) where s 1 = ξω n + ω n ξ 2 1 < (2.3.41) s 2 = ξω n ω n ξ 2 1 < (2.3.42) Therefore, from Equations and 2.3.7, x(t) = A 1 e s 1t + B 1 e s 2t + Asin(ωt φ) (2.3.43) At t =, x() = A 1 + B 1 Asin φ (2.3.44) Differentiating Equation , ẋ(t) = A 1 s 1 e s 1t + B 1 s 2 e s 2t + ωacos(ωt φ) (2.3.45) At t =, ẋ() = A 1 s 1 + B 1 s 2 + ωacos φ (2.3.46) Solving Equations and , and A 1 = s 2(x() + Asin φ) (ẋ() ωacos φ) s 2 s 1 (2.3.47) B 1 = s 1(x() + Asin φ) + (ẋ() ωacos φ) s 2 s 1 (2.3.48) The total response x(t) = x h (t) + x p (t) is shown in Figure for ξ>1.

113 Vibration of an SDOF System under Constant and Purely Harmonic Excitation Steady State Response For all cases of damping c eq > orξ> (Equations , , and 2.3.4) x h (t) as t (2.3.49) Therefore, the homogeneous part x h (t) is also called transient response. In the steady state (t ), x(t) = x h (t) + x p (t) x p (t) as t (2.3.5) Therefore, the particular integral x p (t) is also called the steady state response x ss (t): where (from Equations and ), and x ss (t) = Asin(ωt φ) (2.3.51) A 1 = (2.3.52) f /k eq (1 r 2 ) 2 + (2ξr) 2 tan φ = 2ξr (2.3.53) 1 r 2 Note that A is the amplitude of the steady state response. And φ is the phase of the steady state response which is the angle by which the steady state response lags behind the forcing function. Amplitude (A) and phase (φ) are plotted in Figures and as functions of the frequency ratio r for different values of the damping ratio ξ. For all values of the damping ratio ξ, A asr. Near resonance condition, the amplitude can be very large, particularly for a small value of the damping. To find the peak amplitude, Equation is differentiated with respect to r, k eq da f dr = 1 2 [(1 r 2 ) 2 + (2ξr) 2 ] 3 2 [2(1 r 2 )( 2r) + (2ξ) 2 2r] (2.3.54)

114 96 Vibration of Mechanical Systems ξ = Amplitude, A(f /k eq ) ξ =.3 ξ =.5 ξ =.77.5 ξ = 2 ξ = Frequency ratio, r = ω/ω n Figure Amplitude versus frequency ratio (direct excitation on mass) The condition for the maximum value of the amplitude A is da dr = (2.3.55) Therefore, from Equations and , (1 r 2 ) + 2ξ 2 = (2.3.56) or r = 1 2ξ 2 provided ξ 1 2 =.77 (2.3.57) Equation yields the frequency ratio at which the amplitude is maximum. The corresponding peak amplitude (A p ) is given by A p f /k eq = 1 2ξ ; ξ.77 (2.3.58) 1 ξ 2

115 Vibration of an SDOF System under Constant and Purely Harmonic Excitation ξ = ξ =.5 14 ξ =.3 Phase angle (φ), degrees ξ = 1 ξ =.77 ξ =.5 ξ =.3 ξ =.5 ξ =.77 ξ = 1 ξ = 2 2 ξ = Frequency ratio, r = ω/ω n Figure Phase versus frequency ratio (direct excitation on mass) It should be noted that the maximum value of the amplitude occurs at r = forξ>.77. For the phase plot (Figure 2.3.4), two important points should be noted: a. The phase angle φ = 9 at ω = ω n for all values of damping ratios greater than zero. In other words, the phase angle φ of a damped (ξ >) system is always 9 when the excitation frequency ω equals the undamped natural frequency ω n of the system. b. The phase angles φ of an undamped (ξ = ) system are and 18 for ω<ω n and ω>ω n, respectively. In other words, the phase angle φ changes abruptly from to 18 across ω = ω n. It should be noted that the phase angle φ of an undamped system is not defined for ω = ω n,asasin(ωt φ) is not the particular integral in this case.

116 98 Vibration of Mechanical Systems f sin ωt x (t) x(t) Turbine blade Displacement shape (a) (b) Figure Turbine blade subjected to sinusoidal excitation Example 2.3.1: Steady State Response of a Turbine Blade Consider a turbine blade, which is subjected to a sinusoidal force f sin ωt (Figure 2.3.5a). For an assumed displacement shape of the turbine blade (Figure 2.3.5b), the equivalent SDOF system is shown in Figure 2.3.6, where m eq =.114 kg and k eq = 43, N/m (Griffin and Sinha, 1985). Assuming that the damping ratio ξ =.1 and the force amplitude f = 1 N, determine the steady state amplitudes and the phases for the following values of frequency ratio:.4,.95, 1, and 2. Also, plot the response with zero initial conditions for each frequency ratio. x(t) k eq c eq m eq f sin ωt Figure Equivalent SDOF model for the turbine blade

117 Vibration of an SDOF System under Constant and Purely Harmonic Excitation 99 Solution a. r =.4 A = ω n = f k eq = m k eq m eq = 6,141.6 rad/sec f /k eq (1 r 2 ) 2 + (2ξr) 2 = m b. r =.95 φ = tan 1 A = 2ξr.8 = tan 1 =.95 rad 1 r 2.84 f /k eq (1 r 2 ) 2 + (2ξr) 2 = m c. r = 1 φ = tan 1 A = 2ξr.19 = tan 1 =.1925 rad 1 r f /k eq (1 r 2 ) 2 + (2ξr) 2 = m d. r = 2 A = φ = tan 1 2ξr.2 = tan 1 = π 1 r 2 2 rad f /k eq (1 r 2 ) 2 + (2ξr) 2 = m φ = tan 1 2ξr.4 = tan 1 = rad 1 r 2 3 For each frequency ratio, the response is computed with zero initial conditions (Equation ). The steady state response and the excitation force are plotted together in Figure 2.3.7, primarily to

118 1 Vibration of Mechanical Systems 2 1 r =.4 x(t)/(f /k eq ) f(t) 2 1 r =.95 x(t)/(f /k eq ) 5 f(t) x(t)/(f 8 /k eq ) 1 x(t)/(f /k eq ) r = 1 3 f(t) r = 2 6 f(t) Nondimensional time, ω n t Nondimensional time, ω n t Figure Steady state vibration of a turbine blade see the phase relationships. For r = 1, the excitation force is maximum when x(t) = and ẋ(t) >. This confirms that the steady state response lags behind the forcing function by 9. The subplot for r =.95 in Figure is re-plotted in Figure 2.3.8, where it is estimated that ω n t =.2 rad where t is the time difference for peaks of excitation and steady state response to occur. Therefore, ω t = ω.2 rad =.19 rad ω n This is the phase difference between the excitation and the steady state response. Also, the excitation peak occurs before the response peak. Therefore, the steady state response lags behind the excitation

119 Vibration of an SDOF System under Constant and Purely Harmonic Excitation r = x(t)/(f /k eq ) 5 f(t) Nondimensional time, ω n t Figure A zoomed subplot in Figure by.19 rad, which matches well with the theoretical result. The slight difference between the numerical and the theoretical result is due to approximations involved in obtaining ω n t from the plot Force Transmissibility When a force is applied to the mass, it is important to determine the force transmitted to the support. Therefore, free body diagrams are constructed for mass, spring, and damper in Figure From these free body diagrams in Figure 2.3.9, force transmitted to the support = k eq (x + ) + c eq ẋ = mg + k eq x + c eq ẋ (2.3.59) The time-varying part of the force transmitted to the support is given by f T (t) = k eq x + c eq ẋ (2.3.6)

120 12 Vibration of Mechanical Systems Support c eq x& k eq (x + Δ) ceq keq c eq x& k eq (x + Δ) m eq f sin ωt mg Figure Free body diagram of each element in Figure In the steady state, the force transmitted to the support is obtained by substituting Equation into Equation 2.3.6, f T (t) = k eq Asin(ωt φ) + c eq ωacos(ωt φ) = f T sin(ωt ψ ) (2.3.61) where ψ = φ θ (2.3.62) f T = A keq 2 + c2 eq ω2 (2.3.63a) and tan θ = c eqω k eq (2.3.63b) Substituting Equation into Equation a, and using Equation , f T 1 + (2ξr) 2 = (2.3.64) f (1 r 2 ) 2 + (2ξr) 2

121 Vibration of an SDOF System under Constant and Purely Harmonic Excitation ξ = ξ =.3 ξ = Force transmissibility ξ = 2 1 ξ =.5.77 ξ = 2 ξ = 1.5 r = ξ = ξ =.3 ξ =.5 ξ = Frequency ratio, r = ω/ω n Figure Force transmissibility as a function of frequency ratio Using Equations , , and , ψ = φ θ = tan 1 2ξr 1 r 2 tan 1 2ξr = tan 1 2ξr r 2 (4ξ 2 1) (2.3.65) Since f and f T are, respectively, the amplitudes of the applied sinusoidal force and the time-varying part of the force transmitted to the support, the ratio f T /f is known as the force transmissibility, which is plotted in Figure as a function of the frequency ratio r for many values of the damping ratio ξ. As the applied force and the time-varying part of the force transmitted to the support are f sin ωt and f T sin(ωt ψ) respectively, ψ is the phase angle by which the transmitted force lags behind the applied force. The phase angle ψ has been plotted in Figure as a function of the frequency ratio r for many values of the damping ratio ξ.

122 14 Vibration of Mechanical Systems ξ =.5 Phase angle (ψ), degrees ξ =.3 ξ =.5 ξ =.77 ξ = 1 ξ = Frequency ratio, r = ω/ω n Figure Phase plot for force transmissibility It is often desired to select the spring stiffness and the damping coefficient such that the force transmissibility is as small as possible, but never greater than one. To determine the frequency ratio at which f T /f is one, from Equation , 1 + (2ξr) 2 = 1 (2.3.66) (1 r 2 ) 2 + (2ξr) 2 Squaring both sides of Equation , 1 + (2ξr) 2 (1 r 2 ) 2 + (2ξr) = 1 (2.3.67) 2 After some algebra, Equation yields r 2 (r 2 2) = (2.3.68)

123 Vibration of an SDOF System under Constant and Purely Harmonic Excitation 15 Solving Equation , r = or r = 2 (2.3.69) Therefore, the force transmissibility equals one at r = and r = 2 for all values of the damping ratio. In Figure 2.3.1, it can also be seen that the force transmissibility is greater than one for < r < 2, whereas it is less than one for r > 2. Further, for r > 2, the force transmissibility increases as the damping ratio increases. Therefore, a support system, that is, the spring stiffness and the damping constant, should be designed such that r > 2 and the damping constant c is as small as possible. For r > 2, ω > 2 ωn 2 ω < ω2 n 2 k eq < m eqω 2 (2.3.7) 2 In general, a design guideline is to choose small values for the spring stiffness and the damping constant. Rubber pads are often used to minimize the force transmitted to the support because rubber material has small values for the stiffness and the damping constant. Example 2.3.2: Force Transmitted to Turbine Blade Support For the frequency ratio = 1, find the steady state force transmitted to the support of the turbine blade in Example Solution Here, r = 1 and ξ =.1. f T = f 1 + (2ξr) 2 (1 r 2 ) 2 + (2ξr) 2 = f (2ξ) 2 = 5.1 N ψ = tan 1 2ξr r 2 (4ξ 2 1) = 1 tan 1 = rad 2ξ The steady state force transmitted to the support is f T (t) = f T sin(ωt ψ) = 5.1 sin(6141.6t 1.558) N

124 16 Vibration of Mechanical Systems Q = 1 Amplitude, A/A r r 1 r Frequency ratio, r = ω/ω n Figure Frequency ratios for A/A r = Quality Factor and Bandwidth Here, two widely used terms, quality factor (Q) and bandwidth, are defined. Quality Factor The amplitude A r at r = 1 is obtained from Equation , A r = 1 (2.3.71) f /k eq 2ξ For the definition of quality factor, it is important to determine the frequency ratios where the values of A are 1/ 2 times the amplitude A r (Figure ). Using Equations and , the amplitude ratio (A/A r ) is determined and set equal to 1/ 2 for this purpose: A 2ξ = A r (1 r 2 ) 2 + (2ξr) = 1 (2.3.72) 2 2 Squaring both sides of Equation , (r 2 ) 2 (2 4ξ 2 )r 2 + (1 8ξ 2 ) = (2.3.73)

125 Vibration of an SDOF System under Constant and Purely Harmonic Excitation 17 Solving this quadratic equation in r 2, r 2 = (1 2ξ 2 ) ± 2ξ 1 ξ 2 (2.3.74) For a small ξ, 1 ξ 2 1 and 1 2ξ 2 1, that is, Using binomial expansion (Appendix B), r 2 1 ± 2ξ (2.3.75) r (1 ± 2ξ) 1 2 = 1 ± ξ ± higher power terms of ξ (2.3.76) Neglecting higher powers of ξ, frequencies ω 1 and ω 2 where the values of A/(f /k eq )are1/ 2 times the value at r = 1 can be approximated for small ξ as r 1 = ω 1 ω n = 1 ξ (2.3.77) r 2 = ω 2 = 1 + ξ (2.3.78) ω n The quality factor Q is defined as Q = From Equations , ω n ω 2 ω 1 (2.3.79) Q = 1 2ξ (2.3.8) A higher value of Q implies a lower value of the damping ratio ξ and vice versa. Bandwidth The bandwidth of a system is defined as the frequency below which the steady state amplitude is above 1/ 2(=.77) times the steady state amplitude at zero frequency or the steady state amplitude under constant force, for example (Figure ). The bandwidth is a measure of the frequency range for which the system responds strongly to the forcing function.

126 18 Vibration of Mechanical Systems Amplitude, A/(f /k eq ) Bandwidth Frequency ratio, r = ω/ω n Figure Definition of bandwidth The bandwidth (frequency) is obtained by setting A 1 = f /k eq (1 r 2 ) 2 + (2ξr) = 1 (2.3.81) 2 2 Squaring both sides of Equation , the following quadratic equation in r 2 is obtained: (r 2 ) 2 (2 4ξ 2 )r 2 1 = (2.3.82) Solving Equation , r 2 = (2 4ξ 2 ) ± (2 4ξ 2 ) (2.3.83) The negative sign in front of the square root sign in Equation will lead to a negative value of r 2, which is meaningless. Therefore, the positive sign in front of the square root sign is chosen to obtain r 2 = 1 2ξ 2 + 4ξ 4 4ξ (2.3.84)

127 Vibration of an SDOF System under Constant and Purely Harmonic Excitation Bandwidth/ω n Damping ratio (ξ) Figure Bandwidth versus damping ratio Therefore, bandwidth = 1 2ξ ω 2 + 4ξ 4 4ξ (2.3.85) n The bandwidth is plotted as a function of the damping ratio in Figure As the damping is increased, the bandwidth decreases. It is interesting to note that the bandwidth equals the undamped natural frequency ω n when the damping ratio ξ equals 1/ 2(=.77). 2.4 ROTATING UNBALANCE In a rotating machine such as a motor, a generator, a turbine, and so on, there is always an unbalance because the center of rotation never coincides with the center of the rotor mass. Even though this eccentricity is small, this results in a rotating centrifugal force on the rotor with a significant amplitude because the centrifugal force is proportional to the square of the angular speed. Further, a rotating

128 11 Vibration of Mechanical Systems m eq m u e ωt esin ωt x(t) m eq x(t) Static Equilibrium c eq x& k eq x c eq Free body diagram k eq Figure A spring mass damper system with rotating unbalance centrifugal force results in a sinusoidal excitation on the structure, and hence can lead to a large magnitude of the structural vibration because of the resonance phenomenon. Consider the system in which an unbalance mass m u with an eccentricity e is rotating at a speed of ω rad/sec (Figure 2.4.1). Let the total mass be m eq, which includes the unbalance mass m u. From the free body diagram in Figure 2.4.1, net force in x-direction = k eq x c eq ẋ (2.4.1) And the acceleration of the mass (m eq m u )isẍ. The displacement of the unbalance mass m u is x + e sin ωt. As a result, the acceleration of the unbalance mass will be the second time derivative of x + e sin ωt. Applying Newton s second law of motion, k eq x c eq ẋ = (m eq m u )ẍ + m u d 2 (x + e sin ωt) (2.4.2) dt2

129 Vibration of an SDOF System under Constant and Purely Harmonic Excitation 111 After some simple algebra, m eq ẍ + c eq ẋ + k eq x = m u eω 2 sin ωt (2.4.3) Comparing Equations and 2.3.3, f = m u eω 2 (2.4.4) Following the developments in Section 2.3, the steady state response x ss (t) is again given by x ss (t) = Asin(ωt φ) (2.4.5) where the amplitude A and the phase angle φ are given by Equations and : A 1 = (2.4.6) f /k eq (1 r 2 ) 2 + (2ξr) 2 and tan φ = 2ξr (2.4.7) 1 r 2 It should be noted that the complete solution is still given by Equations , , and for underdamped, critically damped, and overdamped systems, respectively. Using Equation 2.4.4, f = m ueω 2 = em u m eq ω 2 = em u ω 2 = em u r 2 (2.4.8) k eq k eq m eq k eq m eq ωn 2 m eq Substituting Equation into Equation 2.4.6, the steady state amplitude due to the rotating unbalance is given by m eq m u A e = r 2 (1 r 2 ) 2 + (2ξr) 2 (2.4.9) The steady state amplitude (A) due to the rotating unbalance is plotted in Figure as a function of the frequency ratio for many values of the damping ratio. The plot for the phase angle is exactly the same as the one shown in Figure Here, the amplitude (A) equals zero

130 112 Vibration of Mechanical Systems ξ = ξ = Amplitude, A.m eq /e.m u ξ =.3 ξ =.5 ξ =.77.5 ξ = 1 ξ = Frequency ratio, r = ω/ω n Figure Steady state amplitude versus frequency ratio (rotating unbalance) at r = and equals (m u e/m eq ) for all damping ratios as r. Near resonance condition, the amplitude (A) can be large for small values of the damping ratio. To find the maximum amplitude, Equation is differentiated with respect to r, m eq da m u e dr = r 2 [2(1 r 2 )( 2r) + (2ξ) 2 2r] 2r + 2[(1 r 2 ) 2 + (2ξr) 2 ] 1.5 [(1 r 2 ) 2 + (2ξr) 2 ].5 (2.4.1) The condition for the maximum value of the amplitude A is da dr = (2.4.11) Therefore, from Equations and , r 2 [ (1 r 2 )( 2r) + (2ξ) 2 r ] + 2r [ (1 r 2 ) 2 + (2ξr) 2] = (2.4.12)

131 Vibration of an SDOF System under Constant and Purely Harmonic Excitation 113 ω = 85 rpm x(t) ω? x(t) k eq k eq ceq ω meq x = x& > ceq ω 45 meq x = x& > (a) (b) Figure Rotating unbalance position under stroboscopic light After some algebra, Equation yields r = 1 1 2ξ 2 provided ξ 1 2 =.77 (2.4.13) It should be noted that the maximum amplitude occurs for r = when ξ.77. For ξ.77, substituting Equation into Equation 2.4.9, m eq A p m u e = 1 2ξ (2.4.14) 1 ξ 2 where A p is the peak amplitude. It is interesting to note that righthand sides of Equations and are identical. Using Equation , the amplitude f T of the force transmitted to the support in steady state will be given by f T 1 + (2ξr) m u eω = 2 (2.4.15) (1 2 r 2 ) 2 + (2ξr) 2 Example 2.4.1: Identification of Damping Ratio and Natural Frequency Consider an SDOF system with the rotating unbalance =.1 kg meter and the equivalent mass m eq = 2 kg (Figure 2.4.3a). At a speed of 85 rpm under stroboscopic light, the configuration of the eccentric mass is horizontal, the displacement x is zero, and the velocity ẋ is positive. The steady state amplitude at the speed of 85 rpm is found to be 25 mm. Determine the damping ratio and the undamped

132 114 Vibration of Mechanical Systems natural frequency of the system. Also, determine the angular speed ω when the angular position of the rotating unbalance as shown in Figure 2.4.3b when the displacement x is zero, and the velocity ẋ is positive. Solution The configuration in Figure 2.4.3a indicates that the response x(t) lags behind the excitation force by 9. Therefore, the frequency ratio r = 1 from Equation 2.4.7, that is, Given: From Equation 2.4.9, ω n = ω = 85 rpm = rad/sec m u e =.1 kg-m and A =.25 m m eq A m u e = 1 2ξ ξ =.1 The configuration in Figure 2.4.3b indicates that the response x(t) lags behind the excitation force by 135. Therefore, tan φ = 1 1 From Equation 2.4.7, 2ξr r 2 1 = 1 r 2.2r 1 = Solving this quadratic equation: r = 1.1 and.99. Since the negative value of r is meaningless, r = ω ω n = 1.1 ω = 1.1ω n = rpm Example 2.4.2: A Rotor Shaft with Mass Unbalance Consider the rotor shaft system shown in Figure 1.3.1, but with a casing around the rotor as shown in Figure The clearance δ between

133 Vibration of an SDOF System under Constant and Purely Harmonic Excitation 115 l 2 l 2 Casing δ Clearance = δ Rotor with mass = m δ Simply supported shaft Figure Rotor shaft system with casing the rotor and the casing is 1 mm. The length and the diameter of the circular steel shaft are.5 m and 3 cm, respectively. The mass and the unbalance of the turbine rotor are 12 kg and.25 kg-cm, respectively. a. Determine the critical speed of the rotor. b. If the rotor operates at the critical speed, find the time after which the rotor will hit the casing. Assume that the initial conditions are zeros. c. Assume that the operating speed of the rotor is higher than its critical value. Then, the rotor must pass through its critical speed before it acquires the desired speed. Recommend a safe value of angular acceleration to cross the critical speed. a. Area moment of inertia I = π(.3)4 64 = m 4 For steel, E = N/m 2 k eq = 48EI l 3 ω n = l =.5m = N/m k eq m eq = rad/sec critical speed ω = ω n = rad/sec

134 116 Vibration of Mechanical Systems b. Unbalance m u e = kg-m f = m u eω 2 = N Equation is rewritten here. Therefore, or x(t) = f 2k eq sin ω n t f ω n 2k eq t cos ωt x(t) = sin ω n t.525t cos ω n t x(t).525t cos ω n t Let the time after which the rotor hits the casing be t h. Then, c. Recommended acceleration =.525t h =.1 t h =.195 sec desired speed (.195/2) rad/sec 2. The operating speed will reach the desired speed from rest in half the time that it takes for the rotor to hit the casing at the critical speed. Therefore, the rotor will not hit the casing. 2.5 BASE EXCITATION There are many practical situations where the base is not fixed and the vibration is caused by the displacement of the base, for example, automobile vibration caused by uneven road profile, building vibration during earthquake, and so on. Consider the SDOF spring mass damper system with the base having a displacement y(t) (Figure 2.5.1). From the free body diagram, net force in x-direction = k eq (x y) c eq (ẋ ẏ) (2.5.1)

135 Vibration of an SDOF System under Constant and Purely Harmonic Excitation 117 m eq x(t) Static equilibrium m eq x(t) c eq k eq c eq ( x& y) & k eq ( x y) y( t) = y sin ωt Free body diagram Figure A spring mass damper system with base excitation Applying Newton s second law of motion, k eq (x y) c eq (ẋ ẏ) = m eq ẍ (2.5.2) or, m eq ẍ + c eq ẋ + k eq x = c eq ẏ + k eq y (2.5.3) With y(t) = y sin ωt, the right-hand side of Equation can be written as c eq ẏ + k eq y = c eq ωy cos ωt + k eq y sin ωt (2.5.4) Let Equation be represented as c eq ẏ + k eq y = f sin(ωt + θ) (2.5.5) Using Equation 2.5.4, c eq ωy cos ωt + k eq y sin ωt = f sin(ωt + θ) = f cos θ sin ωt + f sin θ cos ωt (2.5.6)

136 118 Vibration of Mechanical Systems Comparing the coefficients of cos ωt and sin ωt on both sides, and f sin θ = c eq ωy (2.5.7) f cos θ = k eq y (2.5.8) Squaring Equations and and adding them, f = y k 2 eq + (c eqω) 2 (2.5.9) Dividing Equation by Equation and using Equation , tan θ = c eqω = 2ξr (2.5.1) k eq Dividing Equation by k eq and using Equation , ( ) f ceq ω 2 = y 1 + = y 1 + (2ξr) k 2 (2.5.11) eq k eq Using Equations and 2.5.5, the differential equation of motion is written as m eq ẍ + c eq ẋ + k eq x = f sin(ωt + θ) (2.5.12) where f and θ are defined by Equations and 2.5.1, respectively. Following the developments in Section 2.3, the steady state response x ss (t) is given by x ss (t) = Asin(ωt + θ φ) (2.5.13) where the amplitude A and the phase angle φ are given by Equations and A 1 = (2.5.14) f /k eq (1 r 2 ) 2 + (2ξr) 2 and tan φ = 2ξr 1 r 2 (2.5.15)

137 Vibration of an SDOF System under Constant and Purely Harmonic Excitation 119 It should be noted that the complete solutions are given by Equations , , and for underdamped, critically damped, and overdamped systems, respectively. Substituting Equation into Equation , A 1 + (2ξr) 2 = (2.5.16) y (1 r 2 ) 2 + (2ξr) 2 The ratio of the steady state amplitude A of the mass and the input base amplitude y is known as the displacement transmissibility. It is important to note that the expressions for displacement and force transmissibilities are identical. Compare Equations and Therefore, the plot of transmissibility (Figure 2.3.1) holds for base excitation also. And a support system, that is, the spring stiffness and the damping constant, should be designed such that r > 2 and the damping constant c is as small as possible. Equation is valid here also. Equation can be written as x ss (t) = Asin(ωt ψ) (2.5.17) where ψ = φ θ (2.5.18) Since the input base displacement is y(t) = y sin ωt, the phase ψ is the angle by which the steady state displacement x ss (t) lags behind the base displacement. From Equations and 2.5.1, ψ = φ θ = tan 1 2ξr 1 r 2 tan 1 2ξr = tan 1 2ξr r 2 (4ξ 2 1) (2.5.19) Again, note that the expression of the phase lag ψ for base excitation is the same as that for the force transmitted to the support when the mass is directly excited by the sinusoidal force. Compare Equations and The plot of the phase lag ψ (Figure ) holds for base excitation also.

138 12 Vibration of Mechanical Systems Example 2.5.1: Microgravity Isolation Systems In a spacecraft, a number of scientific experiments are conducted to utilize the microgravity environment; that is, the acceleration due to gravity in space is 1 6 g where g is the acceleration due to gravity on earth. However, because of various disturbances such as crew motion, thruster firing, and so on, the acceleration due to gravity in space can be as high as 1 3 g. Design a spring damper suspension system for an experiment module, which has a mass of 1.5 kg. The disturbance frequencies lie between.1 and.5 Hz, and assume that the damping ratio is ξ =.2. Solution In the context of microgravity isolation system, m eq in Figure is the mass of the experiment module. The base acceleration, ÿ(t) is caused by various disturbances on the spacecraft. The displacement and acceleration transmissibilities are identical because ω 2 A ω 2 y = A y The desired transmissibility is 1 3 because ω 2 A and ω 2 y are 1 6 g and 1 3 g, respectively. Using Equation , A 1 + (2ξr) 2 = = 1 3 y (1 r 2 ) 2 + (2ξr) 2 This equation leads to the following quadratic equation in r 2 : (r 2 ) r = There are two roots: r 2 = and.62. Since a negative value of r 2 is meaningless, r 2 = and r = 4. When r > 2, the transmissibility decreases as r increases. Therefore, ω ω n 4 ω n ω 4

139 Vibration of an SDOF System under Constant and Purely Harmonic Excitation 121 y( t) = y sin ωt c eq k eq c eq ( x& y) & k eq ( x y) Static equilibrium y( t) = y sin ωt x(t) m eq Pointer Scale y( t) = y sin ωt x(t) m eq Free body diagram Vibrating structure Figure A vibration measuring instrument Here,.1 Hz ω.5 Hz. Therefore, Therefore, ω n =.1 2π 4 =.157 rad/sec k eq = m eq ω 2 n = N/m c eq = 2ξm eq ω n = N-sec/m 2.6 VIBRATION MEASURING INSTRUMENTS Here, basic theories for designing instruments that measure amplitudes of vibratory displacements and acceleration are presented. These instruments are composed of a spring mass damper system as shown in Figure 2.6.1, and are rigidly attached to the vibratory structure with the displacement y(t) = y sin ωt (2.6.1) and therefore the acceleration ÿ = ω 2 y sin ωt (2.6.2)

140 122 Vibration of Mechanical Systems The position of the pointer attached to the mass, which is the relative displacement z = x y (2.6.3) is available from the scale reading. By proper selection of mass, stiffness, and damping coefficient, the amplitude of z = x y can serve as a good estimate of either the displacement amplitude (y )orthe acceleration amplitude (ω 2 y ). The instruments that measure the displacement amplitude (y ) and the acceleration amplitude (ω 2 y )are called the vibrometer and the accelerometer, respectively. From the free body diagram in Figure 2.6.1, net force in x-direction = k eq (x y) c eq (ẋ ẏ) (2.6.4) Applying Newton s second law of motion, k eq (x y) c eq (ẋ ẏ) = m eq ẍ (2.6.5) From Equations and 2.6.2, ẍ = z + ÿ = z ω 2 y sin ωt (2.6.6) Substituting Equations and into Equation 2.6.5, m eq z + c eq ż + k eq z(t) = f sin ωt (2.6.7) where f = m eq ω 2 y (2.6.8) Therefore, f k eq = m eqω 2 y k eq = ω2 y ω 2 n (2.6.9) The steady state z ss (t) = z sin(ωt φ) (2.6.1)

141 Vibration of an SDOF System under Constant and Purely Harmonic Excitation 123 where the amplitude z is obtained by using Equations and 2.6.9: z = f /k eq (1 r 2 ) 2 + (2ξr) = ω 2 y /ωn 2 (2.6.11) 2 (1 r 2 ) 2 + (2ξr) 2 where r is the frequency ratio ω/ω n. The phase angle φ is given by Equation : φ = tan 1 2ξr 1 r 2 (2.6.12) From Equation , Vibrometer z r 2 = (2.6.13) y (1 r 2 ) 2 + (2ξr) 2 The plot of the ratio in Equation is shown in Figure It should be noted that the right-hand side of Equation is identical to the relationship for the steady state amplitude of the rotating unbalance problem (Equation 2.4.9). For large r, z 1 or z y (2.6.14) y It should be recalled that z is directly available from the scale reading. Equation establishes the fact that z can be a good estimate of the amplitude of vibration y when the frequency ratio r = ω/ω n is large. A large value of r = ω/ω n implies a small value of the natural frequency ω n, which is achieved by having a small stiffness and/or a large mass. Example 2.6.1: Design of a Vibrometer A vibrometer is to be designed such that the error in the estimated vibration amplitude is less than 4%. Determine the undamped natural frequency when the frequency of vibration lies between 2 and 5 Hz. Assume that the damping ratio is.3.

142 124 Vibration of Mechanical Systems Vibrometer ξ = ξ = Amplitude ratio, z /y ξ =.3 ξ =.5 Large r for z to be almost equal to y ξ = 1.5 ξ = Frequency ratio, r = ω/ω n Figure Plot for vibrometer design Solution It is required that.96 z y 1.4 The plot of z /y is shown in Figure for ξ =.3. The line z /y = 1.4 intersects the plot at two points, whereas z /y =.96 intersects the plot at only one point. The intersection point corresponding to z /y =.96 is not important for vibrometer design. The error is less than 4% when the frequency ratio is greater than the higher of the two values of the frequency ratio where z /y = 1.4 intersects the plot. To determine this frequency ratio, z r 2 = y (1 r 2 ) 2 + (2ξr) = 1.4 2

143 Vibration of an SDOF System under Constant and Purely Harmonic Excitation Accelerometer ξ = Modified amplitude ratio, ω 2 2 n z /ω y Small r for ω 2 n z to be almost equal to ω 2 y ξ =.3 ξ =.5 ξ = ξ =.77 ξ = Frequency ratio, r = ω/ω n Figure Plot for accelerometer design With ξ =.3,.816(r 2 ) r = This is a quadratic equation in r 2. Two roots are: r 2 = and.6279, or equivalently, r = and Therefore, r = ω ω n ω ω n Here 2 Hz ω 5 Hz. The undamped natural frequency ω n which will satisfy the above inequality ω n ω/ for all signal frequencies ω is given as ω n = 2 2π rad/sec = rad/sec

144 126 Vibration of Mechanical Systems From Equation , Accelerometer ωn 2z 1 = (2.6.15) ω 2 y (1 r 2 ) 2 + (2ξr) 2 The plot of the ratio in Equation is shown in Figure It should be noted that the right-hand side of Equation is identical to the relationship for the steady state amplitude of the direct force excitation problem (Equation ). For small r, ωn 2z 1 or ω ω 2 n 2 y z ω 2 y (2.6.16) It should be again recalled that z is directly available from the scale reading. Equation establishes the fact that ωn 2z can be a good estimate of the amplitude of the acceleration ω 2 y when the frequency ratio r = ω/ω n is small. A small value of r = ω/ω n implies a large value of the natural frequency ω n, which is achieved by having a large stiffness and/or a small mass. Example 2.6.2: Design of an Accelerometer An accelerometer is to be designed such that the error in the estimated acceleration amplitude is less than 4%. Determine the undamped natural frequency when the frequency of vibration lies between 2 and 5 Hz. Assume that the damping ratio is.3. Solution It is required that.96 ω2 n z 1.4 ω 2 y The plot of ωn 2z /ω 2 y is shown in Figure for ξ =.3. The line ωn 2z /ω 2 y = 1.4 intersects the plot at two points, whereas the line ωn 2z /ω 2 y =.96 intersects the plot at only one point. The intersection point corresponding to ωn 2z /ω 2 y =.96 is not important for the accelerometer design. The error is less than 4% when the frequency

145 Vibration of an SDOF System under Constant and Purely Harmonic Excitation Amplitude ratio, ω n z /ω y r =.2177 ξ = Frequency ratio, r = ω/ω n Figure Accelerometer design with damping ratio ξ =.3 ratio is smaller than the lower of the two values of frequency ratio where ωn 2z /ω 2 y = 1.4 intersects the plot. To determine this frequency ratio, With ξ =.3, ωn 2z 1 = ω 2 y (1 r 2 ) 2 + (2ξr) = (r 2 ) r = This is a quadratic equation in r 2. Two roots are: r 2 = and.474 or equivalently, r = and Therefore, r = ω.2177 ω n ω ω n.2177 Here 2 Hz ω 5 Hz. The undamped natural frequency ω n, which will satisfy the above inequality for all signal frequencies ω, is given as ω n = 5 2π.2177 rad/sec = rad/sec

146 128 Vibration of Mechanical Systems 2.7 EQUIVALENT VISCOUS DAMPING FOR NONVISCOUS ENERGY DISSIPATION There are many forms of nonviscous damping found in applications; for example, Coulomb friction, where the damping force is not proportional to the velocity. In such cases, it is useful to determine equivalent viscous damping so that the linear analysis developed in previous sections can be used. Let W nv be the energy dissipated by nonviscous damping per cycle of oscillation having an amplitude A and a frequency ω. Then, the equivalent viscous damping c eq is defined by equating W nv with the energy dissipated by equivalent viscous damper per cycle of oscillation having the same amplitude A and frequency ω. Using Equation , or πc eq ωa 2 = W nv (2.7.1) c eq = W nv (2.7.2) πωa 2 If the damping force is f d (t), then the energy dissipated per cycle of oscillation is calculated as follows: W nv = f d (t)dx (2.7.3) It should be noted that the integral in Equation is evaluated for one complete cycle of oscillation. A typical plot of f d (t) versus displacement x(t) is shown in Figure Hence, the integral on the right-hand side of Equation is the shaded area in Figure In other words, the shaded area is the energy dissipated (W nv ) per cycle of oscillation. Example Forced Response of a Frictionally Damped Spring Mass System Consider a spring mass system in Figure where one side of the mass is pushed against the wall by the normal load N. Assuming that

147 Vibration of an SDOF System under Constant and Purely Harmonic Excitation 129 f d ( t) dx f d (t) x(t) Figure Damper force f d (t) versus oscillatory displacement x(t) the coefficient of friction is μ, the friction force f d (t) will be μn in the direction opposite to the velocity ẋ(t). The external force on the mass is f sin ωt. It is assumed that the steady state response is sinusoidal, that is, x(t) = Asin(ωt φ) (2.7.4) where amplitude A and phase φ are to be determined. Coefficient of friction μ m k Static equilibrium x(t) f sin ωt Normal load N Figure Frictionally damped spring mass system under sinusoidal excitation

148 13 Vibration of Mechanical Systems f d (t) μn A A x(t) μn Figure Coulomb friction force f d (t) versus steady state displacement x(t) Figure shows the friction force versus the steady state displacement plot. Therefore, the energy dissipated per cycle of oscillation is W nv = f d (t)dx = 4μNA (2.7.5) From Equation 2.7.2, the equivalent viscous damping is given by c eq = 4μN (2.7.6) πωa Using the equivalent viscous damping c eq, the system shown in Figure can be approximated as a standard spring mass damper system shown in Figure The application of Equation yields A = f [(k mω 2 ) 2 + (c eq ω) 2 ] Substituting Equation into Equation 2.7.7, A = f k ( 4μN 1 πf (1 r 2 ) 2 ) 2.5 (2.7.7) (2.7.8)

149 Vibration of an SDOF System under Constant and Purely Harmonic Excitation 131 c eq m k Static equilibrium x(t) f sin ωt Figure Spring mass system with the equivalent viscous damper where r is the frequency ratio ω/ω n. For the amplitude A to be a real number, it is required that f > 4μN π (2.7.9) The inequality in Equation is the condition for the validity of the equivalent viscous damping approach. From Equation 2.7.8, the following results are derived: a. At resonance condition (r = 1orω = ω n ), the steady state amplitude of vibration is unbounded. In other words, Coulomb friction damping is unable to bring any change to the response of a spring mass system. This is a reflection of the fact that the energy dissipated by Coulomb friction per cycle is proportional to the amplitude. Therefore, the Coulomb friction is a weaker form of damping in comparison with the viscous damping for which the energy dissipated per cycle is proportional to the square of the amplitude. b. At nonresonance condition (r 1orω ω n ), the steady state amplitude of vibration does get reduced due to Coulomb friction, when compared with the amplitude of an undamped system.

150 132 Vibration of Mechanical Systems EXERCISE PROBLEMS P2.1 Consider the system shown in Figure P2.1a where a = 25 cm, l 1 = 5 cm, and l 2 = 3 cm. When the force f (t) is a step function of magnitude 1 N, the response is as shown in Figure P2.1b. Determine the mass m, the stiffness k, and the damping constant c. A a l 1 c k l 2 Massless and rigid bar f (t) m Figure P2.1a Spring mass damper system subjected to step forcing function Displacement of mass (meter) Time (sec) Figure P2.1b Unit step response for system in Figure P2.1a

151 Vibration of an SDOF System under Constant and Purely Harmonic Excitation 133 P2.2 Consider a spring mass-damper system (Figure P2.2) with m eq = 1 kg, k eq = 1, N/m, and c eq = 2 N-sec/m. m1 m2 m eq c eq k eq x(t) Static equilibrium Figure P2.2 Spring mass-damper system It is required to place a 1 kg mass on the main mass m eq such that the new static equilibrium is reached without any vibration. Develop a strategy to achieve this goal. You are allowed to use two separate masses m 1 and m 2 such that m 1 + m 2 = 1 kg. P2.3 Consider the system shown in Figure P2.1a where a = 25 cm, l 1 = 5 cm, and l 2 = 3 cm. Here, k = 1,1 N/m and.5kg< m < 2 kg. The force f (t) is a step function. Find a value of damping constant c such that the steady state is reached without any overshoot. P2.4 Consider the system shown in Figure P1.12. a. First the damper is detached and the mass is excited by a force f (t) = 2 sin ωtn. Find and plot responses when ω =.8ω n,ω n, and 1.5ω n. Assume that the initial conditions are zero. Compare your results from analysis to those from MATLAB ODE23 or ODE45. b. Reattach the damper, and the mass is again excited by a force f(t) = 2 sin ωtn. Find and plot responses when ω =.8ω n,ω n, and 1.5ω n. Assume that the initial conditions are zero. Compare your results from the analysis to those from MATLAB ODE23 or ODE45. Also, show the phase lags of the responses in your plots.

152 134 Vibration of Mechanical Systems P2.5 Consider a rotor on a massless and rigid shaft, which is supported by ball bearings at the ends (Figure P2.5). l 2 l 2 Bearing Rotor with mass = m and eccentricity = e Massless and rigid shaft Bearing Figure P2.5 Rotor on massless and rigid shaft The mass of the rotor is 1 kg and the eccentricity is.5 cm. If the operating speed of the rotor is 4,2 rpm, what should be the stiffnesses of the bearings such that the amplitude of rotor vibration does not exceed.5 cm? P2.6 Consider a rotor on a massless and flexible steel shaft, which is simply supported at the ends (Figure P2.6). l 2 l 2 Simply supported shaft Rotor with mass = m Figure P2.6 Rotor on massless and flexible shaft The mass of the rotor is 1 kg and the eccentricity is.5 cm. The length and the diameter of the shaft are 5 cm and 5 cm, respectively. a. Compute the critical speed of the rotor. b. If the rotor operates at critical speed, how much time will it take for the maximum bending stress in the shaft to be about 7% of its yield stress? Assume that the rotor starts from rest.

153 Vibration of an SDOF System under Constant and Purely Harmonic Excitation 135 c. Assuming that the rotor starts from rest, suggest an angular acceleration of the rotor to reach a speed that equals two times the critical speed. It is desired that the maximum bending stress in the shaft is below 5% of its yield stress. Verify your result using numerical integration of differential equation. d. For part c, plot the force transmitted to each support as a function of time. P2.7 An instrument with mass = 13 kg is to be isolated from aircraft engine vibrations ranging from 18, to 2,3 cpm. What should be the stiffness of an isolator for at least 65% isolation? Assume that the damping ratio is.45. Assuming that the initial conditions are zero, demonstrate the performance of your isolator for both the extreme frequencies (1,8 and 2,3 cpm) using the MATLAB routine ODE23 or ODE45. For each frequency, plot displacements of the radio and the support in a single figure. Then, also verify the analytical expression of the phase of the steady state response of the instrument. Massless and rigid bar A a l 1 c k l 2 y( t) = y sin ωt x(t) m Figure P2.8 A spring mass damper system with sinusoidal base displacement P2.8 Determine the amplitude and the phase of the steady state response of the mass m in Figure P2.8. P2.9 A vehicle with mass m eq = 1,5 kg and suspension stiffness k eq = 435, N/m is traveling with a velocity V on a sinusoidal

154 136 Vibration of Mechanical Systems road surface with amplitude =.11 m and a wavelength of 5.3 m (Figure P2.9). a. Determine the critical speed of the vehicle. b. Select an appropriate amount of damping for the suspension system. c. With the damping selected in part b, should the vehicle be operated above or below the critical speed so that the amplitude of the vehicle is small? Justify your answer. d. With the damping selected in part b, should the vehicle be operated above or below the critical speed so that the amplitude of the vehicle is small relative to the road profile? Justify your answer. V m eq k eq Road profile Figure P2.9 A vehicle moving over a rough road P2.1 The natural frequency and the damping ratio of a vibrometer are 6 Hz and.22 Hz, respectively. What is the range of frequencies for the measurement error to be below 3%? Corroborate the validity of your design using MATLAB ODE23 or ODE45 for a signal frequency. Show the signal that your instrument will produce after it has been attached to the vibrating structure. P2.11 An accelerometer with mass =.1 kg and a damping ratio =.77 is to be designed. What should be the undamped natural

155 Vibration of an SDOF System under Constant and Purely Harmonic Excitation 137 frequency of the system so that the measurement error never exceeds 2%? The vibration signal, which is to be measured, can have a frequency as high as 2 Hz. Corroborate the validity of your design using MATLAB ODE23 or ODE45 for the signal frequency = 8 Hz. Show the signal that your instrument will produce after it has been attached to the vibrating structure. P2.12 The force-deflection curve for a structure is experimentally obtained (Figure P2.12). What is the equivalent viscous damping if the frequency of oscillation is 1 Hz? Force (N) Deflection (mm) 794 Figure P2.12 Force-deflection curve

156 3 RESPONSES OF AN SDOF SPRING MASS DAMPER SYSTEM TO PERIODIC AND ARBITRARY FORCES In Chapter 2, the response has been calculated when the excitation is either constant or sinusoidal. Here, a general form of periodic excitation, which repeats itself after a finite period of time, is considered. The periodic function is expanded in a Fourier series, and it is shown how the response can be calculated from the responses to many sinusoidal excitations. Next, a unit impulse function is described and the response of the single-degree-of-freedom (SDOF) system to a unit impulse forcing function is derived. Then, the concept of the convolution integral, which is based on the superposition of responses to many impulses, is developed to compute the response of an SDOF system to any arbitrary type of excitation. Last, the Laplace transform technique is presented. The concepts of transfer function, poles, zeros, and frequency response function are also introduced. The connection between the steady-state response to sinusoidal excitation and the frequency response function is shown. 3.1 RESPONSE OF AN SDOF SYSTEM TO A PERIODIC FORCE The procedure of a Fourier series expansion of a periodic function is described first. The concepts of odd and even functions are introduced next to facilitate the computation of the Fourier coefficients. It is also shown how can a Fourier series expansion be interpreted and used 138

157 Responses of an SDOF Spring Mass Damper System 139 f (t) L L 2T T T 2T 3T t Figure A periodic function for a function with a finite duration. Last, the particular integral of an SDOF system subjected to a periodic excitation is obtained by computing the response due to each term in the Fourier series expansion and then using the principle of superposition Periodic Function and its Fourier Series Expansion Consider a periodic function f (t) with the time period T, that is, the function repeats itself after time T. Therefore, f (t + nt) = f (t); n = 1, 2, 3,..., (3.1.1) and f (t nt) = f (t); n = 1, 2, 3,..., (3.1.2) It should be noted that a periodic function is defined for t (Figure 3.1.1). Sine and cosine functions are the simplest examples of periodic functions. The fundamental frequency ω of a periodic function with the time period T is defined as follows: ω = 2π T (3.1.3) The Fourier series expansion of a periodic function f (t) is defined as follows: f (t) = a + a n cos(nωt) + b n sin(nωt) (3.1.4) n=1 n=1

158 14 Vibration of Mechanical Systems where a n = 2 T T a = 1 T T f (t)dt = 1 T f (t) cos(nωt)dt = 2 T T/2 T/2 T/2 f (t)dt (3.1.5) f (t) cos(nωt)dt (3.1.6) T/2 and b n = 2 T T f (t)sin(nωt)dt = 2 T T/2 T/2 f (t)sin(nωt)dt (3.1.7) Derivations of Equations are based on the following facts: a. Integrals of cosine and sine functions over the time period T is zero, that is, T cos(nωt)dt = T/2 T/2 cos(nωt)dt = ; n (3.1.8) T sin(lωt)dt = T/2 T/2 sin(lωt)dt = (3.1.9) b. Orthogonality of cos nωt and sin lωt in the following sense: T cos(nωt)sin(lωt)dt = T/2 T/2 cos(nωt)sin(lωt)dt = (3.1.1) c. Orthogonality of sin nωt and sin lωt in the following sense: T sin(nωt)sin(lωt)dt = T/2 T/2 sin(nωt)sin(lωt)dt = ; n l (3.1.11)

159 Responses of an SDOF Spring Mass Damper System 141 d. Orthogonality of cos nωt and cos lωt in the following sense: T cos(nωt) cos(lωt)dt = T/2 cos(nωt) cos(lωt)dt = ; n l T/2 (3.1.12) The expression for a is derived by integrating both sides of Equation over a full time period as follows: T f (t)dt = T a dt + n=1 a n Using Equations and 3.1.9, T T cos(nωt)dt + n=1 b n T sin(nωt)dt (3.1.13) f (t)dt = a T + + (3.1.14) It is easily seen that Equation yields Equation The expression for a l is derived by multiplying both sides of Equation by cos(lωt) and then integrating over a full time period as follows: T f (t) cos(lωt)dt = T + a cos(lωt)dt + n=1 b n Using Equations , T T f (t) cos(lωt)dt = + a l = a l 2 n=1 a n T cos(nωt) cos(lωt)dt sin(nωt) cos(lωt)dt (3.1.15) T T cos 2 (lωt)dt cos(2lωt)dt = a l 2 T (3.1.16) It is easily seen that Equation yields Equation It should also be noted that the equation (3.1.4) is multiplied by a l where l can

160 142 Vibration of Mechanical Systems be any value from 1 to. The subscript l is chosen to be different from n, which is a counter for the summation. The expression for b l is derived by multiplying both sides of Equation by sin(lωt) and then integrating over a full time period as follows: T T f (t)sin(lωt)dt = a sin(lωt)dt + n=1 T + b n n=1 Using Equations , T T a n T cos(nωt)sin(lωt)dt sin(nωt)sin(lωt)dt (3.1.17) f (t)sin(lωt)dt = + + b l sin 2 (lωt)dt = b l 2 T 1 cos(2lωt)dt = b l 2 T (3.1.18) It is easily seen that Equation yields Equation For an even function g e (t), Even and Odd Periodic Functions Because of the property in Equation , T/2 g e (t) = g e ( t) (3.1.19) g e (t)dt = 2 T/2 g e (t)dt (3.1.2) T/2 Cosine functions are even functions because cos(nωt) = cos( nωt). For an odd function g o (t), g o (t) = g o ( t) (3.1.21)

161 Responses of an SDOF Spring Mass Damper System 143 Even Even Even Even Odd Odd Odd Even Odd Odd Odd Even Figure Multiplication of odd and even functions Because of the property in Equation , T/2 T/2 g o (t)dt = (3.1.22) Sine functions are odd because sin(nωt) = sin( nωt). When odd and even functions are multiplied among each other, the result can be either odd or even functions (Figure 3.1.2). Fourier Coefficients for Even Periodic Functions For an even periodic function, the computational effort needed to obtain the Fourier coefficients can be significantly reduced. First, using Equation 3.1.2, a = 1 T T f (t)dt = 1 T T/2 f (t)dt = 2 T T/2 f (t)dt (3.1.23) T/2 Since cosine is an even function, f (t) cos(nωt) will be an even function according to the information in Figure Therefore, using Equation again, a n = 2 T T f (t) cos(nωt)dt = 2 T T/2 f (t) cos(nωt)dt T/2 = 4 T/2 f (t) cos(nωt)dt (3.1.24) T

162 144 Vibration of Mechanical Systems f (t) A L L T T 2 T 2 T t Figure An even periodic function Since sine is an odd function, f (t)sin(nωt) will be an odd function according to the information in Figure Therefore, using Equation , b n = 2 T T f (t)sin(nωt)dt = 2 T T/2 T/2 f (t)sin(nωt)dt = (3.1.25) Example 3.1.2: Fourier Series Expansion of Triangular Waveform Consider the even periodic function shown in Figure Therefore, b n = ; n = 1, 2, 3,..., (3.1.26) To evaluate the integrals for a and a n in Equations and , the function f (t) is defined between and T/2 as follows: f (t) = A T/2 t = 2A T t; t T 2 (3.1.27) Using Equation , a = 2 T T/2 2A 4A tdt = T T 2 ( ) T = A 2 (3.1.28) Using Equation , a n = 4 T T/2 [ 2A 8A t t cos(nωt)dt = T T 2 nω sin(nωt) + 1 ] T/2 (nω) cos(nωt) 2 (3.1.29)

163 Responses of an SDOF Spring Mass Damper System 145 or a n = 8A [ 1 2nωT sin ( nωt 2 ) 1 + (nωt) cos 2 ( nωt 2 ) ] 1 T/2 (nωt) 2 (3.1.3) Because ωt = 2π (Equation 3.1.3) [ 1 a n = 8A 4nπ sin(nπ) + 1 4π 2 n cos(nπ) 1 ] (3.1.31) 2 4π 2 n 2 For odd and even n, cos nπ = 1 and +1, respectively. And sin nπ = for all n. Therefore, a n = 4A ; n = 1, 3, 5,..., (3.1.32) n 2 π 2 and Therefore, the Fourier series expansion is f (t) = A 2 4A 4A cos ωt π 2 9π a n = ; n = 2, 4, 6,..., (3.1.33) 2 cos 3ωt 4A cos 5ωt (3.1.34) 25π 2 Fourier Coefficients for Odd Periodic Functions For an odd periodic function, the computational effort needed to obtain the Fourier coefficients can also be significantly reduced. First, using Equation , a = 1 T T f (t)dt = 1 T T/2 T/2 f (t)dt = (3.1.35) Since cosine is an even function, f (t) cos(nωt) will be an odd function according to the information in Figure Therefore, using Equation again, a n = 2 T T f (t) cos(nωt)dt = 2 T T/2 T/2 f (t) cos(nωt)dt = (3.1.36)

164 146 Vibration of Mechanical Systems T T 2 f (t) A L L t A T 2 T Figure Square waveform: An odd periodic function Since sine is an odd function, f (t)sin(nωt) will be an even function according to the information in Figure Therefore, using Equation 3.1.2, b n = 2 T T f (t)sin(nωt)dt = 2 T T/2 f (t)sin(nωt)dt T/2 = 4 T/2 f (t)sin(nωt)dt (3.1.37) T Example 3.1.3: Fourier Series Expansion of a Square Waveform Consider the square waveform (Figure 3.1.4), which is an odd periodic function. Therefore, a = (3.1.38) and a n = ; n = 1, 2, 3,..., (3.1.39) To evaluate the integrals for b n in Equation , the function f (t)is defined between and T/2 as follows: f (t) = A; t T 2 (3.1.4)

165 Responses of an SDOF Spring Mass Damper System 147 f (t) td t Using Equation , Figure A finite duration function b n = 4 T T/2 = 4A nωt Asin(nωt)dt = 4A T [ cos ( nωt 2 [ cos(nωt) ] T/2 nω ) ] + 1 (3.1.41) Because ωt = 2π (Equation 3.1.3), b n = 2A (1 cos nπ) (3.1.42) nπ For odd and even n, cos nπ = 1 and +1, respectively. Therefore, and b n = 4A ; n = 1, 3, 5,..., (3.1.43) nπ Therefore, the Fourier series expansion is b n = ; n = 2, 4, 6,..., (3.1.44) f (t) = 4A π sin ωt + 4A 3π 4A sin 3ωt + sin 5ωt + (3.1.45) 5π Fourier Series Expansion of a Function with a Finite Duration Consider a function f (t) of finite duration t d (Figure 3.1.5). Then, it can also be expanded in a Fourier series by treating this function as one period of a fictitious periodic function g(t) (Figure 3.1.6).

166 148 Vibration of Mechanical Systems Fictitious g(t) Fictitious L L 2td td t d 2t 3t d d t Figure Fictitious periodic function corresponding to a finite duration function The Fourier series expansion of this fictitious periodic function g(t) can be defined as follows: g(t) = a + a n cos(nωt) + b n sin(nωt) (3.1.46) n=1 n=1 where the fundamental frequency ω is given by ω = 2π (3.1.47) t d However, the Fourier series (Equation ) should only be used for t t d, as the actual function f (t) is zero for t < and t > t d. Therefore, f (t) = a + a n cos(nωt) + b n sin(nωt); t t d (3.1.48) n=1 n=1 Example 3.1.3: Fourier Series Expansion of a Triangular Pulse Consider the triangular pulse f (t) as shown in Figure The corresponding fictitious periodic function g(t) is shown in Figure 3.1.8, which is identical to the periodic function shown in Figure Therefore, from Equation , f (t) = A 2 4A 4A cos ωt π 2 9π where 2 cos 3ωt 4A 25π 2 cos 5ωt ; t t d (3.1.49) ω = 2π t d (3.1.5)

167 Responses of an SDOF Spring Mass Damper System 149 f (t) A td 2 t d t Figure A triangular pulse Example 3.1.4: Turbomachinery Blade Excitation Consider the rotor and the stator of a turbomachine (Figure 3.1.9). The schematic drawings of the stator and the rotor are shown in Figure The stator consists of four equi-spaced nozzles, with each nozzle having a 45 degrees sector. The rotor is a bladed disk with a rotational speed of cps (Hz). When the blade comes in front of the nozzle, it experiences a constant force p due to fluid flow. The temporal variation of the force experienced by each blade is periodic and is shown in Figure The force pattern is repeated four times during one full rotation. As a result, the time period T of the forcing function is T = 1 4 sec And the fundamental frequency ω = 2π T = 8π rad/sec The periodic force shown in Figure is neither odd nor even. To evaluate the integrals for a, a n, and b n in Equation 3.1.4, the function Fictitious g(t) Fictitious L A L 2t d td t d 2t d t Figure Fictitious periodic function for a triangular pulse

168 15 Vibration of Mechanical Systems Stator Rotor Fluid flow Shaft Ω Fluid flow Figure Fluid flow through a rotor/stator f (t) is defined as follows: { p for t <.5T f (t) = for.5t t < T Using Equation 3.1.5, a = 1 T T/2 pdt + = p 2 (3.1.51) (3.1.52) Using Equation 3.1.6, a n = 2 T T/2 Because ωt = 2π, p cos(nωt)dt + = 2p [ ] sin(nωt) T/2 T nω = 2p nωt sin ( ) nωt 2 (3.1.53) a n = (3.1.54) Stator Nozzle# 4 Rotor Blades Nozzle# 3 Nozzle# 1 Disk Nozzle# 2 Figure Descriptions of rotor and stator

169 Responses of an SDOF Spring Mass Damper System 151 f (t) p.5t.5t T.5T T 1.25Ω sec 1 Ω sec One full rotation t Figure Force experienced by each blade Next, using Equation 3.1.7, b n = 2 T/2 p sin(nωt)dt + = 2p T T = 2p [ ( ) ] nωt cos + 1 nωt 2 Because ωt = 2π, [ cos(nωt) ] T/2 nω (3.1.55) b n = p (1 cos nπ) (3.1.56) nπ For odd and even n, cos nπ = 1 and +1, respectively. Therefore, b n = 2p ; n = 1, 3, 5,..., (3.1.57) nπ and Therefore, the Fourier series expansion is f (t) = p 2 + 2p π b n = ; n = 2, 4, 6,..., (3.1.58) 2p 2p sin ωt + sin 3ωt + sin 5ωt + (3.1.59) 3π 5π Particular Integral (Steady-State Response with Damping) Under Periodic Excitation Consider the spring mass damper system subjected to a periodic force f (t) with the fundamental frequency ω. Using the Fourier series

170 152 Vibration of Mechanical Systems expansion, the differential equation of motion can be written as m eq ẍ + c eq ẋ + k eq x = f (t) = a + a n cos(nωt) + b n sin(nωt) n=1 n=1 (3.1.6) First the particular integral will be obtained for each term on the righthand side separately. a. Constant term a Let the particular integral due to the constant term be x. Then, m eq ẍ + c eq ẋ + k eq x = a (3.1.61) The particular integral in this case will be a constant p, that is, x = p (3.1.62) Substituting Equation into Equation , because ṗ = and p =. In other words, b. Term a n cos(nωt) Let the particular integral be x cn (t). Then, k eq p = a (3.1.63) x = p = a k eq (3.1.64) m eq ẍ cn + c eq ẋ cn + k eq x cn = a n cos(nωt) (3.1.65) Using Equation , x cn (t) = a n /k eq (1 n2 r 2 ) 2 + (2ξnr) 2 cos(nωt φ n) (3.1.66) where φ n = tan 1 2ξnr 1 (nr) 2 (3.1.67)

171 Responses of an SDOF Spring Mass Damper System 153 and r = ω ω n (3.1.68) c. Term b n sin(nωt) Let the particular integral be x sn (t). Then, Using Equation , x sn (t) = m eq ẍ sn + c eq ẋ sn + k eq x sn = b n sin(nωt) (3.1.69) b n /k eq (1 n2 r 2 ) 2 + (2ξnr) 2 sin(nωt φ n) (3.1.7) where φ n and r are given by Equations and Having obtained the steady-state response or the particular integral due to each term on the right-hand side of Equation 3.1.6, the complete particular integral x p (t) is obtained by using the principle of superposition: or x p (t) = a k eq + + n=1 x p (t) = x + x cn (t) + x sn (t) (3.1.71) n=1 n=1 n=1 a n /k eq (1 n2 r 2 ) 2 + (2ξnr) 2 cos(nωt φ n) b n /k eq (1 n2 r 2 ) 2 + (2ξnr) 2 sin(nωt φ n) (3.1.72) where φ n and r are given by Equations and It should be noted that x p (t) will be the steady-state response when c eq >. Example 3.1.5: Steady-State Response of a Turbine Blade Consider the turbomachinery problem of Example The turbine rotates at 6 Hz. The natural frequency of the system is 16 Hz. Find the maximum steady-state amplitude in terms of p/k eq assuming that the damping ratio ξ is.1.

172 154 Vibration of Mechanical Systems Using Equations and , x p (t) p/k eq = where n=1,3,... 2/(nπ) (1 n2 r 2 ) 2 + (2ξnr) 2 sin(nωt φ n) (3.1.73) r = ω ω n = 6 16 = 3 8 and ξ=.1 n = 1: 2/n (1 n2 r 2 ) 2 + (2ξnr) 2 = ; φ 1 =.87 rad n = 3: 2/n (1 n2 r 2 ) 2 + (2ξnr) 2 = 2.58; φ 3 = rad n = 5: 2/n (1 n2 r 2 ) 2 + (2ξnr) 2 =.159; φ 5 = rad n = 7: 2/n (1 n2 r 2 ) 2 + (2ξnr) 2 =.485; φ 7 = rad Neglecting n = 7 and higher terms, x p (t) = sin(ωt φ 1 ) sin(3ωt φ 3 ) p/k eq π π sin(5ωt φ 5 ) π The plot of x p (t)/(p/k eq ) is shown in Figure The maximum value of the steady-state response x p (t)/(p/k eq )is RESPONSE TO AN EXCITATION WITH ARBITRARY NATURE The response to an arbitrary type of excitation is obtained via an impulsive force, which has a large magnitude and a small duration.

173 Responses of an SDOF Spring Mass Damper System x p (t)/(p/2*k eq ) Time (sec) Figure Steady-state response with n = 5 in Equation The time integral ˆ f of an impulsive force f (t) is finite, and is defined as the impulse of the force: f ˆ = f (t)dt (3.2.1) In other words, the impulse of a force fˆ is the area under the force time plot. For a unit impulse, f ˆ = 1. Drawing an analogy with this impulsive force, a mathematical impulse unit function is defined as follows Unit Impulse Function δ(t a) Consider a constant function of the magnitude 1/ε of duration ε (Figure 3.2.1a). Therefore, f ˆ = f (t)dt = 1 ε ε = 1 (3.2.2) The function shown in Figure 3.2.1a has a unit impulse. Having ε, the unit impulse function δ(t a) is obtained (Figure 3.2.1b) with the following properties:

174 156 Vibration of Mechanical Systems ε δ(t a) 1 ε ε a (a) ε t a (b) t Figure Unit impulse function 1. δ(t a) = when t a (3.2.3) 2. δ(t a) = when t = a, but with the following constraint δ(t a)dt = 1 (3.2.4) 3. For any function g(t), g(t)δ(t a)dt = g(a) (3.2.5) It should be noted that the unit impulse function is not necessarily a force function. It could be defined for any variable, such as displacement and velocity Unit Impulse Response of an SDOF System with Zero Initial Conditions Consider a spring mass damper system subjected to the unit impulse force δ(t) (Figure 3.2.2). The differential equation of motion is m eq ẍ + c eq ẋ + k eq x = δ(t) (3.2.6) This unit impulse force is applied at t =. Since the duration of this impulse function is zero, symbols and + are introduced to denote instants just before and after the application of the force, respectively.

175 Responses of an SDOF Spring Mass Damper System 157 c eq k eq Static equilibrium x(t) m eq δ(t) Figure Spring mass damper subjected to unit impulse force Zero initial conditions will then be represented as x( ) = and ẋ( ) = (3.2.7) Due to the impulsive force, change in momentum = m ẋ( + ) mẋ( ) = mẋ( + ) (3.2.8) Since the change in the momentum equals the magnitude of the impulse, mẋ( + ) = 1 (3.2.9) or, ẋ( + ) = 1 m (3.2.1) In other words, the velocity has changed instantaneously from to 1/m. It can be shown that the displacement remains unchanged, that is, x( + ) = x( ) = (3.2.11)

176 158 Vibration of Mechanical Systems Note that there is no force for t +. Hence, the response due to the unit impulse force can be obtained by solving the following free vibration problem: m eq ẍ + c eq ẋ + k eq x = ; x( + ) = and ẋ( + ) = 1 m (3.2.12) Case I: Undamped and Underdamped System ( ξ<1) From Equation , the free vibration of the underdamped system is given by x(t) = e ξωnt (A 1 cos ω d t + B 1 sin ω d t) (3.2.13) where A 1 = x( + ) = (3.2.14) B 1 = ẋ(+ ) + ξω n x( + ) = 1 (3.2.15) ω d mω d Substituting Equations and into Equation , x(t) = 1 e ξωnt sin ω d t (3.2.16) mω d The unit impulse response (the response due to the unit impulse force) of an underdamped system is given by Equation and will be represented by a new symbol g(t), that is, g(t) = 1 e ξωnt sin ω d t (3.2.17) mω d For an undamped system (ξ = ), g(t) = 1 sin ω n t (3.2.18) mω n Case II: Critically Damped (ξ = 1 or c eq = c c ) From Equation , the free vibration of the critically damped system is given by x(t) = x( + )e ω nt + [ ẋ ( +) + ω n x ( +)] te ω nt = 1 m te ω nt (3.2.19)

177 Responses of an SDOF Spring Mass Damper System 159 The unit impulse response (the response due to the unit impulse force) of a critically damped system is given by Equation and will be represented by a new symbol g(t), that is, g(t) = 1 m te ω nt (3.2.2) Case III: Overdamped (ξ>1 or c eq >c c ) From Equation , the free vibration of the overdamped system is given by x(t) = A 1 e s 1t + B 1 e s 2t (3.2.21) where s 1 = ξω n + ω n ξ 2 1 < (3.2.22) s 2 = ξω n ω n ξ 2 1 < (3.2.23) Using Equations and , A 1 = s 2x( + ) ẋ( + ) 1 = s 2 s 1 m(s 2 s 1 ) = 1 2mω n ξ 2 1 (3.2.24) B 1 = s 1x( + ) + ẋ( + ) 1 = s 2 s 1 m(s 2 s 1 ) = 1 2mω n ξ 2 1 (3.2.25) The unit impulse response (the response due to the unit impulse force) of an overdamped system is given by Equation and will be represented by a new symbol g(t), that is, g(t) = 1 2mω n ξ 2 1 (es 1t e s 2t ) (3.2.26) Nondimensional unit impulse responses, g(t)mω n, are plotted in Figure for all cases of damping.

178 16 Vibration of Mechanical Systems Nondimensional unit impulse response (g(t)m eq α n ) Nondimensional time, ω n t ξ = < ξ < 1 ξ = 1 ξ > 1 25 Figure Unit impulse response for undamped, underdamped, critically damped, and overdamped systems Convolution Integral: Response to an Arbitrary Excitation with Zero Initial Conditions Consider a forcing function f (t) of an arbitrary nature (Figure 3.2.4). Let us determine the response at t = t. First, the following points should be noted: a. The entire forcing function before t = t will have an influence on the response at t = t. f (t) Δτ L L L t Δτ τ τ t a t Figure An arbitrary forcing function

179 Responses of an SDOF Spring Mass Damper System 161 b. The forcing function after t = t will not have any influence on the response at t = t. The forcing function before the instant t = t is divided into many strips of small time width τ. Each strip can be viewed as an impulse function as shown in Figure Let us consider one such impulse function at t = τ. The magnitude of the impulse is ˆ f τ = f (τ) τ (3.2.27) The contribution of this impulse to the response at t = t is x τ (t ) = g(t τ) ˆ f τ = g(t τ)f (τ) τ (3.2.28) where g(.) is the unit impulse response. The response at t = t is then found by summing up the contributions from all the impulses (strips of width τ ): x(t ) = τ=t τ τ= x τ (t ) = τ=t τ τ= g(t τ)f (τ) τ (3.2.29) When τ, the summation in Equation becomes the integral as follows: x(t ) = τ=t g(t τ)f (τ)dτ (3.2.3) τ= Since t is arbitrarily chosen, the response at any time t is given by x(t) = τ=t g(t τ)f (τ)dτ (3.2.31) τ= This is the convolution integral and yields the response for zero initial conditions. This represents the complete solution, that is, it contains both the homogeneous part and the particular integral. Example 3.2.1: Step Response of an Undamped System Consider the undamped spring mass system subjected to a step forcing function (Figure 3.2.5). Assume that the initial conditions are

180 162 Vibration of Mechanical Systems x(t) Static equilibrium m eq f (t) k eq f (t) f t Figure Undamped SDOF system subjected to step forcing function zeros. Then, f (t) = f for t > (3.2.32) Using Equations , , and , x(t) = τ=t τ= 1 mω n sin(ω n (t τ))f dτ = f mω n After some simple algebra, [ cos(ωn (t τ)) ω n ] τ=t τ= (3.2.33) x(t) = f [1 cos(ω mωn 2 n t)] = f k [1 cos(ω nt)] ; t (3.2.34) Note that Equation is identical to Equation with ξ =. Example 3.2.2: Underdamped SDOF System Subjected to a Rectangular Pulse Consider an underdamped spring mass damper system in which the mass is subjected to the force f (t) as shown in Figure Assuming that all initial conditions are zero, find the response using the convolution integral. The differential equation of motion is m eq ẍ + c eq ẋ + k eq x(t) = f (t); x() =, ẋ() = (3.2.35)

181 Responses of an SDOF Spring Mass Damper System 163 x(t) Static equilibrium k eq m eq f (t) c eq f (t) P a b t Figure Underdamped SDOF system and rectangular pulse Case I: t a x(t) = τ=t τ= g(t τ)f (τ)dτ = τ=t τ= g(t τ)dτ = (3.2.36) Case II: a t b x(t) = = Therefore, x(t) = τ=t τ= τ=t τ=a τ=t τ=a g(t τ)f (τ)dτ = τ=a τ= g(t τ)dτ + τ=t τ=a g(t τ) Pdτ g(t τ) Pdτ (3.2.37) g(t τ) Pdτ = P m eq ω d τ=t τ=a Substituting ν = t τ into Equation , e ξω n(t τ) sin(ω d (t τ))dτ (3.2.38) x(t) = P m eq ω d ν=t a ν= e ξω nν sin(ω d ν)dν (3.2.39)

182 164 Vibration of Mechanical Systems or x(t) = P m eq ω d [ e ξω nν ξ 2 ωn 2 + ( ξω n sin ω d ν ω d cos ω d ν)] ω2 d ν=t a ν= (3.2.4) or [ ] x(t) = P e ξω n(t a) (ξω m eq ω d ωn 2 n sin ω d (t a) + ω d cos ω d (t a)) + P (3.2.41) m eq ωn 2 Simplifying Equation , x(t) = P [ 1 ω ] n e ξωn(t a) sin(ω m eq ωn 2 d (t a) + φ) (3.2.42) ω d where cos φ = ξ Case III: t b x(t) = τ=t g(t τ)f (τ)dτ = τ= τ=a τ= Therefore, x(t) = g(t τ)dτ + τ=b τ=a τ=b τ=a g(t τ) Pdτ + g(t τ) Pdτ = P m eq ω d τ=b τ=a τ=t τ=b g(t τ)dτ (3.2.43) e ξω n(t τ) sin(ω d (t τ))dτ (3.2.44) or [ x(t) = P e ξω nν ν=t a m eq ω d ξ 2 ωn 2 + ( ξω n sin ω d ν ω d cos ω d ν)] ω2 d ν=t b (3.2.45)

183 Responses of an SDOF Spring Mass Damper System 165 Simplifying Equation , x(t) = P [ e ξωn(t b) sin(ω d (t b) + φ) m eq ω d ω n ] e ξωn(t a) sin(ω d (t a) + φ) (3.2.46) where cos φ = ξ Convolution Integral: Response to an Arbitrary Excitation with Nonzero Initial Conditions Consider again the same equivalent SDOF system considered in Section 3.2.3, that is, m eq ẍ + c eq ẋ + k eq x = f (t) (3.2.47) Assume that x() and/or ẋ() are not zero. In this case, the following two problems are separately solved: 1. Forcing function with zero initial conditions m eq ẍ 1 + c eq ẋ 1 + k eq x 1 = f (t); x 1 () = and ẋ 1 () = (3.2.48) This problem has already been solved in Section Therefore, x 1 (t) = τ=t g(t τ)f (τ)dτ (3.2.49) τ= 2. Nonzero initial conditions without any forcing function m eq ẍ 2 + c eq ẋ 2 + k eq x 2 (t) = ; x 2 () = x() and ẋ 2 () = ẋ() (3.2.5) The problem in Equation represents the free vibration of a damped or undamped SDOF system. As seen in Chapter 2, the solution to Equation depends on the damping values as follows.

184 166 Vibration of Mechanical Systems Case I: Undamped and Underdamped ( ξ<1 or c eq <c c ) where x 2 (t) = e ξω nt (A 1 cos ω d t + B 1 sin ω d t) (3.2.51) A 1 = x() (3.2.52) and B 1 = ẋ() + ξω nx() ω d (3.2.53) Case II: Critically Damped (ξ = 1 or c eq = c c ) x 2 (t) = x()e ω nt + [ẋ() + ω n x()]te ω nt (3.2.54) Case III: Overdamped (ξ >1 or c eq > c c ) x 2 (t) = A 1 e s 1t + B 1 e s 2t (3.2.55) where s 1 = ξω n + ω n ξ 2 1 < (3.2.56) and s 2 = ξω n ω n ξ 2 1 < (3.2.57) A 1 = s 2x() ẋ() s 2 s 1 (3.2.58) B 1 = s 1x() + ẋ() (3.2.59) s 2 s 1 Last, the solution to Equation with an arbitrary forcing function and nonzero initial conditions is x(t) = x 1 (t) + x 2 (t) (3.2.6)

185 Responses of an SDOF Spring Mass Damper System 167 x(t) Velocity m eq Static equilibrium y(t) k eq x( ) x&( ) = y t = Position Figure Vehicle moving over a step bump where x 1 (t) is given by Equation and x 2 (t) is given by Equations , , and for ξ<1, ξ = 1, and ξ>1, respectively. It should also be remembered that the expression of the unit impulse response g(t), in the solution to Equation for x 1 (t) is given by Equations , 3.2.2, and for ξ<1, ξ = 1, and ξ>1, respectively. Example 3.2.3: A Vehicle Past a Step Bump The differential equation of motion (Figure 3.2.7) is m eq ẍ + k eq x = f (t) (3.2.61) where f (t) = k eq y(t) (3.2.62) Using Equations 3.2.6, , and , the response is x(t) = x 1 (t) + x 2 (t) (3.2.63) where x 1 (t) = τ=t g(t τ)f (τ)dτ (3.2.64) τ=

186 168 Vibration of Mechanical Systems and Here, From Equations and , x 1 (t) = 1 m eq ω n x 2 (t) = x() cos ω n t (3.2.65) g(t) = 1 m eq ω n sin ω n t (3.2.66) τ=t τ= τ=t = 1 m eq ω n τ= Evaluating the integral in Equation , sin(ω n (t τ))k eq y(τ)dτ Last, from Equations , , and , sin(ω n (t τ))k eq y dτ (3.2.67) x 1 (t) = y (1 cos ω n t) (3.2.68) x(t) = y (1 cos ω n t) + x() cos ω n t (3.2.69) 3.3 LAPLACE TRANSFORMATION Consider a function f (t)u s (t) in time-domain where u s (t) is the unit step function (Figure 3.3.1). In other words, { f (t) for t f (t)u s (t) = (3.3.1) for t < The Laplace transform of f (t)u s (t) is defined as f (s) = L(f (t)u s (t)) = f (t)e st dt (3.3.2)

187 Responses of an SDOF Spring Mass Damper System 169 u s (t) 1 t Figure Unit step function And f (t)u s (t) is defined as the inverse Laplace transformation of f (s), that is, L 1 (f (s)) = f (t)u s (t) (3.3.3) a. Linearity Properties of Laplace Transformation L(αf 1 (t) + β f 2 (t)) = αl(f 1 (t)) + βl(f 2 (t)) (3.3.4) where α and β are any real or complex numbers. b. Derivatives ( ) df L = sl(f (t)) f () (3.3.5) dt ( d 2 ) f L dt 2 = s 2 L(f (t)) sf() df () (3.3.6) dt c. Integrals L t f (τ)dτ = 1 L(f (t)) (3.3.7) s d. Shifting in time-domain L(f (t a)u s (t a)) = e as L(f (t)) (3.3.8)

188 17 Vibration of Mechanical Systems e. Shifting in s-domain f. Final value theorem L(e αt f (t)u s (t)) = f (s α) (3.3.9) lim f (t) = lim sf(s) t s (3.3.1) provided sf(s) is analytic on the imaginary axis and in the right half of the complex s-plane. g. Convolution integral t L f 1 (t τ)f 2 (τ)dτ = L(f 1 (t)).l(f 2 (t)) = f 1 (s)f 2 (s) (3.3.11) Therefore, the convolution integral in time-domain is equivalent to a simple multiplication in s-domain Response of an SDOF System via Laplace Transformation The differential equation of motion for the system shown in Figure is m eq ẍ + c eq ẋ + k eq x = f (t) (3.3.12) where the nature of forcing function f (t) is arbitrary. Taking Laplace transformation on both sides, L(m eq ẍ + c eq ẋ + k eq x) = L(f (t)) (3.3.13) Using the linearity property in Equation 3.3.4, m eq L(ẍ(t)) + c eq L(ẋ(t)) + k eq L(x(t)) = L(f (t)) Using properties in Equations and 3.3.6, m eq [s 2 x(s) sx() ẋ()] + c eq [sx(s) x()] + k eq x(s) = f (s) (3.3.14) or ( meq s 2 ) + c eq s + k eq x(s) meq ẋ() (m eq s + c eq )x() = f (s) (3.3.15)

189 Responses of an SDOF Spring Mass Damper System 171 Solving the algebraic Equation , x(s) = or, x(s) = f (s) + (m eqs + c eq )x() + m eq s 2 + c eq s + k eq m eq s 2 + c eq s + k eq f (s) [ ] + m eq s 2 + c eq m eq s + k eq m eq Using Equations and , ( s + c eq m eq ) x() [ s 2 + c eq m eq s + k eq m eq ] + m eq ẋ() m eq s 2 + c eq s + k eq (3.3.16) ẋ() [ ] s 2 + c eq m eq s + k eq m eq (3.3.17) f (s) x(s) = ( ) + (s + 2ξω n)x() ẋ() ( + ( m eq s2 + 2ξω n s + ωn 2 s2 + 2ξω n s + ωn) 2 s2 + 2ξω n s + ωn) 2 (3.3.18) Last, the response x(t) is obtained by taking the inverse Laplace transformation of x(s). Example 3.3.1: Rectangular Pulse as the Force Consider an underdamped SDOF system subjected to the force shown in Figure To determine the Laplace transform of the forcing function f(s), via the table in Appendix C, the function f (t) is represented as a difference between the two forcing functions f 1 (t) and f 2 (t) shown in Figure 3.3.3, that is, Using the property in Equation 3.3.4, From the table in Appendix C, f (t) = f 1 (t) f 2 (t) (3.3.19) f (s) = f 1 (s) f 2 (s) (3.3.2) L(u s (t)) = 1 s (3.3.21)

190 172 Vibration of Mechanical Systems f (t) P a b t Figure Rectangular pulse as a forcing function for SDOF System Therefore, using the property in Equation 3.3.8, and Therefore, from Equation 3.3.2, L(f 1 (t)) = L(Pu s (t a)) = P s e as (3.3.22) L(f 2 (t)) = L(Pu s (t b)) = P s e bs (3.3.23) f (s) = P s (e as e bs ) (3.3.24) With zero initial conditions, Equation yields Pe as Pe bs x(s) = m eq s ( ) s 2 + 2ξω n s + ωn 2 m eq s ( ) (3.3.25) s 2 + 2ξω n s + ωn 2 From the table in Appendix C, [ ] L 1 ωn 2 [ s ( ) = 1 ω ] n e ξωnt sin(ω s 2 + 2ξω n s + ωn 2 d t + φ) u s (t) ω d (3.3.26) f 1( t ) P _ f 2( t ) P a t b t Figure Rectangular pulse as a difference between two delayed step functions

191 Responses of an SDOF Spring Mass Damper System 173 where φ = cos 1 ξ. Using the property in Equation and the result in Equation , the inverse Laplace transformation of Equation yields [ x(t) = 1 ω n ] e ξωn(t a) sin(ω d (t a) + φ) ω d [ 1 ω n e ξωn(t b) sin(ω d (t b) + φ) ω d P u m eq ωn 2 s (t a) ] P u m eq ωn 2 s (t b) (3.3.27) Transfer Function and Frequency Response Function A general second-order differential equation can be written as ẍ + a 1 ẋ + a x(t) = b 2 ü + b 1 u + b u(t) (3.3.28) where x(t) and u(t) are defined as the output and the input of the system, respectively. Taking Laplace transform of Equation , s 2 x(s) sx() ẋ() + a 1 (sx(s) x()) + a x(s) = s 2 u(s) su() u() + b 1 (su(s) u()) + b u(s) (3.3.29) Setting all initial conditions (x(), ẋ(), u(), and u()) to be zero, Equation yields s 2 x(s) + a 1 sx(s) + a x(s) = b 2 s 2 u(s) + b 1 su(s) + b u(s) (3.3.3) The transfer function G(s) is then defined as G(s) = x(s) u(s) = b 2s 2 + b 1 s + b s 2 + a 1 s + a (3.3.31)

192 174 Vibration of Mechanical Systems u(s) G(s) x(s) Figure Block diagram Equation is represented by a block diagram, shown in Figure 3.3.4, where x(s) = G(s)u(s) (3.3.32) Example 3.3.2: Direct Excitation Problem For the direct excitation problem, the governing differential equation is given by Equation Comparing Equations and , u(t) = f (t) (3.3.33) b = 1 m eq, b 1 =, b 2 = (3.3.34) a = k eq m eq, a 1 = c eq m eq (3.3.35) The transfer function is given by G(s) = x(s) u(s) = b s 2 + a 1 s + a (3.3.36) Example 3.3.3: Base Excitation Problem For the base excitation problem, governing differential equation is Equation Comparing Equations and , u(t) = y(t) (3.3.37) b 2 =, b 1 = c eq m eq, b = k eq m eq (3.3.38) a = k eq m eq, a 1 = c eq m eq (3.3.39)

193 Responses of an SDOF Spring Mass Damper System 175 The transfer function is given by G(s) = x(s) u(s) = b 1s + b s 2 + a 1 s + a (3.3.4) Significance of Transfer Function Let u(t) = δ(t), the unit impulse function. In this case, u(s) = 1 and Equation yields x(s) = G(s), where x(s) is the Laplace transform of the response to the unit impulse input under zero initial conditions, known as the unit impulse response. In other words, the transfer function is the Laplace transform of the unit impulse response of the system. Taking the inverse Laplace transform of Equation , x(t) = t g(t τ)u(τ)dτ (3.3.41) Equation is the same convolution integral that has been derived in Section Poles and Zeros of Transfer Function A transfer function G(s) can be written as a ratio of two polynomials N(s) and D(s): G(s) = N(s) (3.3.42) D(s) Poles are defined as the roots of the denominator polynomial equation D(s) =, whereas zeros are defined as the roots of the numerator polynomial equation N(s) = In general, D(s) = is also the characteristic equation. For example, poles are the roots of s 2 + a 1 s + a = for both direct force and base excitation transfer functions (Equations and 3.3.4). They are the same as the characteristic roots defined in Chapter 1. There is no finite zero for the direct force excitation transfer function (Equation ). However, there is a real zero at b /b 1 for the base excitation transfer function (Equation 3.3.4). The transfer function is called stable when all poles are located in the left half of the complex s-plane (Figure 1.6.1).

194 176 Vibration of Mechanical Systems Frequency Response Function The frequency response function G( jω) is obtained by substituting s = jω into the transfer function G(s), where ω is the frequency. The frequency response function for the transfer function in Equation is given by G( jω) = b 1 jω + b b + jb 1 ω = ( jω) 2 + a 1 jω + a (a ω 2 ) + ja 1 ω (3.3.43) For a specified frequency ω, G( jω) is a complex number, for which the magnitude and the phase (or angle) can be expressed as b G( jω) 2 + b2 1 ω2 = (3.3.44) (a ω 2 ) 2 + (a 1 ω) 2 G( jω) = tan 1 b 1ω b tan 1 a 1 ω (a ω 2 ) (3.3.45) When the magnitude in decibels, db (2 log 1 G( jω) ) and the phase are plotted as a function of the frequency ω, the resulting diagrams are known as Bode plots. The MATLAB routine bode can be readily used to make Bode plots. For a stable G(s), the magnitude and the phase of the frequency response function have the following physical significance: a. The magnitude G( jω) is the ratio of the amplitudes of the steadystate output and the sinusoidal input with frequency ω. b. The angle G( jω) is the phase difference between the steady-state sinusoidal output and the sinusoidal input with frequency ω. Example 3.3.4: Frequency Response Function for Direct Excitation Problem The frequency response function for the direct force transfer function (Equation ) is as follows: G( jω) = b ( jω) 2 + ja 1 ω + a (3.3.46)

195 Responses of an SDOF Spring Mass Damper System 177 For a linear differential equation with characteristic roots in the left half of the complex plane, the steady-state response x(t) is sinusoidal when the input u(t) is sinusoidal. Let u(t) = u sin ωt (3.3.47) Then the steady-state response x(t) will be expressed as x(t) = Asin(ωt φ) (3.3.48) where φ is the phase lag between the steady-state response x(t) and the input u(t). The magnitude and the phase of the complex number G( jω) are related to the steady-state amplitude and the phase as follows: G( jω) A = (3.3.49) u and G( jω) = φ (3.3.5) For the direct force excitation problem, substitution of the parameters in Equations and yields G( jω) = 1 ( keq m eq ω 2) + jc eq ω (3.3.51) From Equation , A = G( jω) 1 = u (keq m eqω 2) 2 ( ) 2 + ceqω (3.3.52) From Equation 3.3.5, φ = G( jω) = tan 1 c eq ω k eq m eq ω 2 (3.3.53) Note that Equations and are same as Equations and

196 178 Vibration of Mechanical Systems Bode diagram Magnitude (db) Phase (deg) Frequency (rad/sec) Figure Bode plot for direct excitation problem Example 3.3.5: Bode Plot via MATLAB Consider the Example for which m eq = 1kg, k eq = 2, 5 N/m, and c eq = 25 N-sec/meter. Construct Bode plots for this system. Using Equation , the transfer function of the system is G(s) = 1 s s + 2, 5 (3.3.54) The Bode plots are shown in Figure The MATLAB program is given in Program MATLAB Program 3.3.1: Bode Plots num=1; den=[ ]; sysg=tf(num,den); bode(sysg) grid

197 Responses of an SDOF Spring Mass Damper System 179 EXERCISE PROBLEMS P3.1 Find the Fourier series expansions of the periodic functions shown in Figures P3.1a c. P f (t) Half sine wave T T 2T t Figure P3.1a Periodic function with half sine waves f (t) P T T 2 P P T 2 T t Figure P3.1b Periodic function with triangular waves f (t) P 2T T.3T T 2T t Figure P3.1c A periodic function with ramps P3.2 Consider a spring mass damper system with mass = 1.2 kg and damping ratio =.5, which is subjected to the periodic force shown in Figures P3.1a c. Let the natural frequency of the system be π/t.

198 18 Vibration of Mechanical Systems a. Determine the steady-state response for each forcing function. b. Verify your analytical results by numerical integration of the differential equations. P3.3 In Example 3.1.4, the second nozzle is inadvertently closed. Find the Fourier coefficients of the force experienced by each blade. Also, find the maximum steady-state amplitude using the natural frequency and the damping ratio provided in Example P3.4 A turbine blade in a rotor of a gas turbine experiences the force (Table P3.4) during each rotation. An SDOF model of the turbine blade is described in Figure P3.4a where m = 1 kg, k = 8, N/m, and c = 3 N-sec/m. The clearance is.2 m. The force f (t) is described by data in Table P3.4, where the locations of the pressure measurements are shown in P3.4b. Table P3.4 Force (N) as a function of angular position Locations 1 through Locations 11 through Locations 21 through Locations 31 through Locations 41 through Locations 51 through Locations 61 through Locations 71 through Locations 81 through Locations 91 through

199 Responses of an SDOF Spring Mass Damper System 181 k x(t) Casing f (t) m c Clearance Figure P3.4a Model of a turbine blade n n = number of force measurements Δθ = 2π n rad Figure P3.4b Locations of force measurements Ω 6 Hz t Figure P3.4c Angular velocity profile The gas turbine is stationary. You are in charge of starting the engine and operating the gas turbine at a steady rotational speed equal to 6 Hz. Using the velocity profile (Figure P3.4c), determine the smallest angular acceleration of the turbine so that the blade never hits the casing.

200 182 Vibration of Mechanical Systems At the steady speed of 6 Hz, compute the Fourier coefficients and the steady-state response analytically. Compare your steady-state response to that from ODE23 or ODE45. P3.5 Consider an SDOF spring mass damper system subjected to the step forcing function (Figures and 2.1.2). Using the convolution integral, derive Equations , , and for underdamped, critically damped, and overdamped system, respectively. P.3.6 Using the convolution integral, derive the expression for the response of an undamped (c = ) spring mass system subjected to the forcing function f(t) shown in Figure P3.6. Assume that all initial conditions are zero. f(t) Q x(t) k P f (t) m c a t Figure P3.6 Spring mass system subjected to staircase function with two steps P3.7 Consider the cantilever of the atomic force microscope (Binning and Quate, 1986) with length =1 μm, thickness =.8 μm, and width = 2 μm. The material of the cantilever is silicon nitride having E = Pa and density = 3.29 gm/cc. A force f(t) is applied at the tip of the cantilever. The force f(t) is as shown in Figure P3.7 for which P = 1 μn, Q =.5 μn, a = 1 sec, b = 2 sec, d = 3 sec, and g = 5 sec. a. On the basis of an equivalent SDOF model, find the unit impulse response function for zero initial conditions. Assume the damping ratio =.1.

201 Responses of an SDOF Spring Mass Damper System 183 f (t) P Q a b d g t Figure P3.7 Forcing function for atomic force microscope problem b. Using the convolution integral, determine x(t) fort >. Assume that x() =.5 μm, and dx/dt =. Using MATLAB, plot x(t) versus t. P3.8 The base displacement y(t) in Figure P3.8a is described in Figure P3.8b. Using the convolution integral, determine the response x(t). x(t) Static equilibrium m eq x( ) y(t) y k eq x( ) = y(t) a b t (a) (b) Figure P3.8 (a) Base excitation problem (b) Specified base displacement P3.9 Consider an SDOF spring mass damper system subjected to the step forcing function (Figures and 2.1.2). Using the Laplace transformation technique, derive Equations , , and for underdamped, critically damped, and overdamped systems, respectively.

202 184 Vibration of Mechanical Systems P3.1 Solve the problem P3.7 using the Laplace transformation technique. P3.11 Solve the problem P3.8 using the Laplace transformation technique. P3.12 Consider the system shown in Figure P3.12, where a = 25 cm, l 1 = 5 cm, and l 2 = 3 cm. Here, k = 1, 1 N/m, m = 2 kg, and the damping ratio is.1. A y(t) a l 1 c k Massless and rigid bar l 2 x(t) m Figure P3.12 A massless bar with mass, spring, and damper a. Derive the transfer function x(s) y(s). b. Find poles and zeros of the transfer function. c. Construct Bode plots for the frequency response function. P3.13 The dynamics of pedestrian bridge interaction is given (Newland, 24) by Mÿ + Ky(t) + mαβ ÿ(t ) + 2ξ KMẏ = mβẍ(t) where M : Mass of the bridge, K : Stiffness of the bridge, ξ : damping ratio, Y : displacement of the pavement, x : movement of the center of mass caused by pedestrian walking, m : mass of the pedestrian, and : time lag. α and β are constants.

203 Responses of an SDOF Spring Mass Damper System 185 a. Derive the transfer function G(s) = y(s) x(s) b. Plot αg( jω) versus frequency ratio ω ω n where ω : input frequency, K ω n = M for ω =, π 2, π and π αβm. Assume that =.1 and 2 M ξ =.1.

204 4 VIBRATION OF TWO-DEGREE-OF- FREEDOM-SYSTEMS For a two-degree-of-freedom (2DOF) system, the number of independent second-order differential equations is two. With respect to a vector composed of the displacements associated with each degree of freedom, these two differential equations are represented as a single equation. In this vector equation, the coefficients of acceleration, velocity, and displacement vectors are defined as the mass matrix, the damping matrix, and the stiffness matrix, respectively. Next, the method to compute the natural frequencies and the modal vectors, also known as mode shapes, is presented. The number of natural frequencies equals the number of degrees of freedom, which is two. Unlike in a single-degree-of-freedom (SDOF) system, there is a mode shape associated with each natural frequency. Next, free and forced vibration of both undamped and damped 2DOF systems are analyzed. Using these techniques, vibration absorbers are designed next. A vibration absorber consists of a spring, a mass, and a damper, and is attached to an SDOF main system experiencing vibration problems. After the addition of a vibration absorber to an SDOF main system, the complete system has two degrees of freedom. Last, the response is represented as a linear combination of the modal vectors, and it is shown that the response of each mode of vibration is equivalent to the response of an SDOF system. 186

205 Vibration of Two-Degree-of-Freedom-Systems 187 x 1 ( t) x 2 ( t) k 1 k 2 m 1 m 2 k 3 c 1 c 2 f 1( t ) f 2 ( t ) Figure Two-degree-of-freedom system 4.1 MASS, STIFFNESS, AND DAMPING MATRICES Let x 1 (t) and x 2 (t) be the displacements (linear or angular) associated with two degrees of freedom. Then, displacement x(t), velocity ẋ(t), and acceleration ẍ(t) vectors are defined as follows: [ ] [ ] [ ] x 1 (t) ẋ 1 (t) ẍ 1 (t) x(t) = ; ẋ(t) = ; and ẍ(t) = (4.1.1a c) x 2 (t) ẋ 2 (t) ẍ 2 (t) Let f 1 (t) and f 2 (t) be the forces (or the torques) associated with each degree of freedom. Then, the force vector f(t) is described as follows: [ ] f 1 (t) f(t) = (4.1.2) f 2 (t) The dynamics of a 2DOF system is governed by a set of two coupled second-order differential equations, which is written in the matrix form as follows: Mẍ + Cẋ + Kx(t) = f(t) (4.1.3) where the matrices M, C, and K are known as mass, damping, and stiffness matrices, respectively. Example 4.1.1: Two Degree of Freedom System Consider the 2DOF system shown in Figure The free body diagram of each mass is shown in Figure

206 188 Vibration of Mechanical Systems c 1x& 1 k 1 x 1 m 1 k2( x1 x2 ) c 2( x& 1 x& 2 ) k2( x2 x1) c 2( x& 2 x& 1 ) f 1( t) f 2( t ) m 2 k 3 x 2 Figure Free body diagrams for system in Figure From the free body diagram of the mass m 1 in Figure 4.1.2, Newton s second law of motion yields f 1 (t) k 1 x 1 c 1 ẋ 1 k 2 (x 1 x 2 ) c 2 (ẋ 1 ẋ 2 ) = m 1 ẍ 1 (4.1.4) From the free body diagram of the mass m 2 in Figure 4.1.2, Newton s second law of motion yields f 2 (t) k 3 x 2 k 2 (x 2 x 1 ) c 2 (ẋ 2 ẋ 1 ) = m 2 ẍ 2 (4.1.5) Equations and are rearranged as follows: m 1 ẍ 1 + (c 1 + c 2 )ẋ 1 c 2 ẋ 2 + (k 1 + k 2 )x 1 k 2 x 2 = f 1 (t) (4.1.6) m 2 ẍ 2 c 2 ẋ 1 + c 2 ẋ 2 k 2 x 1 + (k 2 + k 3 )x 2 = f 2 (t) (4.1.7) Putting Equations in the matrix form, [ ][ ] [ ][ ] m 1 ẍ 1 c 1 + c 2 c 2 ẋ 1 + m 2 ẍ 2 c 2 c 2 ẋ 2 [ ][ ] [ ] k 1 + k 2 k 2 x 1 f 1 (t) + = k 2 k 2 + k 3 x 2 f 2 (t) (4.1.8) Equation has the form of Equation 4.1.3, Mẍ + Cẋ + Kx = f(t) (4.1.9) where [ ] [ ] x 1 (t) f 1 (t) x(t) = ; f(t) = x 2 (t) f 2 (t) (4.1.1)

207 Vibration of Two-Degree-of-Freedom-Systems 189 A B k l 1 k c l 1 k l l 1 cg l l 1 θ1 θ2 Figure Double pendulum [ ] [ ] m 1 c 1 + c 2 c 2 M =, C =, and K = m 2 c 2 c 2 [ k 1 + k 2 k 2 k 2 k 2 + k 3 ] (4.1.11) The matrices M, C, and K are mass, damping, and stiffness matrices, respectively. Example 4.1.2: Double Pendulum Consider a double pendulum as shown in Figure The mass and the mass-moment of inertia about the support point of each pendulum are m and Ɣ, with the center of mass located at a distance l from the support joint. Derive the differential equations of motion and obtain the mass and stiffness matrices. The free body diagram of each pendulum is shown in Figure 4.1.4, where R A and R B are the unknown reaction forces at the support joints A and B. Taking moments about the support joint A, kl 1 θ 1 l 1 k c (l 1 θ 1 l 1 θ 2 )l 1 mglθ 1 = Ɣ θ 1 (4.1.12) Taking moments about the support joint B, kl 1 θ 2 l 1 k c (l 1 θ 2 l 1 θ 1 )l 1 mglθ 2 = Ɣ θ 2 (4.1.13)

208 19 Vibration of Mechanical Systems R B R A kl 1 θ 1 θ 1 k c ( l1θ1 l1θ 2) k c cg θ 2 k c ( l1θ 2 l1θ1) kl 1 θ 2 cg mg mg Figure Free body diagram from each pendulum Equations and can be put in the matrix form as [ ][ ] [ ][ ] [ ] Ɣ θ 1 (k + k c )l mgl k cl 2 1 θ 1 Ɣ θ 2 k c l 2 1 (k + k c )l mgl = θ 2 (4.1.14) Equation yields mass (M) and stiffness (K) matrices as [ ] Ɣ M = (4.1.15) Ɣ and [ ] (k + k c )l 2 1 K = + mgl k cl 2 1 k c l 2 1 (k + k c )l mgl (4.1.16) Example 4.1.3: Combined Translational and Rotational Motion Consider a rigid bar (Figure 4.1.5) with the mass m and the massmoment of inertia I c about the center of mass C, which is located at distances l 1 and l 2 from left and right ends, respectively. Further, the bar is supported by springs with stiffnesses k 1 and k 2 at its left and right ends, respectively. Derive the differential equations of motion and obtain the mass and stiffness matrices. Note: This rigid bar and springs can be viewed as an automobile chassis and tires. To help with your imagination, dotted lines are drawn.

209 Vibration of Two-Degree-of-Freedom-Systems 191 l1 l2 x c θ k1 k2 C: Center of mass Figure A rigid bar supported by springs at both ends The free body diagram of a rigid bar is shown in Figure 4.1.6, where x is the displacement of the center of mass C and θ is the clockwise angular displacement of the bar. Summing all forces along x-directions, k 1 (x l 1 θ) k 2 (x + l 2 θ) = mẍ (4.1.17) Taking moments about the center of mass C, k 1 (x l 1 θ)l 1 k 2 (x + l 2 θ)l 2 = I c θ (4.1.18) Equations and are represented in the matrix form as [ ][ ] [ ][ ] [ ] m ẍ k 1 + k 2 (k 1 l 1 k 2 l 2 ) x + I c θ (k 1 l 1 k 2 l 2 ) k 1 l k = 2l 2 2 θ (4.1.19) Equation yields mass (M) and stiffness (K) matrices as [ ] m M = (4.1.2) I c x l 1 θ k1( x l1θ ) θ l1 l 2 c x k2( x + l 2θ ) x + l 2 θ Figure Free body diagram of rigid bar supported by springs

210 192 Vibration of Mechanical Systems and [ ] k 1 + k 2 (k 1 l 1 k 2 l 2 ) K = (k 1 l 1 k 2 l 2 ) k 1 l k 2l 2 2 (4.1.21) 4.2 NATURAL FREQUENCIES AND MODE SHAPES There is a mode shape or a modal vector associated with a natural frequency. The method to compute the natural frequencies and the mode shapes is as follows. Ignoring damping and external force terms, Equation can be written as Mẍ + Kx = (4.2.1) Let x(t) = [ A 1 A 2 ] sin(ωt + φ) (4.2.2) where amplitudes A 1, A 2, and the phase φ are to be determined. Differentiating Equation twice with respect to time, [ ] ẍ = ω 2 A 1 sin(ωt + φ) (4.2.3) A 2 Substituting Equations and into Equation 4.2.1, [ ] [ ] (K ω 2 A 1 M) = A 2 (4.2.4) For a nonzero or a nontrivial solution of A 1 and A 2, det(k ω 2 M) = (4.2.5) which will be a quadratic equation in ω 2. The solution of Equation yields two values (ω1 2 and ω2 2 )forω2 where ω 1 and ω 2 are the natural frequencies. In other words, there are two natural frequencies for a 2DOF system. Corresponding to each natural frequency, there is

211 Vibration of Two-Degree-of-Freedom-Systems 193 a mode shape which is obtained from Equation after substituting ω = ω 1 or ω = ω 2, that is, [ ] [ ] A 1,i P i = ; i = 1, 2 (4.2.6) A 2,i where [ ] P i = (K ωi 2 M) = p 11,i p 12,i ; i = 1, 2 (4.2.7) p 21,i p 22,i Substituting Equation into Equation 4.2.6, the following two linear equations are obtained: p 11,i A 1,i + p 12,i A 2,i = (4.2.8) p 21,i A 1,i + p 22,i A 2,i = (4.2.9) Because det P i =, the rank (Strang, 1988) of the matrix P i cannot be two, and both the rows of the matrix P i are not independent, or equivalently Equations and are essentially the same. In other words, there is only one equation in two unknowns A 1,i and A 2,i. Therefore, only the ratio of A 1,i and A 2,i can be found, for example, using Equation 4.2.8, A 2,i = p 11,i (4.2.1) A 1,i p 12,i One of the variables, that is, either A 1,i or A 2,i, can be chosen arbitrarily. For example, let A 1,i = 1. Then, A 2,i = p 11,i p 12,i (4.2.11) And the modal vector v i corresponding to the natural frequency ω i can be defined as v i = [ A 1,i A 2,i ] [ 1 = p 11,i p 12,i ] ; i = 1, 2 (4.2.12) This modal vector v i is also defined as the mode shape corresponding to the natural frequency ω = ω i.

212 194 Vibration of Mechanical Systems x 1( t ) x 2( t ) k 1 m k 2 m k 3 Figure Undamped spring mass system Example 4.2.1: Natural Frequencies and Mode Shapes of the System in Figure Consider the 2DOF system shown in Figure with the following parameters: c 1 = c 2 = and m 1 = m 2 = m. The resulting spring mass system is shown in Figure Determine the natural frequencies and the mode shapes for the two cases: k 1 = k 2 = k 3 = k, and k 1 = k 2 = k and k 3 = 2k. Case I: k 1 = k 2 = k 3 = k Equation yields the following mass and stiffness matrices: [ ] [ ] m 2k k M = and K = (4.2.13) m k 2k Then, K ω 2 M = [ 2k ω 2 m k k 2k ω 2 m ] (4.2.14) Therefore, from Equation , det(k ω 2 M) = (2k ω 2 m) 2 k 2 = (k ω 2 m)(3k ω 2 m) = (4.2.15) Solving Equation , the two natural frequencies are: ω 1 = k m and ω 2 = 3k m (4.2.16)

213 Vibration of Two-Degree-of-Freedom-Systems 195 Mode shape (modal vector) corresponding to the natural frequency ω 1 From Equation 4.2.7, [ ] [ ] P 1 = (K ω1 2 M) = p 11,1 p 12,1 k k = (4.2.17) p 21,1 p 22,1 k k Equation yields the following eigenvector: [ ] 1 v 1 = 1 (4.2.18) Mode shape (modal vector) corresponding to the natural frequency ω 2 From Equation 4.2.7, [ ] [ ] P 2 = (K ω2 2 M) = p 11,2 p 12,2 k k = (4.2.19) p 21,2 p 22,2 k k Equation yields the following eigenvector: [ ] 1 v 2 = 1 (4.2.2) Case II: k 1 = k 2 = k and k 3 = 2k Equation yields the following mass and stiffness matrices: [ ] [ ] m 2k k M = and K = (4.2.21) m k 3k Therefore, from Equation , [ K ω 2 2k ω 2 m M = k From Equation , k 3k ω 2 m ] (4.2.22) det(k ω 2 M) = (2k ω 2 m)(3k ω 2 m) k 2 = (4.2.23) or m 2 (ω 2 ) 2 5kmω 2 + 5k 2 = (4.2.24)

214 196 Vibration of Mechanical Systems Solving Equation , ω 2 = 5km ± km 5 (4.2.25) 2m 2 Hence, the two natural frequencies are: 1.382k 3.618k ω 1 = and ω 2 = (4.2.26) m m Mode shape (Modal vector) corresponding to the natural frequency ω 1 From Equation 4.2.7, [ ] [ ] P 1 = (K ω1 2 M) = p 11,1 p 12,1.618k k = (4.2.27) p 21,1 p 22,1 k 1.618k Equation yields the following eigenvector: [ ] 1 v 1 = (4.2.28).618 Mode shape (Modal vector) corresponding to the natural frequency ω 2 From Equation 4.2.7, [ ] [ ] P 2 = (K ω2 2 M) = p 11,2 p 12, k k = p 21,2 p 22,2 k.618k (4.2.29) Equation yields the following eigenvector: [ ] 1 v 2 = (4.2.3) Example 4.2.2: Natural Frequencies and Mode Shapes of a Double Pendulum Find the natural frequencies and the mode shapes of the double pendulum shown in Figure From Equations and , [ ] (K ω 2 α ω 2 Ɣ β M) = (4.2.31) β α ω 2 Ɣ

215 Vibration of Two-Degree-of-Freedom-Systems 197 where From Equation , α = (k + k c )l mgl and β = k cl 2 1 (4.2.32) det(k ω 2 M) = (α ω 2 Ɣ) 2 β 2 = (α + β ω 2 Ɣ)(α β ω 2 Ɣ) = (4.2.33) Solving Equation , the two natural frequencies are ω 2 1 = α β Ɣ and ω 2 2 = α + β Ɣ (4.2.34) Mode shape (Modal vector) corresponding to the natural frequency ω 1 From Equation 4.2.7, [ ] [ ] P 1 = (K ω1 2 M) = p 11,1 p 12,1 β β = (4.2.35) p 21,1 p 22,1 β β Equation yields the following modal vector: [ ] 1 v 1 = 1 (4.2.36) Mode shape (Modal vector) corresponding to the natural frequency ω 2 From Equation 4.2.7, [ ] [ ] P 2 = (K ω2 2 M) = p 11,2 p 12,2 β β = (4.2.37) p 21,2 p 22,2 β β Equation yields the following modal vector: [ ] 1 v 2 = 1 (4.2.38) Eigenvalue/Eigenvector Interpretation From Equation 4.2.4, K [ A 1 A 2 ] = ω 2 M [ A 1 A 2 ] (4.2.39)

216 198 Vibration of Mechanical Systems Pre-multiplying both sides of Equation by M 1, M 1 Kv = ω 2 v (4.2.4) where v = [ A 1 A 2 ] (4.2.41) Therefore, the square of the natural frequency ω 2 and the corresponding modal vector v are the eigenvalue and the eigenvector (Boyce and DiPrima, 25; Strang, 1988) of the matrix M 1 K, respectively. The eigenvalue/eigenvector interpretation is convenient from the computation point of view, as there are standard routines available, for example, the MATLAB routine eig. 4.3 FREE RESPONSE OF AN UNDAMPED 2DOF SYSTEM In general, if the initial conditions are arbitrarily chosen, the free vibration will contain both the natural frequencies ω 1 and ω 2.Onthe basis of the assumed solution (Equation 4.2.2) and the fact that the response can be expressed as a linear combination of the modal vectors, the general solution can be written as x(t) = α 1 v 1 sin(ω 1 t + φ 1 ) + α 2 v 2 sin(ω 2 t + φ 2 ) (4.3.1) where v 1 and v 2 are the modal vectors (or mode shapes) corresponding to the natural frequencies ω 1 and ω 2, respectively. Constants α 1, α 2, φ 1, and φ 2 can be determined from the initial conditions x() and ẋ(). From Equation 4.3.1, x() = v 1 α 1 sin φ 1 + v 2 α 2 sin φ 2 (4.3.2) or [ ] α 1 sin φ 1 x() = Q α 2 sin φ 2 (4.3.3)

217 Vibration of Two-Degree-of-Freedom-Systems 199 where Q is a 2 2 matrix defined as follows: [ ] Q = v 1 v 2 (4.3.4) From Equation 4.3.3, [ ] α 1 sin φ 1 = Q 1 x() (4.3.5) α 2 sin φ 2 Differentiating Equation with respect to time, ẋ(t) = α 1 v 1 ω 1 cos(ω 1 t + φ 1 ) + α 2 v 2 ω 2 cos(ω 2 t + φ 2 ) (4.3.6) Substituting t = into Equation 4.3.6, ẋ() = α 1 v 1 ω 1 cos φ 1 + α 2 v 2 ω 2 cos φ 2 (4.3.7) Using the definition of Q given in Equation 4.3.4, [ ] ω 1 α 1 cos φ 1 ẋ() = Q ω 2 α 2 cos φ 2 (4.3.8) Therefore, [ ] ω 1 α 1 cos φ 1 = Q 1 ẋ() (4.3.9) ω 2 α 2 cos φ 2 In summary, α 1 sin φ 1, α 2 sin φ 2, α 1 cos φ 1, and α 2 cos φ 2 are first obtained from Equations and Then, α 1, α 2, φ 1, and φ 2 are determined. As an example, let the initial conditions be as follows: x() = χv 1 and ẋ() = (4.3.1) where χ is a constant. Then, from Equations and 4.3.9, [ ] [ ] [ ] [ ] α 1 sin φ 1 χ α 1 cos φ 1 = and = α 2 sin φ 2 α 2 cos φ 2 (4.3.11)

218 2 Vibration of Mechanical Systems Therefore, α 1 = χ, φ 1 = π 2, and α 2 = (4.3.12) Hence, from Equations and , the response would be ( x(t) = χv 1 sin ω 1 t + π ) = χv 1 cos ω 1 t (4.3.13) 2 The response is purely sinusoidal with the frequency equal to the first natural frequency. This is due to the fact that the initial conditions match with the first modal vector and do not contain the second modal vector. If the initial conditions match the second modal vector, the free vibration will be purely sinusoidal with the frequency equal to the second natural frequency. Example 4.3.1: Free Vibration of a Double Pendulum Consider the double pendulum shown in Figure Numerical values of the parameters are as follows: m =.5kg, Ɣ =.6kg m 2, k = k c = 1, N/m, l =.8m,l 1 =.3 m. Initial conditions are given as: θ 1 () =.1rad, θ 2 () =.5 rad, θ 1 () = 1 rad/sec, θ 2 () = 15 rad/ sec. Determine the equation governing the free vibration. Solution From Equation , ω 1 = rad/sec and ω 2 = rad/sec From Equation 4.3.4, Therefore, [ ] 1 1 Q = 1 1 Q 1 = [ ]

219 Vibration of Two-Degree-of-Freedom-Systems 21 From Equation 4.3.5, [ ] α 1 sin φ 1 α 2 sin φ 2 From Equation 4.3.9, [ ] ω 1 α 1 cos φ 1 [ ] = Q 1.75 x() = rad.25 [ ] = Q ẋ() = rad/ sec ω 2 α 2 cos φ Considering α 1 sin φ 1 =.75 and ω 1 α 1 cos φ 1 = 2.5, α 1 =.989, φ 1 = rad Considering α 2 sin φ 2 =.25 and ω 2 α 2 cos φ 2 = 12.5, α 2 =.1879, φ 2 =.1335 rad From Equation 4.3.1, [ ] [ ] θ 1 (t) 1 =.989 sin(38.814t ) θ 2 (t) 1 [ ] sin(67.13t ) FORCED RESPONSE OF AN UNDAMPED 2DOF SYSTEM UNDER SINUSOIDAL EXCITATION The differential equations of motion of an undamped 2DOF system can be represented as [ M ẍ 1 ẍ 2 ] + K [ x 1 x 2 ] = [ f 1 f 2 ] sin ωt (4.4.1) where M and K are mass and stiffness matrices, respectively. The amplitudes of sinusoidal excitations are f 1 and f 2. Assume the forced response (particular integral) to be [ ] [ ] x 1 (t) A 1 = sin ωt (4.4.2) x 2 (t) A 2

220 22 Vibration of Mechanical Systems x 1( t ) x 2( t ) k m k m 2k f sinωt f sin ωt Figure Two-degree-of-freedom system under sinusoidal excitation Here, the amplitudes A 1 and A 2 are allowed to be negative. A negative value of the amplitude represents the phase angle equal to 18 degrees. Substituting Equation into Equation 4.4.1, and equating the coefficients of sin ωt on both sides, (K ω 2 M) [ A 1 A 2 ] = [ f 1 f 2 ] (4.4.3) From Equation 4.4.3, [ A 1 ] = (K ω 2 M) 1 [ f 1 A 2 f 2 ] (4.4.4) Example 4.4.1: Forced Response of a 2DOF System Consider the 2DOF system shown in Figure which is the same model as the one shown in Figure with the following parameters: c 1 = c 2 =, m 1 = m 2 = m, k 1 = k 2 = k, k 3 = 2k, and f 1 (t) = f 2 (t) = f sin ωt. With respect to Equation 4.4.1, f 1 = f 2 = f (4.4.5) [ ] m M = m [ 2k and K = k ] k 3k (4.4.6)

221 Vibration of Two-Degree-of-Freedom-Systems 23 Therefore, from Equation 4.4.4, [ ] [ A 1 2k ω 2 m = k A 2 k 3k ω 2 m ] 1 [ f f ] (4.4.7) Equation yields [ ] [ ] A 1 f 4k ω 2 m = (2k ω 2 m)(3k ω 2 m) k 2 3k ω 2 m A 2 (4.4.8) After some algebra, the amplitudes of masses are found to be and where ω z1 = A 1 f /k =.8[1 (ω/ω z1 ) 2 ] [1 (ω/ω 1 ) 2 ][1 (ω/ω 2 ) 2 ] A 2 f /k =.6[1 (ω/ω z2 ) 2 ] [1 (ω/ω 1 ) 2 ][1 (ω/ω 2 ) 2 ] (4.4.9) (4.4.1) 4k 3k 1.382k 3.618k m,ω z 2 = m,ω 1 = m, and ω 2 = m (4.4.11) It should be noted that ω 1 and ω 2 are the natural frequencies (Equation ). When ω = ω 1 or ω = ω 2, the amplitudes A 1 and A 2 are infinite (Figures and 4.4.3). Furthermore, A 1 = atω = ω z1 and A 2 = atω = ω z FREE VIBRATION OF A DAMPED 2DOF SYSTEM The free vibration of a damped 2DOF system is governed by the following differential equations: Mẍ + Cẋ + Kx = (4.5.1) where M, C, and K are mass, damping, and stiffness matrices, respectively. Here, the initial displacement x() and/or velocity ẋ() will be nonzero.

222 A 1 k/f ω = ω z1 Phase = 18 5 ω = ω ω/ω 1 Figure Amplitude of mass with displacement equal to x 1 in Figure ω = ω z2 A 2 k/f ω = ω ω/ω 1 Figure Amplitude of mass with displacement equal to x 2 in Figure 4.4.1

223 Vibration of Two-Degree-of-Freedom-Systems 25 The solution can be written as [ x(t) = A 1 A 2 ] e st (4.5.2) where s is to be determined. Substituting Equation into Equation 4.5.1, (Ms 2 + Cs + K) [ A 1 A 2 ] [ ] = (4.5.3) For a nontrivial solution of Equation 4.5.3, det(ms 2 + Cs + K) = (4.5.4) When C =, det(ms 2 + K) = (4.5.5) It can be shown that the roots of this equation are purely imaginary. In fact, by substituting s =±jω, Equation reduces to det( Mω 2 + K) = (4.5.6) which is the same as Equation In the case of C, the roots of Equation can be obtained via the eigenvalue/eigenvector formulation as follows: Define y 1 = x(t) and y 2 = ẋ(t) (4.5.7) Then, ẏ 1 = y 2 (4.5.8) and from Equation 4.5.1, ẏ 2 = ẍ = M 1 Kx(t) M 1 Cẋ(t) = M 1 Ky 1 (t) M 1 Cy 2 (t) (4.5.9)

224 26 Vibration of Mechanical Systems Equations and can be written in the matrix form as ẏ(t) = Ry(t) (4.5.1) where and [ ] y 1 (t) y(t) = y 2 (t) [ ] I R = M 1 K M 1 C (4.5.11) (4.5.12) It can be shown that the eigenvalues of the matrix R are the same as the roots of Equation These eigenvalues can be complex or real. A set of complex conjugate eigenvalues ( σ ± jω) correspond to an underdamped SDOF system and can be written as σ ± jω = ξω n ± ω n 1 ξ 2 j (4.5.13) where ξ and ω n are the damping ratio and the undamped natural frequency respectively. Therefore, ω n = σ 2 + ω 2 (4.5.14) and ξ = σ σ 2 + ω 2 (4.5.15) The undamped natural frequency (Equation ) will be one of the roots of det(k ω 2 M). The solution of Equation can be described as where e Rt is the matrix exponential described as y(t) = e Rt y() (4.5.16) e Rt = I + Rt + R 2 t2 t3 + R3 + (4.5.17) 2! 3!

225 Vibration of Two-Degree-of-Freedom-Systems 27 and R 2 = RR, R 3 = R 2 R,... (4.5.18) In MATLAB, the matrix exponential can be calculated by using the command expm. It should also be noted that [ ] [ ] y 1 () x() y() = = (4.5.19) y 2 () ẋ() That is, y() is composed of the initial displacement x() and the velocity ẋ() vectors. Example 4.5.1: Free Vibration of a Damped 2DOF Freedom System Consider the spring mass damper model in Figure with f 1 (t) = f 2 (t) =. Let m 1 = m 2 =.5kg, k 1 = k 2 = k 3 = 1, N/m, c 1 = 1 N sec/m, and c 2 =. Obtain x 1 (t) and x 2 (t) when the initial conditions are as follows: x 1 () = 1.5, x 2 () =.5, ẋ 1 () =, and ẋ 2 () =. From Equation 4.1.8, [ ] m 1 M = = m 2 [.5.5 [ ] [ ] c 1 + c 2 c 2 1 C = = c 2 c 2 ] and [ ] 2, 1, K = 1, 2, The eigenvalues of the matrix R (Equation ) are presented in Table 4.1. The response is obtained from the MATLAB program and is presented in Figure

226 28 Vibration of Mechanical Systems Table 4.1 Eigenvalues of matrix R Characteristic roots ( σ ± jω) ξ ω n (rad/sec) 5.1 ± j ± i MATLAB Program 4.5.1: Free Vibration of a Damped 2DOF System % clear all close all % M=[.5 ;.5]; C=[1 ; ]; K=[2 1;-1 2]; % MI=inv(M); % R=[zeros(2,2) eye(2);-mi K -MI C] % ada=eig(r); ogn1=abs(ada(1)); xi1=-real(ada(1))/ogn1; ogn2=abs(ada(3)); xi2=-real(ada(3))/ogn2; % %Initial conditions y=[1.5 ] ; % T=2 pi/ogn1; delt=t/4; t=-delt;

227 Vibration of Two-Degree-of-Freedom-Systems 29 1 x 1 (t) x 2 (t) time (sec) Figure Free vibration of a 2DOF system for i=1:1 t=t+delt; tv(i)=t; y(:,i)=expm(r t) y; end plot(tv,y(1,:),tv,y(2,:), -- ) xlabel( time(sec.) ) legend( x_1(t), x_2(t) ) grid 4.6 STEADY-STATE RESPONSE OF A DAMPED 2DOF SYSTEM UNDER SINUSOIDAL EXCITATION The vibration of a damped 2DOF system is governed by the differential Equation that is rewritten here: Mẍ + Cẋ + Kx = f(t) (4.6.1)

228 21 Vibration of Mechanical Systems where the external excitation vector f(t) is written as [ ] f 1 (t) f(t) = f 2 (t) (4.6.2) It is assumed that both f 1 (t) and f 2 (t) are sinusoidal with the same frequency. Typically, or f 1 = ˆ F 1 sin(ωt + ψ 1 ) and f 2 = ˆ F 2 sin(ωt + ψ 2 ) (4.6.3) f 1 = ˆ F 1 cos(ωt + ψ 1 ) and f 2 = ˆ F 2 cos(ωt + ψ 2 ) (4.6.4) To facilitate the analysis, it will be assumed for any of these two cases, [ ] [ ] f 1 (t) F 1 f(t) = = e jωt ; j = 1 (4.6.5) f 2 (t) F 2 where [ F 1 F 2 ] [ ˆ = F 1 e jψ 1 Fˆ 2 e jψ 2 ] (4.6.6) The steady-state solution of Equation is assumed to be [ ] [ ] x 1 (t) X 1 = e jωt (4.6.7) x 2 (t) X 2 Therefore, and [ ] [ ẋ 1 (t) ẋ(t) = = ẋ 2 (t) [ ] [ ẍ 1 (t) ẍ(t) = = ẍ 2 (t) X 1 X 2 X 1 X 2 ] ] jωe jωt (4.6.8) ( ω 2 )e jωt (4.6.9) Substituting Equations 4.6.5, into Equation 4.6.1, [ ] [ ] [(K ω 2 M) + jcω] X 1 X 2 e jωt = F 1 F 2 e jωt (4.6.1)

229 Vibration of Two-Degree-of-Freedom-Systems 211 Equating the coefficients of e jωt on both sides, [ ] [ [(K ω 2 M) + jcω] X 1 X 2 = F 1 F 2 ] (4.6.11) Therefore, [ X 1 ] = [(K ω 2 M) + jcω] 1 [ F 1 X 2 F 2 ] (4.6.12) Corresponding to Equation 4.6.3, [ ] [ ] x 1 (t) A 1 sin(ωt + φ 1 ) = x 2 (t) A 2 sin(ωt + φ 2 ) (4.6.13) where A 1 = X 1, A 2 = X 2,φ 1 = X 1, and φ 2 = X 2 (4.6.14) If excitations were in the form of Equation 4.6.4, [ ] [ ] x 1 (t) A 1 cos(ωt + φ 1 ) = x 2 (t) A 2 cos(ωt + φ 2 ) (4.6.15) where A 1, A 2, φ 1, and φ 2 are given by Equation Example 4.6.1: Steady-State Response of a Damped 2DOF System Find the steady-state response of system in Figure for which f 1 (t) = 1 sin(24t) N and f 2 (t) = 1 cos(24t) N. As in Example 4.5.1, let m 1 = m 2 =.5kg, k 1 = k 2 = k 3 = 1, N/m, c 1 = 1 N sec/m, and c 2 =. Using K, M, and C in Example 4.5.1, [ ] (K ω 2 M) + jωc = j Therefore, [ ] [(K ω 2 M) + jωc] 1 = j j j j.2513

230 212 Vibration of Mechanical Systems With respect to Equation 4.6.3, ˆ F 1 = 1, ψ 1 =, ˆ F 2 = 1, and ψ 2 = π 2 Therefore, [ F 1 F 2 ] [ ˆ = F 1 e jψ 1 Fˆ 2 e jψ 2 ] [ ] 1 = j1 From Equation , [ ] [ = [(K ω 2 M) + jcω] 1 X 1 F 1 X 2 F 2 ] [ ] j.217 =.275 j.133 Next, A 1 = X 1 =.325, A 2 = X 2 =.35, φ 1 = X 1 =.736 rad, and φ 2 = X 2 = rad Last, Equation yields the following steady-state response: [ ] [ ] x 1 (t).325 sin(24t +.736) = m x 2 (t).35 sin(24t ) 4.7 VIBRATION ABSORBER In this section, design techniques for two types (undamped and damped) of vibration absorbers are presented Undamped Vibration Absorber Consider an undamped spring mass system with the stiffness k 1 and the mass m 1 subjected to a sinusoidal excitation f sin ωt (Figure 4.7.1). When ω = k 1 /m 1, there will be resonance and a large amount of vibration. In many applications, it is not possible to add damping, or to change the natural frequency, or the excitation frequency. In these situations, another spring mass system with the stiffness k 2 and the

231 Vibration of Two-Degree-of-Freedom-Systems 213 k 1 f sin ωt x 1 m 1 Static equilibrium Vibration absorber k 2 x 2 m 2 Static equilibrium Figure An undamped vibration absorber mass m 2 is added to suppress this vibration. The spring mass system with the stiffness k 2 and the mass m 2 is called the undamped vibration absorber. The objectives are to select the absorber stiffness k 2 and the absorber mass m 2 for the suppression of vibration of the main mass m 1 in a reliable manner. Applying Newton s second law of motion to free body diagrams in Figure 4.7.2, f sin ωt k 1 x 1 k 2 (x 1 x 2 ) = m 1 ẍ 1 (4.7.1) k 2 (x 2 x 1 ) = m 2 ẍ 2 (4.7.2) Rearranging the differential equations of motion (Equations and 4.7.2), m 1 ẍ 1 + (k 1 + k 2 )x 1 k 2 x 2 = f sin ωt (4.7.3) m 2 ẍ 2 k 2 x 1 + k 2 x 2 = (4.7.4)

232 214 Vibration of Mechanical Systems f sin ωt k 1 x 1 m 1 k2( x2 x1 ) k2( x1 x2 ) m 2 Figure Free body diagrams of masses in Figure Therefore, the differential equations of motion in matrix form can be written as [ ] [ ] [ ] ẍ 1 x 1 f M + K = sin ωt (4.7.5) ẍ 2 where [ ] m 1 M = m 2 and [ ] k 1 + k 2 k 2 K = k 2 k 2 From Equations and 4.7.7, [ ] K ω 2 k 1 + k 2 ω 2 m 1 k 2 M = k 2 k 2 ω 2 m 2 From Equation 4.7.8, [ ] (K ω 2 M) 1 = 1 k 2 ω 2 m 2 +k 2 +k 2 k 1 + k 2 ω 2 m 1 where x 2 (4.7.6) (4.7.7) (4.7.8) (4.7.9) = det(k ω 2 M) = (k 1 + k 2 ω 2 m 1 )(k 2 ω 2 m 2 ) k 2 2 (4.7.1) From Equation 4.4.4, [ ] [ ][ ] A 1 = 1 k 2 ω 2 m 2 k 2 f A 2 k 2 k 1 + k 2 ω 2 m 1 [ ] = 1 (k 2 ω 2 m 2 )f k 2 f (4.7.11)

233 Vibration of Two-Degree-of-Freedom-Systems 215 where A 1 and A 2 are the amplitudes of x 1 and x 2, respectively. From Equations and 4.7.1, and (k 2 ω 2 m 2 )f A 1 = (k 1 + k 2 ω 2 m 1 )(k 2 ω 2 m 2 ) k2 2 (4.7.12) k 2 f A 2 = (4.7.13) (k 1 + k 2 ω 2 m 1 )(k 2 ω 2 m 2 ) k2 2 The amplitude A 1 of the main mass m 1 will be zero when the absorber parameters k 2 and m 2 are chosen such that Define and k 2 m 2 = ω 2 (4.7.14) k 1 m 1 = ω 11 (4.7.15) k 2 m 2 = ω 22 (4.7.16) The condition in Equation can then be expressed as ω = ω 22 (4.7.17) In other words, whenever ω 22 is chosen such that it equals the excitation frequency ω, A 1 =, that is, the vibration of the main mass will be completely suppressed. The parameter ω 22 is designed such that the vibration of the main mass will be suppressed at the resonance condition of the original SDOF system, that is, ω = ω 11. As a result, the design condition is ω 22 = ω 11 (4.7.18) From the definitions in Equations and , the condition in Equation is also expressed as k 2 m 2 = k 1 m 1 (4.7.19)

234 216 Vibration of Mechanical Systems In summary, when k 2 and m 2 are selected to satisfy Equation , A 1 = when ω = ω 11. And the amplitude of the absorber mass at this condition ω = ω 11 = ω 22 will be Define the mass ratio μ as A 2 = f k 2 (4.7.2) μ = m 2 (4.7.21) m 1 Then, from Equation , k 2 = m 2 = μ (4.7.22) k 1 m 1 Using Equation , Equations and are represented as follows: A 1 k 1 (1 r 2 ) = (4.7.23) f (1 + μ r 2 )(1 r 2 ) μ and A 2 k 1 1 = f (1 + μ r 2 )(1 r 2 ) μ (4.7.24) where r = ω = ω (4.7.25) ω 11 ω 22 In Figure 4.7.3, the magnitude of the amplitude A 1 is plotted as a function of ω with the condition in Equation It should be noted that A 1 when ω ω 11. In fact, the amplitude A 1 is infinite when ω equals one of the natural frequencies of the 2DOF system, which are the roots of Equation as follows: = det(k ω 2 M) = (k 1 + k 2 ω 2 m 1 )(k 2 ω 2 m 2 ) k2 2 = (4.7.26) In Figure 4.7.4, the magnitude of the amplitude A 2 is plotted as a function of ω with the condition in Equation Dividing

235 Vibration of Two-Degree-of-Freedom-Systems ω 11 = ω 22 la 1 l k 1 /f r 1 r ω/ω 11 Figure Amplitude of the main mass versus excitation frequencies 25 2 ω 11 = ω 22 la 2 l k 1 /f r 1 r ω/ω 11 Figure Amplitude of the absorber mass versus excitation frequencies

236 218 Vibration of Mechanical Systems Equation by k 1 k 2, ( 1 + k 2 ω 2 m )( 1 1 ω 2 m ) 2 k 2 = (4.7.27) k 1 k 1 k 2 k 1 Using Equations and , (1 + μ r 2 )(1 r 2 ) μ = (4.7.28) After some algebra, Equation yields (r 2 ) 2 (2 + μ)r = (4.7.29) The roots of the quadratic Equation are as follows: where r 2 1 = (2 + μ) μ 2 + 4μ 2 r 2 2 = (2 + μ) + μ 2 + 4μ 2 (4.7.3) (4.7.31) r 1 = ω 1 ω 11 and r 2 = ω 2 ω 11 (4.7.32a, b) Note that ω 1 and ω 2 are the natural frequencies of the 2DOF system. These natural frequencies are plotted as a function of the mass ratio μ in Figure Example 4.7.1: An Undamped Vibration Absorber When a fan with 1, kg mass operates at a speed of 2,4 rpm on the roof of a room (Figure 4.7.6), there is a large amount of vibration. Design an undamped vibration absorber, which can satisfactorily perform even when there is about 1% fluctuation in the angular speed of the fan. Solution ω 11 = ω = 2, 4 2π 6 = 8π rad/sec

237 Vibration of Two-Degree-of-Freedom-Systems ω 11 = ω 22 ω 1 /ω 11 and ω 2 /ω r 2 = ω 2 /ω 11 r 1 = ω 1 /ω Mass ratio (μ) Figure Natural frequencies versus mass ratio To ensure a good margin of safety, let us keep the natural frequencies of the 2DOF system to be at least 2% away from ω 11 (Figure 4.7.5). In this case, r =.8 in the following equation: (r 2 ) 2 (2 + μ)r = or μ = r r 2 2 = (.8)4 + 1 (.8) 2 2 =.225 ω k 2 Vibration absorber m 2 Figure Undamped vibration absorber attached to the ceiling of room

238 22 Vibration of Mechanical Systems It should be noted that r =.8 has been chosen because the lower natural frequencies in Figure are closer to the r = 1 line. Therefore, and Last, μ = m 2 m 1 =.225 m 2 = 22.5kg k 2 m 2 = ω 2 11 = (8π)2 k 2 = m 2 (8π) 2 = 6, 4π = N/m Damped Vibration Absorber An undamped vibration absorber yields complete suppression of vibration at the design frequency of excitation, which is generally selected to be the resonance condition of the original system. However, if the excitation frequency is a variable, unbounded response is obtained when the excitation frequency matches either of the two natural frequencies of the 2DOF system. In order to alleviate this situation, a damper is included in the absorber design as shown in Figure 4.7.7, and the objective is to choose damper parameters such that the vibration of the main mass is minimized over all excitation frequencies. When the damping constant c =, the amplitude of the main mass can be unbounded. Similarly, when the damping constant is extremely large, both masses will become as if they are welded together, and the system will effectively become an SDOF system, that is, the response can again be unbounded. In other words, neither a small c nor an extremely large c will be desirable. There exists an optimal value of the damping constant c for which the vibration of the main mass is minimized over all excitation frequencies. From the free body diagram in Figure 4.7.7, f sin ωt k 1 x 1 k 2 (x 1 x 2 ) c(ẋ 1 ẋ 2 ) = m 1 ẍ 1 (4.7.33) k 2 (x 2 x 1 ) c(ẋ 2 ẋ 1 ) = m 2 ẍ 2 (4.7.34)

239 Vibration of Two-Degree-of-Freedom-Systems 221 k 1 f sin ωt k 1 x 1 f sin ωt m 1 Damped vibration absorber c m 1 k 2 x 1 Static equilibrium c( x 1 x 2 ) c( x 2 x 1 ) k2( x1 x2 ) k2( x2 x1 ) m 2 x 2 m 2 Static equilibrium Figure Damped vibration absorber and associated free body diagrams or m 1 ẍ 1 + cẋ 1 cẋ 2 + (k 1 + k 2 )x 1 k 2 x 2 = f sin ωt (4.7.35) m 2 ẍ 2 cẋ 1 + cẋ 2 k 2 x 1 + k 2 x 2 = (4.7.36) Equations are put in the matrix form as Mẍ + Cẋ + Kx(t) = f(t) (4.7.37) where [ ] x 1 (t) x(t) = ; f(t) = x 2 (t) [ ] f sin ωt (4.7.38) [ ] [ m 1 c M =, C = m 2 c ] [ ] c k 1 + k 2 k 2, and K = c k 2 k 2 (4.7.39)

240 222 Vibration of Mechanical Systems The steady-state response is computed by following the procedure outlined in Section 4.6. To use Equation , [ ] (K ω 2 k 1 + k 2 ω 2 m 1 + jωc k 2 jωc M) + jcω = k 2 jωc k 2 ω 2 m 2 + jωc Therefore, [(K ω 2 M) + jωc] 1 = 1 where [ k 2 ω 2 m 2 + jωc k 2 + jωc k 2 + jωc (4.7.4) k 1 + k 2 ω 2 m 1 + jωc = det(k ω 2 M + jωc) = (k 1 + k 2 ω 2 m 1 + jωc)(k 2 ω 2 m 2 + jωc) (k 2 + jωc) 2 After some algebra, ] (4.7.41) (4.7.42) = [(k 1 ω 2 m 1 )(k 2 ω 2 m 2 ) ω 2 k 2 m 2 ] + jωc(k 1 ω 2 (m 1 + m 2 )) (4.7.43) Furthermore [ ] [ ] F 1 f = (4.7.44) F 2 From Equation , [ ] [ ] X 1 = 1 (k 2 ω 2 m 2 + jωc)f X 2 (k 2 + jωc)f The amplitude of the main mass is given by (k 2 ω 2 m 2 + jωc)f A 1 = X 1 = (4.7.45) (4.7.46)

241 Vibration of Two-Degree-of-Freedom-Systems 223 Using Equation , A 1 = or f (k2 ω 2 m 2 ) 2 + (cω) 2 [(k1 ω 2 m 1 )(k 2 ω 2 m 2 ) ω 2 k 2 m 2 ] 2 + [ωc(k 1 ω 2 (m 1 + m 2 )] 2 A 1 k 1 f = [ (k1 ω 2 m 1 )(k 2 ω 2 m 2 ) k 2 1 ( ) 2 ( k2 k 1 ω2 m 2 k 1 + ω2 k 2 m 2 k 2 1 ) 2 cω k 1 (4.7.47) ] 2 [ ] (4.7.48) 2 + ωc(k1 ω 2 (m 1 +m 2 )) Define the following nondimensional variables: μ = m 2 k 1 k 2,ω 11 =,ω 22 =, f = ω 22 (4.7.49) m 1 m 1 m 2 ω 11 k 2 1 Then, g = ω c and ξ = (4.7.5) ω 11 2m 2 ω 11 k 2 k 1 = k 2 m 2 m 1 k 1 m 2 m 1 = ( ω22 ω 11 ) 2 μ = f 2 μ (4.7.51) ω 2 m 2 k 1 = ω2 m 1 k 1 m 2 m 1 = g 2 μ (4.7.52) cω = 2cω ω 11 m 2 = 2ξ ω = 2ξgμ (4.7.53) k 1 2m 2 ω 11 k 1 ω 11 (ωc)(k 1 ω 2 ( ) ( (m 1 + m 2 )) ωc = 1 ω2 k 2 1 (k 1 ω 2 m 1 )(k 2 ω 2 m 2 ) k 2 1 k 1 ω 2 11 ) ω2 μ ω11 2 = (2ξgμ)(1 g 2 μg 2 ) (4.7.54) ( ) = (1 g 2 k2 ) ω2 m 2 k 1 k 1 = (1 g 2 )(f 2 g 2 )μ (4.7.55)

242 224 Vibration of Mechanical Systems ω 2 k 2 m 2 k 2 1 = ω2 k 2 k 1 m 1 k 1 m 2 m 1 = g 2 f 2 μ 2 (4.7.56) Substituting Equations into Equation , A 1 k 1 f = (f 2 g 2 ) 2 + (2ξg) 2 [(1 g2 )(f 2 g 2 ) g 2 f 2 μ] 2 + (2ξg) 2 (1 g 2 μg 2 ) 2 (4.7.57) Den Hartog (1956) has obtained optimal absorber parameters to minimize the following function: I = max g A 1(g) (4.7.58) Here, A 1 (g) is a representation of the fact that A 1 is a function of frequency ratio g. The objective function I is the maximum value of A 1 with respect to the variation in g. Case I: Tuned Case (f = 1 or ω 22 = ω 11 ) For the minimum value of I, the optimal value of ξ is ξ 2 = μ(μ + 3)(1 + μ/(μ + 2) 8(1 + μ) (4.7.59) And the objective function I is I = max g A 1(g) = f k 1 1 ( μ + (1 + μ) μ/(μ + 2)) (4.7.6) The amplitude of this main mass due to this optimal absorber is shown in Figure Case II: No restriction on f (Absorber not tuned to main system) For the minimum value of I, f = 1 (4.7.61) 1 + μ

243 Vibration of Two-Degree-of-Freedom-Systems 225 A 1 k 1 /t μ =.1 f = 1 ξ =.1 ξ = ξ opt ξ =.3 ξ opt = g = ω/ω 11 Figure Amplitude of the main mass for various damping ratios (Absorber tuned to main system, f = 1) and The objective function I is ξ 2 = 3μ 8(1 + μ) 3 (4.7.62) I = max g A 1(g) = f 1 + 2/μ (4.7.63) k 1 The amplitude of this main mass due to this optimal absorber is shown in Figure Example 4.7.2: A Damped Vibration Absorber When a fan with 1, kg mass operates at a speed of 2,4 rpm on the roof of a room (Figure 4.7.1), there is a large amount of vibration. Design an optimally damped vibration absorber with the mass ratio equal to.225, which is same as that in Example

244 226 Vibration of Mechanical Systems A 1 k 1 /t μ =.1 f = 1/(1+ μ) ξ =.1 ξ = ξ opt ξ =.3 ξ opt = g = ω/ω 11 Figure Amplitude of the main mass for various damping ratios (Absorber not tunedtomainsystem) and From Equation (4.7.61), f = μ = =.8316 k 2 = f 2 ω11 2 = ( π)2 m 2 ω Damped vibration absorber c m 2 k 2 Figure Damped vibration absorber attached to the ceiling of room

245 Vibration of Two-Degree-of-Freedom-Systems 227 Therefore, k 2 = m 2 (f 8π) 2 = 6, 4π 2 f = N/m From Equation (4.7.62), 3μ ξ = 8(1 + μ) =.29 3 c = 2ξm 2 ω 11 = N-sec/m 4.8 MODAL DECOMPOSITION OF RESPONSE The general differential Equation is rewritten Mẍ + Cẋ + Kx = f(t) (4.8.1) Pre-multiplying both sides of Equation by M 1, ẍ + M 1 Cẋ + M 1 Kx = M 1 f(t) (4.8.2) In general, the response x(t) is a linear combination of the modal vectors v 1 and v 2, that is, x(t) = v 1 y 1 (t) + v 2 y 2 (t) (4.8.3) where y 1 (t) and y 2 (t) are the coefficients of modal vectors v 1 and v 2, respectively. Equation can be represented in a compact form as follows: x(t) = Vy(t) (4.8.4) where V = [ v 1 v 2 ] (4.8.5) and [ ] y 1 (t) y(t) = y 2 (t) (4.8.6)

246 228 Vibration of Mechanical Systems Substituting Equation into Equation and pre-multiplying by V 1, ÿ + C d ẏ + K d y = f v (t) (4.8.7) where C d = V 1 M 1 CV, K d = V 1 M 1 KV, and f v (t) = V 1 M 1 f(t) (4.8.8) From Equation and Equation 4.2.4, K d = V 1 M 1 KV = = [ ω1 2 ω2 2 ] (4.8.9) Let [ ] f v1 (t) f v (t) = f v2 (t) (4.8.1) Case I: Undamped System (C = ) Equations and yield ÿ 1 + ω 2 1 y 1 = f v1 (t) (4.8.11) ÿ 2 + ω 2 2 y 2 = f v2 (t) (4.8.12) Equations and can be solved by techniques developed for an SDOF system. After that, the response can be obtained from Equation Case II: Damped System (C ) For a general damping matrix, the matrix C d will not be diagonal, and the differential equations for y 1 (t) and y 2 (t) will be coupled. However, the modal equations are decoupled for special cases, for example, C = αm + βk (4.8.13)

247 Vibration of Two-Degree-of-Freedom-Systems 229 x 1( t ) x 2( t ) k m k m k f sin ωt 1 f sin ωt 2 Figure Two-degree-of-freedom system with sinusoidal excitation and modal damping where α and β are constants. This form of damping is known as Proportional or Rayleigh damping. In this case, from Equation 4.8.8, C d = V 1 M 1 CV = αi + β (4.8.14) Equation yields ÿ 1 + (α + βω1 2 )ẏ 1 + ω1 2 y 1 = f v1 (t) (4.8.15) ÿ 2 + (α + βω 2 2 )ẏ 2 + ω 2 2 y 2 = f v2 (t) (4.8.16) Equations and can be solved by techniques developed for an SDOF system. After that, the response can be obtained from Equation On the basis of Equations and , the modal damping in each mode ξ i ; i = 1, 2, can be described as ξ i = α + βω2 i 2ω i ; i = 1, 2 (4.8.17) Example 4.8.1: Application of Modal Decomposition Consider the system shown in Figure and assume that the damping ratios in modes 1 and 2 are.5 and.1 respectively. Determine the steady-state response via modal decomposition. Following parameters are provided: m = 2kg,k = 1, 8 N/m, f 1 = 1 N, f 2 = 2 N, and ω = 45 rad/sec.

248 23 Vibration of Mechanical Systems Solution From the Example (Case I), [ ] [ ] V = ; M = k ω 1 = m = 3 rad/sec and 3k ω 2 = = rad/sec m Note that ξ 1 =.5, r 1 = ω = 45 ω 1 3 = 1.5 ξ 2 =.1, r 2 = ω ω 2 = =.866 [ ] 1 f(t) = sin 45t 2 From Equation 4.8.8, [ ] [ ] f v (t) = V 1 M 1 f v1 (t) 7.5 f(t) = = sin ωt f v2 (t) 2.5 In steady state, the responses of Equations and are given as y 1 (t) = Y 1 sin(45t + φ 1 ) y 2 (t) = Y 2 sin(45t + φ 2 ) where Y 1 = 7.5 ω1 2[(1 r =.66, 1 2)2 + (2ξ 1 r 1 ) 2 ].5 φ 1 = tan 1 2ξ 1 r 1 = 3.22 rad (1 r1 2 ) 2.5 Y 2 = ω2 2[(1 r =.3, 2 2)2 + (2ξ 2 r 2 ) 2 ].5 φ 2 = tan 1 2ξ 2 r 2 =.658 rad (1 r2 2 )

249 Vibration of Two-Degree-of-Freedom-Systems 231 Last, from Equation 4.8.4, x 1 (t) = y 1 (t) + y 2 (t) x 2 (t) = y 1 (t) y 2 (t) EXERCISE PROBLEMS P4.1 Consider the system in Figure P4.1. x 1 ( t) x 2 ( t) k m k c m k Figure P4.1 Undamped 2DOF system with coupling stiffness k c a. Derive the differential equations of motion and obtain the mass and stiffness matrices. b. Calculate the natural frequencies and the mode shapes. c. Find the initial conditions such that the free vibration is sinusoidal with each natural frequency. P4.2 Consider the system in Figure P4.2. Massless and rigid bar 2m 3m k 2k Figure P4.2 Massless rigid bar with masses lumped at ends and supported on springs

250 232 Vibration of Mechanical Systems a. Derive the differential equations of motion and obtain the mass and stiffness matrices. b. Calculate the natural frequencies and the mode shapes. c. Find the initial conditions such that the free vibration is sinusoidal with each natural frequency. P4.3 Consider the system in Figure P4.3. A l 2 l k Rigid bar mass = m k m Figure P4.3 A rigid bar connected to another mass via a spring a. Derive the differential equations of motion and obtain the mass and stiffness matrices. b. Calculate the natural frequencies and the mode shapes. c. Find the initial conditions such that the free vibration is sinusoidal with each natural frequency. P4.4 Consider the system in Figure P4.4. Box with mass 2m k Cylinder with mass m k Figure P4.4 A cylinder inside a box with spring connections

251 Vibration of Two-Degree-of-Freedom-Systems 233 a. Derive the differential equations of motion and obtain the mass and stiffness matrices. b. Calculate the natural frequencies and the mode shapes. c. Find the initial conditions such that the free vibration is sinusoidal with each natural frequency. P4.5 Obtain and plot free vibration response for the system shown in Figure P4.1 when m = 1kg,k = 53 N/m, k c = 13 N/m. Assume that x 1 () =.1 m, ẋ 1 () = 1m/sec,x 2 () =, and ẋ 2 () =. P4.6 Consider the system in Figure P4.6. Massless and rigid bar c x 1 x 2 2m 3m k 2k Figure P4.6 Massless rigid bar with masses lumped at ends and supported by springs and a damper a. Derive the differential equations of motion and obtain mass, stiffness, and damping matrices. b. Assume that m = 11 kg, k = 4, 511 N/m, and c = 2 N sec/m. Determine the damping ratio and the undamped natural frequency for each mode. c. Obtain and plot response when x 1 () =.1 m, ẋ 1 () = 1m/sec, x 2 () =.2 m, and ẋ 2 () =. P4.7 A quarter car model of an automobile is shown in Figure P4.7. The vehicle is traveling with a velocity V on a sinusoidal road surface with amplitude =.11 m and a wavelength of 5.3 m.

252 234 Vibration of Mechanical Systems V x 1 m 1 Static equilibrium c k 1 Suspension system x 2 m 2 Static equilibrium Tire y k 2 Road surface Figure P4.7 A quarter car model a. Derive the differential equations of motion and obtain mass, stiffness, and damping matrices. Also, obtain the forcing vector. b. Assume that m 1 = 1, 1 kg, m 2 = 76 kg, k 1 = 31, 11 N/m, k 2 = 321, 1 N/m, and c = 4, 98 N sec/m. Determine the damping ratio and the undamped natural frequency for each mode. c. Compute the amplitudes and the phases of steady-state responses when the velocity V = 1 km/h with and without the damper. d. Plot amplitudes of both masses as a function of the velocity V in the presence of a damper. P4.8 A rotor shaft system (Figure P4.8) consisting of torsional stiffness k 1 and mass-moment of inertia J 1 is subjected to a sinusoidal torque with magnitude 1.3 kn-meter. When the excitation frequency equals 8 Hz, there is a large amount of vibration.

253 Vibration of Two-Degree-of-Freedom-Systems 235 k 1 J1 J2 k 2 Torque Vibration absorber Figure P4.8 Undamped torsional vibration absorber Design an undamped vibration absorber with the requirement that the system will be safe for 2% fluctuation in excitation frequency around 8 Hz. Determine the amplitude of the absorber rotor J 2 at 8 Hz. P4.9 A rotor shaft system consisting (Figure P4.9) of torsional stiffness k 1 and mass-moment of inertia J 1 is subjected to a sinusoidal torque with magnitude 1.3 kn-meter. When the excitation frequency equals 8 Hz, there is a large amount of vibration. k 1 J1 k J2 2 Torque c Vibration absorber Figure P4.9 Damped torsional vibration absorber Design an optimally damped vibration absorber. Determine the amplitudes of both rotors as a function of excitation frequencies. P4.1 Determine the response of the two-mass system in Figure P4.1 via modal decomposition when the force f (t) is a step function of magnitude 5 N and the modal damping ratio in the vibratory mode

254 236 Vibration of Mechanical Systems x 1 ( t) x 2 ( t) f (t) m k m Figure P4.1 Two masses connected by a spring is.5. Assume that m = 1.5kg, k = 1, 25 N/m, x 1 () =.2 m, ẋ 1 () = 1m/sec,x 2 () =.1 m, and ẋ 2 () =.

255 5 FINITE AND INFINITE (CONTINUOUS) DIMENSIONAL SYSTEMS This chapter begins with the computation of the natural frequencies and the mode shapes of a discrete multi-degree-of-freedom (MDOF) system. It is shown that the natural frequencies and the modal vectors (mode shapes) are computed as the eigenvalues and the eigenvectors of a matrix dependent on mass and stiffness matrices. The orthogonal properties of modal vectors are derived. These orthogonal principles are the foundation of the modal decomposition technique, which leads to a significant reduction in the computational effort required to compute the response. Next, the following cases of continuous systems are considered: transverse vibration of a string, longitudinal vibration of a bar, torsional vibration of a circular shaft, and transverse vibration of a beam. These continuous systems have mass continuously distributed, are infinitely dimensional, and are governed by partial differential equations. The method of separation of variables is used and the natural frequencies and the modal vectors are calculated. Again, modal decomposition is used to compute the response. Last, the finite element method is introduced via examples of the longitudinal vibration of a bar and the transverse vibration of a beam. 5.1 MULTI-DEGREE-OF-FREEDOM SYSTEMS The differential equations of an MDOF system is written as Mẍ + Cẋ + Kx = f(t) (5.1.1) 237

256 238 Vibration of Mechanical Systems x 1 (t) x 2 (t) x 3 (t) k m k m k m k f 1 (t) c f 2 (t) f 3 (t) Figure Three-degree-of-freedom system The mass matrix M, the damping matrix C, and the stiffness matrix K are n n matrices where n is the number of degrees of freedom. The force vector f(t)isn-dimensional. Example 5.1.1: Consider the three-degree-of-freedom system shown in Figure The free body diagram of each mass is shown in Figure Applying Newton s law of motion to each mass, three second-order differential equations are obtained as follows: f 1 (t) kx 1 k(x 1 x 2 ) c(ẋ 1 ẋ 2 ) = mẍ 1 (5.1.2) f 2 (t) k(x 2 x 1 ) c(ẋ 2 ẋ 1 ) k(x 2 x 3 ) = mẍ 2 (5.1.3) f 3 (t) k(x 3 x 2 ) kx 3 = mẍ 3 (5.1.4) With the number of degrees of freedom n equal to 3, Equations can be put in the matrix form (Equation 5.1.1). x 1 (t) x 2 (t) x 3 (t) kx 1 f 1 (t) m k(x 1 x 2 ) c(x& 1 x& 2 ) k(x 2 x 1 ) c(x& 2 x& 1 ) f 2 (t) m k(x 2 x 3 ) k(x 3 x 2 ) f 3 (t) m kx 3 Figure Free body diagram for each mass in Figure 5.1.1

257 Finite and Infinite (Continuous) Dimensional Systems 239 where x 1 (t) f 1 (t) x(t) = x 2 (t) ; f(t) = f 2 (t) (5.1.5) x 3 (t) f 3 (t) m 2k k M = m, K = k 2k k, and m k 2k c c C = c c (5.1.6) Natural Frequencies and Modal Vectors (Mode Shapes) There is a mode shape or a modal vector associated with a natural frequency. A general method to compute the natural frequencies and the mode shapes is as follows. Ignoring damping and external force terms, Equation can be written as Mẍ + Kx = (5.1.7) Let x(t) = a sin(ωt + φ) (5.1.8) where n 1 vector a, the frequency ω, and the phase φ are to be determined. Differentiating Equation twice with respect to time, ẍ = ω 2 a sin(ωt + φ) (5.1.9) Substituting Equations and into Equation 5.1.7, (K ω 2 M)a = (5.1.1)

258 24 Vibration of Mechanical Systems For a nonzero or a nontrivial solution of a, det(k ω 2 M) = (5.1.11) which will be a polynomial equation of degree n in ω 2. Equation can also be written as or Ka = ω 2 Ma (5.1.12) M 1 Ka = ω 2 a (5.1.13) Equation clearly indicates that ω 2 and a are an eigenvalue and an eigenvector (Strang, 1988) of the matrix M 1 K. In addition, Equation suggests that ω 2 and a are generalized eigenvalues and eigenvectors of the stiffness matrix K with respect to the mass matrix M. The MATLAB command for computation of generalized eigenvalues and eigenvectors is eig(k, M). The formulation of the generalized eigenvalue/eigenvector problem is convenient for a large number of degrees of freedom because the inverse of the mass matrix is not required. Example 5.1.2: Eigenvalues and Eigenvectors of Three-Mass Chain From Example 5.1.1, the mass and stiffness matrices are as follows: m 2k k M = m ; K = k 2k k (5.1.14a, b) m k 2k Therefore, 2k ω 2 m k K ω 2 M = k 2k ω 2 m k (5.1.15) k 2k ω 2 m From Equation , det(k ω 2 M) = (2k mω 2 )[(2k mω 2 ) 2 2k 2 ] (5.1.16)

259 Finite and Infinite (Continuous) Dimensional Systems 241 From Equation , three natural frequencies are as follows: (2 2)k 2k ω 1 =, ω 2 = m m, and ω (2 + 2)k 3 = (5.1.17) m Using Equation 5.1.1, 2k ω 2 m k (K ω 2 M)a = k 2k ω 2 m k k 2k ω 2 m a 1 a 2 a 3 = (5.1.18) I. Modal Vector for ω 2 = (2 2)k m From Equation , 2k k k 2k k k 2k a 1 a 2 a 3 = (5.1.19) Two independent equations are 2ka1 ka 2 = and ka 2 + 2ka 3 = (5.1.2a, b) Since there are two equations in three unknowns, a 1 is arbitrarily chosen to be 1. Then, the solutions of Equations 5.1.2a,b yield a 2 = 2 and a 3 = 1. In other words, the modal vector is a = [ 1 2 1] T (5.1.21) II. Modal Vector for ω 2 = 2k/m From Equation , k k k k a 1 a 2 a 3 = (5.1.22)

260 242 Vibration of Mechanical Systems Two independent equations are ka 2 = and ka 1 ka 3 = (5.1.23a, b) Again, a 1 is arbitrarily chosen to be 1. Then, the solutions of Equations a,b yield a 2 = and a 3 = 1. In other words, the modal vector is a = [ 1 1 ] T (5.1.24) III. Modal Vector for ω 2 = (2+ 2)k m From Equation , 2k k k 2k k k 2k Two independent equations are a 1 a 2 a 3 = (5.1.25) 2ka 1 ka 2 = and ka 2 2ka 3 = (5.1.26a, b) Again, a 1 is arbitrarily chosen to be 1. Then, the solutions of Equations a,b yield a 2 = 2 and a 3 = 1. In other words, the modal vector is a = [ ] T (5.1.27) Orthogonality of Eigenvectors for Symmetric Mass and Symmetric Stiffness Matrices The orthogonality of eigenvectors is an important property for the vibration analysis of an MDOF system. The derivation of this property is as follows. Let ωi 2 and v i be the eigenvalue and eigenvector pair where i = 1, 2,...,n. Then, Kv i = ωi 2 Mv i (5.1.28)

261 Finite and Infinite (Continuous) Dimensional Systems 243 and Kv j = ω 2 j Mv j (5.1.29) Pre-multiplying both sides of Equation by v T j, v T j Kv i = ω 2 i vt j Mv i (5.1.3) Pre-multiplying both sides of Equation by vi T the transpose, and then taking ( v T i Kv j ) T = ω 2 j ( v T i Mv j ) T (5.1.31) or v T j KT v i = ω 2 j vt j MT v i (5.1.32) For symmetric mass and symmetric stiffness matrices, and K = K T (5.1.33) M = M T (5.1.34) Using Equations and , Equation yields v T j Kv i = ω 2 j vt j Mv i (5.1.35) Substituting Equation into Equation , ( ω 2 i ω 2 ) j v T j Mv i = (5.1.36) As a result, v T j Mv i = for ω i ω j (5.1.37) From Equations and , v T j Kv i = for ω i ω j (5.1.38) Usually, each eigenvector is scaled such that vi T Mv i = 1; i = 1, 2,...,n (5.1.39)

262 244 Vibration of Mechanical Systems In this case, from Equation 5.1.3, Define a modal matrix V as follows: vi T Kv i = ωi 2 ; i = 1, 2,...,n (5.1.4) V = [ v 1 v 2 v n 1 v n ] (5.1.41) Then, Equations are expressed as and V T MV = I n (5.1.42) V T KV = (5.1.43) where ω1 2 ω 2 2 = ωn 1 2 (5.1.44) ω 2 n Note: The derivations (Equations and ) are shown only for nonrepeated natural frequencies. However, it may be possible to diagonalize when some of the natural frequencies are repeated. Example 5.1.3: Orthogonality of Modal Vectors in Three-Mass Chain Modal vectors in Example are normalized to satisfy Equation as follows: v 1 = , v 2 = 1 1, and v 3 = 1 1 m 2m 2 2 m (5.1.45)

263 Finite and Infinite (Continuous) Dimensional Systems 245 Therefore, the modal matrix is V = [ v 1 v 2 v 3 ] = (5.1.46) 2m Hence, V T KV = 1 2k k k 2k k 2 2 8m k 2k It can be verified that V T KV = k m 2+ 2 = ω1 2 ω2 2 ω3 2 (5.1.47) (5.1.48) Modal Decomposition In general, the response x(t) is a linear combination of the modal vectors v i ; i = 1, 2,...,n, that is, x(t) = v 1 y 1 (t) + v 2 y 2 (t) + +v n y n (t) (5.1.49) where y i (t) is the coefficient of the modal vectors v i ; i = 1, 2,...,n. Equation can be represented in a compact form as follows: x(t) = Vy(t) (5.1.5) where the matrix V is defined by Equation and the vector y(t) is defined as y(t) = [ y 1 y 2 y n 1 y n ] T (5.1.51) Substituting Equation into Equation 5.1.1, and pre-multiplying by V T, V T MVÿ + V T CVẏ + V T KVy = V T f(t) (5.1.52)

264 246 Vibration of Mechanical Systems (t) y i k eq = ω 2 i m eq = 1 T f(t) v i Figure An equivalent undamped SDOF system for each mode Equations are often called modal equations as they are in terms of modal components y i ; i = 1, 2,...,n. Matrices V T MV and V T KV are diagonal, but there is no guarantee that V T CV is diagonal. Two special cases of damping resulting in decoupled modal equations are considered as follows. Case I: Undamped System (C = ) Substituting Equations and into Equation , or ÿ + y = V T f(t) (5.1.53) ÿ i + ωi 2 y i = vi T f(t); i = 1, 2,...,n (5.1.54) Here, modal equations are decoupled and each modal equation in Equation can be viewed as an equivalent undamped singledegree-of-freedom system subjected to the force vi T f(t) (Figure 5.1.3). The quantity vi T f(t) is also known as the modal force. Example 5.1.4: Consider the system in Figure with zero damping. Let m = 2 kg and k = 1, kg, and 1 f(t) =.5 sin 3t N (5.1.55) 2

265 Finite and Infinite (Continuous) Dimensional Systems 247 Let the initial displacement and velocity vectors be x() = [.3.2.4] T m and ẋ() = [3 5 8] T m/sec. The MATLAB program is listed in Program 5.1. This yields and ω1 2 = ω2 2 ω V =.5.5 (5.1.56) = (17.114) 2 ( ) 2 ( ) 2 (5.1.57) The conditions in Equations and have been verified. Next, v T 1 f(t) V T f(t) = v T 2 f(t) =.5 sin 3t (5.1.58) v T 3 f(t).817 Initial conditions are y 1 ().695 y 2 () = V 1 x() =.1 (5.1.59) y 3 ().295 and ẏ 1 () ẏ 2 () = V 1 ẋ() = 5 (5.1.6) ẏ 3 ()

266 248 Vibration of Mechanical Systems Equation yields ÿ 1 + (17.114) 2 y 1 = sin 3t; y 1 () =.695, ẏ 1 () = (5.1.61a) ÿ 2 + ( ) 2 y 2 =.5sin3t; y 2 () =.1, ẏ 2 () = 5 (5.1.61b) ÿ 3 + ( ) 2 y 3 =.817 sin 3t; y 3 () =.295, ẏ 3 () = (5.1.61c) These equations can be easily solved using techniques presented in Chapter 2, and the response of the MDOF system is then given by x 1 (t) y 1 (t) x 2 (t) = V y 2 (t) (5.1.62) x 3 (t) y 3 (t) MATLAB Program 5.1: Modal Vectors and Modal Initial Conditions % clear all close all % m=2; k=1; M=[2 ; 2 ; 2]; K=[2 k -k ;-k 2 k -k; -k 2 k]; f=[1.5 2] ; % [V,D]=eig(K,M); V f y=inv(v) [.3.2.4] %initial value of y dy=inv(v) [3 5 8] %initial value of dy/dt

267 Finite and Infinite (Continuous) Dimensional Systems 249 k eq = ω 2 i (t) y i ceq = α + βω 2 i m eq = 1 v T i f(t) Figure An equivalent damped SDOF system for each mode Case II: Proportional or Rayleigh Damping Assume that the damping matrix has the following form: C = αm + βk (5.1.63) where α and β are the constants. This form of damping is known as Proportional and Rayleigh damping. Substituting Equations , , and into Equation , or ÿ + (αi + β )ẏ + y = V T f(t) (5.1.64) ÿ i + ( α + βωi 2 ) ẏi + ωi 2 y i = vi T f(t); i = 1, 2,...,n (5.1.65) Again, the modal equations are decoupled and each modal equation in Equation can be viewed as an equivalent damped singledegree-of-freedom system subjected to the modal force vi T f(t) (Figure 5.1.4). Example 5.1.5: Nonproportional Damping Consider the Example with c = 5 N-sec/meter. The mass and stiffness matrices are the same as those in Example In this case, V T CV = (5.1.66)

268 25 Vibration of Mechanical Systems w(x, t) x x + dx Zero vibration position Figure Transverse vibration of a string Because V T CV is not diagonal, the damping in the system is not proportional. 5.2 CONTINUOUS SYSTEMS GOVERNED BY WAVE EQUATIONS This section deals with the vibration of a string, the longitudinal vibration of a bar, and the torsional vibration of a circular shaft Transverse Vibration of a String Consider a string (Figure 5.2.1) with tension P, mass per unit length μ, and external force per unit length f l (x, t). Let w(x, t) be the transverse deflection of the string at a position x and time t. The free body diagram of a string section with the length dx is shown in Figure f l ( x, t) dx P θ + dθ θ P w w + dw x x + dx Figure Free body diagram of a string element

269 Finite and Infinite (Continuous) Dimensional Systems 251 Applying Newton s second law of motion to the free body diagram in Figure 5.2.2, f l (x, t)dx + P sin(θ + dθ) P sin θ = μdx 2 w (5.2.1) t 2 where θ is the slope of the function w(x, t), that is, tan θ = w (5.2.2) x For a small θ, sin θ tan θ = w (5.2.3) x and sin(θ + dθ) tan(θ + dθ) = w x + 2 w dx (5.2.4) x2 Substituting Equations and into Equation 5.2.1, 1 μ f l(x, t) + χ 2 2 w x = 2 w (5.2.5) 2 t 2 where χ = P μ (5.2.6) Equation is the governing partial differential equation of motion. Natural Frequencies and Mode Shapes Natural frequencies and mode shapes are the characteristics of free vibration. Therefore, the external force term in Equation is ignored to obtain χ 2 2 w x 2 = 2 w t 2 (5.2.7) The partial differential Equation is known as the wave equation. The solution of the partial differential Equation is represented as follows: w(x, t) = X(x)T(t) (5.2.8)

270 252 Vibration of Mechanical Systems where X(x) is a function of position x only and T(t) is a function of time t only. This technique is known as the separation of variables (Boyce and DiPrima, 25). Differentiating Equation 5.2.8, and 2 w x 2 = d2 X dx 2 T (5.2.9) 2 w t 2 = Xd2 T dt 2 (5.2.1) Substituting Equations and into Equation 5.2.7, χ 2 1 d 2 X X dx = 1 d 2 T (5.2.11) 2 T dt 2 Since the left-hand side of Equation is only a function of x and the right-hand side of Equation is only a function of t, they can be equal only by being a constant. Also, it has been found that this constant must be a negative number for a physically meaningful solution. Denoting this negative constant as ω 2, χ 2 1 d 2 X X dx = 1 d 2 T 2 T dt 2 = ω2 (5.2.12) Equation represent the following two ordinary differential equations: and d 2 T dt 2 + ω2 T = (5.2.13) d 2 X dx + ω2 2 χ X = (5.2.14) 2 Both equations have the same form as the differential equation governing the undamped free vibration of an equivalent single-degreeof-freedom system in Chapter 1. Therefore, the solution of Equation is as follows: T(t) = A 1 sin ωt + B 1 cos ωt (5.2.15)

271 Finite and Infinite (Continuous) Dimensional Systems 253 where the constants A 1 and B 1 depend on the initial position and the velocity of the string. The form of the solution Equation clearly indicates that the constant ω is the natural frequency of vibration. The solution of Equation is as follows: ( ) ( ) ω ω X(x) = A 2 sin χ x + B 2 cos χ x (5.2.16) where the constants A 2 and B 2 depend on the boundary conditions which also yield natural frequencies and associated mode shapes. As an example, a string fixed at both ends is considered as follows. String Fixed at Both Ends Let a string of length l be fixed at both ends. In this case, the boundary conditions are described as and From Equations and , From Equations and , w(, t) = for all t (5.2.17) w(l, t) = for all t (5.2.18) X() = (5.2.19) X(l) = (5.2.2) Imposing the conditions (Equations and 5.2.2) to Equation , B 2 = (5.2.21) and ( ) ω X(l) = A 2 sin χ l = (5.2.22) For a nontrivial solution (A 2 ), ( ) ω sin χ l = (5.2.23)

272 254 Vibration of Mechanical Systems The number of solutions of Equation is infinite as follows: ω l = nπ; χ n = 1, 2, 3,..., (5.2.24) Equation leads to the natural frequencies for the transverse vibration of the string: ω n = nπχ l = nπ l P ; μ n = 1, 2, 3,..., (5.2.25) The number of natural frequencies is infinite as the number of degrees of freedom of a continuous structure is infinite. Substituting Equations and into Equation , ( ) ωn ( X(x) = A 2 sin χ x = A 2 sin nπ x ) l (5.2.26) Setting A 2 arbitrarily equal to one, the mode shape associated with the frequency ω n is written as ( φ n (x) = sin nπ x ) ; n = 1, 2, 3,..., (5.2.27) l These mode shapes are shown in Figure The mode shapes (Equation ) are orthogonal to each other in the following sense: l φ i (x)φ j (x)dx = ; i j (5.2.28) It should also be noted that l φ 2 i (x)dx = l 2 (5.2.29)

273 Finite and Infinite (Continuous) Dimensional Systems Displacement.5 n = n = Displacement 1 1 n = 3 n = x/(string length) x/(string length) Figure Mode shapes φ n (x) of strings fixed at both ends Computation of Response In general, the response can be expressed as a linear combination of mode shapes, that is, w(x, t) = α n (t)φ n (x) (5.2.3) n=1 where time-dependent coefficients α n (t) are to be determined. Differentiating Equation twice and using Equation , χ 2 2 w x 2 = n=1 α n (t)χ 2 d2 φ n dx 2 = α n (t)ωn 2 φ n(x) (5.2.31) n=1 Substituting Equation into the partial differential equation of motion (Equation 5.2.5), α n (t)ωn 2 φ n(x) = n=1 n=1 d 2 α n dt 2 φ n(x) + 1 μ f l(x, t) (5.2.32)

274 256 Vibration of Mechanical Systems Multiplying both sides of Equation by φ j (x) and integrating from to l, α n (t)ωn 2 n=1 l φ n (x)φ j (x)dx = n=1 + 1 μ l d 2 α n dt 2 l φ n (x)φ j (x)dx f l (x, t)φ j (x)dx (5.2.33) Then using the properties in Equations and , or α j (t) l 2 ω2 j = d2 α j dt 2 l l f l (x, t)φ j (x)dx (5.2.34) μ d 2 α j dt 2 l + ω 2 j α j(t) = 2 f l (x, t)φ j (x)dx (5.2.35) μl Initial conditions are obtained from w(x, ) and ẇ(x, ). From Equation 5.2.3, w(x, ) = α n ()φ n (x) (5.2.36) n=1 ẇ(x, ) = α n ()φ n (x) (5.2.37) n=1 Using the properties in Equations and , and l α j () = 2 w(x, )φ j (x)dx (5.2.38) l l α j () = 2 ẇ(x, )φ j (x)dx (5.2.39) l

275 Finite and Infinite (Continuous) Dimensional Systems w(x, ) x/(string length) Figure Initial displacement of a string Example 5.2.1: Free Vibration of a String Consider a string for which the initial displacement is shown in Figure 5.2.4, which can be analytically expressed as ( π x ) ( ) 2π x w(x, ) = sin + sin (5.2.4) l l Assuming that the initial velocity of string ẇ(x, ) =, determine the free response of the string. From Equation , and α 1 () = 2 l α 2 () = 2 l l l [ ( π x ) sin + sin l [ ( π x ) sin + sin l ( 2π x l ( 2π x l )] ( π x ) sin dx = 1 (5.2.41) l )] sin ( 2π x l ) dx = 1 (5.2.42) α j () = ; j = 3, 4, 5,..., (5.2.43)

276 258 Vibration of Mechanical Systems u(x, t) f l (x, t) x dx f l (x, t) : Force per unit length Figure Longitudinal vibration of a bar From Equation , α j () = ; j = 1, 2, 3,..., (5.2.44) The solution of Equation with f l (x, t) = is α j (t) = cos ω j t; j = 1, 2 (5.2.45) α j (t) = ; j = 3, 4, 5,..., (5.2.46) Therefore, from Equation 5.2.3, the response is ( π x ) ( ) 2π x w(x, t) = cos(ω 1 t)sin + cos(ω 2 t)sin l l (5.2.47) Longitudinal Vibration of a Bar Consider a longitudinal bar shown in Figure for which u(x, t) is the axial displacement at a distance x from the left end and at any time t. The force per unit length along the axial direction is f l (x, t). The free body diagram of an element of the length dx is shown in Figure 5.2.6, where P is the force on the element from the part of the bar on the left of the element. Similarly, P + dp is the force on the element from the part of the bar that is on the right side of the element. Applying Newton s second law of motion to the element in Figure 5.2.6, f l (x, t)dx + P + dp P = ρadx 2 u (5.2.48) t 2 where ρ and A are the mass density and the cross-sectional area, respectively. Note that the mass of the element of length dx is ρadx.

277 Finite and Infinite (Continuous) Dimensional Systems 259 dx P P + dp f l(x, t)dx u(x, t) Figure Free body diagram of an element of length dx The strain ε (Crandall et al., 1999) at a position x in Figure is ε = u x Therefore, the stress σ at a position x in Figure is (5.2.49) σ = E u (5.2.5) x where E is the Young s modulus of elasticity. Using Equation 5.2.5, the internal force P at a position x in Figure is P = σ A = EA u x Differentiating Equation , (5.2.51) dp = EA 2 u dx (5.2.52) x2 Substituting Equation into Equation , c 2 2 u x + f l(x, t) 2 ρa = 2 u (5.2.53) t 2 where c = E ρ (5.2.54) The governing partial differential equation of motion (Equation ) is also a wave equation. Example 5.2.2: Find the natural frequencies and the mode shapes of a fixed free longitudinal bar.

278 26 Vibration of Mechanical Systems In this case, the boundary conditions are described as u(, t) = for all t (5.2.55) and du (l, t) = dx for all t (5.2.56) Similar to Equation 5.2.8, u(x, t) = X(x)T(t) (5.2.57) The solutions of T(t) and X(x) are given by Equations and where χ is replaced by c. From Equations and X() = (5.2.58) From Equations and , dx (l) = (5.2.59) dx Imposing the conditions in Equations and to Equation , B 2 = (5.2.6) and dx dx (l) = A ω ( ω ) 2 c cos c l = (5.2.61) For a nontrivial solution (A 2 ), ( ω ) cos c l = (5.2.62) The number of solutions of Equation is infinite as follows: ω c l = 2n 1 π; n = 1, 2, 3,..., (5.2.63) 2

279 Finite and Infinite (Continuous) Dimensional Systems Displacement.5 n = n = Displacement n = n = x/(bar length) x/(bar length) 1 Figure Mode shapes φ n (x) for longitudinal vibration of a bar (fixed-free) Equation leads to the natural frequencies for the longitudinal vibration of a bar: (2n 1)πc (2n 1)π E ω n = = ; n = 1, 2, 3,..., (5.2.64) 2l 2l ρ The mode shapes of the longitudinal bar vibration are ( ωn ) ( ) (2n 1)π φ n (x) = sin c x = sin x 2l (5.2.65) These mode shapes are plotted in Figure Torsional Vibration of a Circular Shaft Consider a shaft with a circular cross section shown in Figure for which θ(x, t) is the angle of twist of a section at a distance x from the left end and at any time t. The torque per unit length along the axial direction is n l (x, t). The free body diagram of an element of the length dx is shown in

280 262 Vibration of Mechanical Systems nl (x, t) : Torque per unit length X θ(x, t) X x dx Figure Torsional vibration of a circular shaft Figure 5.2.9, where M t is the torque on the element from the part of the shaft on the left of the element. Similarly, M t + dm t is the torque on the element from the part of the bar that is on the right side of the element. Applying Newton s second law of motion to the element in Figure 5.2.9, M t + dm t + n l (x, t)dx M t = I dx 2 θ (5.2.66) t 2 where I is the mass-moment of inertia per unit length about the axis XX. It is known (Crandall et al., 1999) that M t = GJ θ x (5.2.67) where G and J are the Shear modulus of elasticity and the area moment of inertia of the circular cross section about the axis XX, dx n l dx M t M t + dm t θ(x, t) θ(x, t) + dθ Figure Free body diagram of an element of length dx

281 Finite and Infinite (Continuous) Dimensional Systems 263 Drill n δ(x l) Torque l Figure A simple model of drill respectively. From Equation , dm t = GJ 2 θ dx (5.2.68) x2 Substituting Equation into Equation , c 2 2 θ x + n l(x, t) = 2 θ (5.2.69) 2 I t 2 where c = GJ I (5.2.7) The governing differential equation of motion (Equation ) is also a wave equation. Example 5.2.3: Forced Response of a Circular Drill Consider a circular drill of length l and diameter d. When the drill makes a hole on a work surface, it experiences a torque n at x = l (Figure 5.2.1). Assuming that θ(x, ) =, θ(x, ), and treating the shaft as fixed free, determine the response θ(x, t), the angle of twist of a section at a distance x from the left. Since the shaft is fixed free, the natural frequencies and the mode shapes can be shown to be given by Equations and , that is, ω j = (2 j 1)πc 2l = (2 j 1)π GJ ; 2l I j = 1, 2, 3,..., (5.2.71)

282 264 Vibration of Mechanical Systems And the mode shapes will be ( ω ) j φ j (x) = sin c x ; j = 1, 2, 3,..., (5.2.72) Following Equations and , the response is given by θ(x, t) = α j (t)φ j (x) (5.2.73) j=1 where d 2 α j + ω 2 dt 2 j α j(t) = 2 l n l (x, t)φ j (x)dx (5.2.74) I l Here, n l (x, t) = n δ(x l) (5.2.75) where δ(x l) is the Dirac delta or the unit impulse function. Substituting Equation into Equation , and using Equation 3.2.5, d 2 α j dt 2 + ω 2 j α j(t) = 2n φ j (l) I l (5.2.76) Because θ(x, ) = and θ(x, ) =, α j () = and α j () = ; j = 1, 2, 3,..., (5.2.77) Following the solution procedure in Chapter 2 (Equation ), α j (t) = 2n φ j (l) (1 cos ω I lω 2 j t); j = 1, 2, 3,..., (5.2.78) j From Equations , , and , θ(x, t) = 2n φ j (l) ( ω ) j (1 cos ω I lω 2 j t)sin j c x j=1 (5.2.79)

283 Finite and Infinite (Continuous) Dimensional Systems 265 Force per unit length f l (x, t) w(x, t) x dx Figure Transverse vibration of a beam 5.3 CONTINUOUS SYSTEMS: TRANSVERSE VIBRATION OF A BEAM Governing Partial Differential Equation of Motion Consider a beam shown in Figure for which w(x, t) is the transverse displacement at a distance x from the left end and at any time t. The force per unit length along the lateral direction is f l (x, t). The free body diagram of an element of the length dx is shown in Figure 5.3.2, where V(x, t) and M(x, t) are the shear force and the bending moment, respectively, on the element from the part of the beam on the left of the element. Similarly, V(x, t) + dv(x, t) and M(x, t) + dm(x, t) are the shear force and the bending moment, respectively, on the element from the part of the beam which is on the right side of the element. dx 2 f l (x, t) dx M(x, t) M (x, t) + dm (x, t) w(x, t) V(x, t) V (x, t) + dv(x, t) dx Figure Free body diagram of a beam element

284 266 Vibration of Mechanical Systems Applying Newton s second law of motion to the beam element in Figure 5.3.2, (V + dv) + f l (x, t)dx + V = ρadx 2 w t 2 (5.3.1) where ρ and A are the mass density and the cross-sectional area, respectively. It is well known (Crandall et al., 1999) that V = M x (5.3.2) Therefore, Substituting Equation into Equation 5.3.1, dv = 2 M dx (5.3.3) x2 2 M x 2 + f l(x, t) = ρa 2 w t 2 (5.3.4) From the elementary beam theory (Crandall et al., 1999), M(x, t) = EI a 2 w x 2 (5.3.5) where E and I a are the Young s modulus of elasticity and the area moment of inertia of the beam cross section, respectively. Substituting Equation into Equation 5.3.4, EI a 4 w x 4 + ρa 2 w t 2 = f l(x, t) (5.3.6) Equation is the governing partial differential equation of motion.

285 Finite and Infinite (Continuous) Dimensional Systems Natural Frequencies and Mode Shapes Setting the external force f l (x, t) =, Equation can be written as where β 2 4 w x 4 = 2 w t 2 (5.3.7) β 2 = EI a ρa (5.3.8) Following the method of separation of variables, assume that w(x, t) = X(x)T(t) (5.3.9) where X(x) is a function of x only and T(t) is a function of time t only. From Equation 5.3.9, and 4 w x 4 2 w t 2 = d4 X dx 4 T (5.3.1) = Xd2 T dt 2 (5.3.11) Substituting Equation and into Equation β 2 1 d 4 X X dx = 1 d 2 T (5.3.12) 4 T dt 2 Since the left-hand side of Equation is only a function of x and the right-hand side of Equation is only a function of t, they can be equal only by being a constant. Also, it has been found that this constant must be a negative number for a physically meaningful solution. Denoting this negative constant as ω 2, β 2 1 d 4 X X dx = 1 d 2 T 4 T dt 2 = ω2 (5.3.13) From Equation , d 2 T dt 2 + ω2 T = (5.3.14)

286 268 Vibration of Mechanical Systems The solution of Equation is T(t) = A 1 sin ωt + B 1 cos ωt (5.3.15) where A 1 and B 1 are the constants. Equation also yields where d 4 X dx 4 γ 4 X = (5.3.16) γ 4 = ω2 (5.3.17) β 2 The solution of Equation is assumed as X(x) = Ae sx (5.3.18) Substituting Equation into Equation , For a nontrivial solution (A ), or (s 4 γ 4 )Ae sx = (5.3.19) s 4 γ 4 = (5.3.2) (s + γ )(s γ )(s + iγ )(s iγ ) = (5.3.21) where i = 1. The four roots of Equation are as follows: s 1 = γ, s 2 = γ, s 3 = iγ, and s 4 = iγ (5.3.22) Therefore, the general solution of Equation is X(x) = A 2 e γ x + B 2 e γ x + C 2 e iγ x + D 2 e iγ x (5.3.23) Equation can also be expressed as X(x) = A 3 cosh γ x + B 3 sinh γ x + C 3 cos γ x + D 3 sin γ x (5.3.24) Here, the constants A 3 and B 3 are related to A 2 and B 2.Also,the constants C 3 and D 3 are related to C 2 and D 2.

287 Finite and Infinite (Continuous) Dimensional Systems 269 w(, t) = w(l, t) = M(, t) = l M(l, t) = Figure A simply supported beam Simply Supported Beam For a simply supported beam (Figure 5.3.3) of length l, deflections at both ends are zero, that is, w(, t) = and w(l, t) = (5.3.25) Also, the bending moments at both ends of a simply supported beam are zero, that is, M(, t) = and M(l, t) = (5.3.26) The boundary conditions in Equations and along with the assumed form of the solution (Equation 5.3.9) and Equation lead to the following four conditions on X(x): X() = X(l) = d 2 X dx () = 2 (5.3.27a) (5.3.27b) (5.3.27c) and d 2 X dx 2 (l) = (5.3.27d) From Equations a, c, and , A 3 + C 3 = (5.3.28) A 3 C 3 = (5.3.29)

288 27 Vibration of Mechanical Systems Equations and imply that A 3 = and C 3 = (5.3.3a, b) Applying Equations b and d to Equation , [ ][ ] [ ] sinh γl sin γl B 3 = sinh γl sin γl D 3 (5.3.31) For a nontrivial solution of Equation , [ ] sinh γl sin γl det = (5.3.32) sinh γl sin γl The condition in Equation yields sin γl= (5.3.33) Substituting Equation into Equation yields B 3 = (5.3.34) Because of Equations 5.3.3a,b and , Equation yields X(x) = D 3 sin γ x (5.3.35) Equations and lead to the natural frequencies and the mode shapes, respectively. From Equation , γl= nπ; n = 1, 2, 3,..., (5.3.36) From Equations and , the natural frequencies of a simply supported beam are ω n = n2 π 2 β ; n = 1, 2, 3,..., (5.3.37) l 2 There are an infinite number of natural frequencies as the number of degrees of freedom of a continuous structure is infinite. The

289 Finite and Infinite (Continuous) Dimensional Systems 271 w(, t) = V(l, t) = dw (, t) = dx l M(l, t) = Figure A cantilever beam associated mode shapes are obtained from the Equation by arbitrarily setting D 3 = 1: ( nπ x ) φ n (x) = sin ; n = 1, 2, 3,..., (5.3.38) l Cantilever Beam For a cantilever beam (Figure 5.3.4) of length l, the deflection and the slope at the left end is zero, that is, w(, t) = and dw (, t) = (5.3.39) dx Also, the bending moment and the shear force at the right end of a cantilever beam are zero, that is, M(l, t) = and V(l, t) = (5.3.4) The boundary conditions in Equations and along with the assumed form of solution (Equations 5.3.9) and Equation lead to the following four conditions on X(x): X() = dx dx () = d 2 X dx (l) = 2 (5.3.41a) (5.3.41b) (5.3.41c) and d 3 X dx 3 (l) = (5.3.41d)

290 272 Vibration of Mechanical Systems Applying the conditions in Equations a d to Equation , A 3 + C 3 = B 3 + D 3 = A 3 cosh γl+ B 3 sinh γl C 3 cos γl D 3 sin γl= A 3 sinh γl+ B 3 cosh γl+ C 3 sin γl D 3 cos γl= (5.3.42a) (5.3.42b) (5.3.42c) (5.3.42d) Because of Equations a and b, Equations c and d can be written as [ ][ ] [ ] (cosh γl+ cos γl) (sinh γl+ sin γl) A 3 = (5.3.43) (sinh γl sin γl) (cosh γl+ cos γl) For a nontrivial solution of Equation , [ ] (cosh γl+ cos γl) (sinh γl+ sin γl) det = (5.3.44) (sinh γl sin γl) (cosh γl+ cos γl) The condition in Equation yields And from Equation , B 3 cosh γlcos γl= 1 (5.3.45) B 3 (cosh γl+ cos γl) = A 3 (sinh γl+ sin γl) From Equation , (5.3.46) X(x) = A 3 [(cosh γ x cos γ x) + (sinh γ x sin γ x)] (5.3.47) where = B 3 (cosh γl+ cos γl) = (5.3.48) A 3 (sinh γl+ sin γl) There are an infinite number of roots of Equation The first four roots of Equation are γ 1 l = , γ 2 l = , γ 3 l = , and γ 4 l = (5.3.49)

291 Finite and Infinite (Continuous) Dimensional Systems Displacement Displacement x/(beam length) x/(beam length) Figure Mode shapes φ n (x) of a cantilever beam From Equations and , the natural frequencies of a cantilever beam are ω n = β l (γ nl) 2 ; n = 1, 2, 3,..., (5.3.5) 2 The associated mode shapes are obtained from Equation by arbitrarily setting A 3 = 1: φ n (x) = (cosh γ n x cos γ n x) + n (sinh γ n x sin γ n x) (5.3.51) where n is given by Equation with γ = γ n. These mode shapes are plotted in Figure Computation of Response In general, the response can be expressed as a linear combination of mode shapes, that is, w(x, t) = α n (t)φ n (x) (5.3.52) n=1

292 274 Vibration of Mechanical Systems where time-dependent coefficients α n (t) are to be determined. Mode shapes φ i (x) and φ j (x) of beams are orthogonal, that is, l φ i (x)φ j (x)dx = ; i j (5.3.53) Differentiating Equation four times and using Equation , 4 w x 4 = n=1 α n (t) d4 φ n dx 4 = α n (t) 1 β 2 ω2 n φ n(x) (5.3.54) From the partial differential equation of motion in Equation 5.3.6, α n (t)ωn 2 φ d 2 α n n(x) + dt φ n(x) = 1 2 ρa f l(x, t) (5.3.55) n=1 n=1 Multiplying both sides of Equation by φ j (x), and integrating from to l, α n (t)ωn 2 n=1 l φ n (x)φ j (x)dx + n=1 n=1 d 2 α n dt 2 l φ n (x)φ j (x)dx = 1 l f l (x, t)φ j (x)dx (5.3.56) ρa Then using the property in Equation , or where α j (t)η j ω 2 j + d2 α j dt η 2 j = 1 l f l (x, t)φ j (x)dx (5.3.57) ρa d 2 α j + ω 2 dt 2 j α j(t) = 1 l f l (x, t)φ j (x)dx (5.3.58) ρaη j η j = l φ 2 j (x)dx (5.3.59)

293 Finite and Infinite (Continuous) Dimensional Systems 275 P sin ωt l h b Cross section Figure A cantilever beam subjected to sinusoidal excitation Initial conditions are obtained from w(x, ) and ẇ(x, ). From Equation , w(x, ) = α n ()φ n (x) (5.3.6) n=1 ẇ(x, ) = α n ()φ n (x) (5.3.61) n=1 Using the properties in Equations and , α j () = 1 η j l w(x, )φ j (x)dx (5.3.62) and α j () = 1 η j l ẇ(x, )φ j (x)dx (5.3.63) Example 5.3.1: Response of a Cantilever Beam Consider a steel cantilever beam of rectangular cross section (Figure 5.3.6) with the width b =.1 m and the thickness h =.5 m. The length l of the beam is.8 m. At the tip of the beam, a sinusoidal force with magnitude P = 1 N and frequency ω = 1 rad/ sec is applied. Determine the steady-state response by assuming that the damping ratio in each mode is.2. For steel, E = N/m 2 and ρ = 7, 85 kg/m 3. A = bh = N/m 2 and I a = 1 12 bh3

294 276 Vibration of Mechanical Systems Then, β = EI a ρa = The first four natural frequencies (Equation ) are as follows: ω 1 = 4 rad/ sec, ω 2 = 25.8 rad/ sec, ω 3 = 72.3 rad/ sec, and ω 4 = rad/ sec Here, f l (x, t) = P sin ωtδ(x l) (5.3.64) where δ(x l) is the Dirac delta or the unit impulse function. After introducing the damping ratio ξ in each mode, Equation becomes where d 2 α j dt 2 + 2ξω j α j + ω 2 j α j(t) = f eq ( j)sinωt (5.3.65) f eq ( j) = Pφ j(l) ρaη j (5.3.66) Using Equation , η j = l 1 φ 2 j (x)dx = l φ 2 j ( x)d x (5.3.67) where x = x l (5.3.68) and φ j ( x) = [cosh(γ j l x) cos(γ j l x) + j (sinh(γ j l x) sin(γ j l x)] (5.3.69) It has been verified numerically that η j = l (5.3.7)

295 Finite and Infinite (Continuous) Dimensional Systems 277 Using results in Chapter 2, the steady-state response of Equation can be written as α j (t) = j sin(ωt θ j ) (5.3.71) where and f eq ( j) j = (ω (5.3.72) 2 j ω2) 2 + (2ξωj ω) 2 θ j = 2ξω jω ω 2 j ω2 (5.3.73) From Equation , the steady-state displacement at x = l is given by n n w(l, t) = φ j (l)α j (t) = φ j (l) j sin(ωt θ j ) (5.3.74) j=1 j=1 After some algebra, Equation is written as w(l, t) = B 1 sin ωt B 2 cos ωt (5.3.75) where and From Equation , B 1 = B 2 = n φ j (l) j cos θ j (5.3.76) j=1 n φ j (l) j sin θ j (5.3.77) j=1 w(l, t) = Bsin(ωt ϕ) (5.3.78) where B = B B2 2 and tan ϕ = B 2 B 1 (5.3.79a, b)

296 278 Vibration of Mechanical Systems From the MATLAB Program 5.2 with ω = 1 rad/ sec, MATLAB Program 5.2 B =.155 m and ϕ = rad %cantilever beam gaml=[ ];%eq.(5.3.49) E=2e11;%Young s Modulus of Elasticity (N/mˆ2) rho=785;%mass density(kg./mˆ3) L=.8;%length of beam(meter) b=.1;%meter h=.5;%meter A=b h;%cross sectional area % P=1;%magnitude of force (Newton) omega=1;%excitation frequency(rad./sec.) m=rho L A;%mass of beam Ia=b hˆ3/12; beta=sqrt(e Ia/(rho A)); sumcos=; sumsin=; for i=1:4 omegan(i)=beta gaml(i)ˆ2/lˆ2;%natural Frequencies(rad/sec.),eq.(5.3.5) lamd=-(cosh(gaml(i))+cos(gaml(i)))/(sinh(gaml(i))+sin(gaml(i)));%eq. %(5.3.48) ang=gaml(i) 1; phil(i)=(cosh(ang)-cos(ang))+lamd (sinh(ang)-sin(ang)); Feq(i)=P phil(i)/(m L);%Equivalent force in mode#i den=sqrt((omegan(i)ˆ2-omegaˆ2)ˆ2+(2.2 omegan(i) omega)ˆ2); Amp(i)=Feq(i)/den;%Amplitude of Response in Mode#i phase(i)=atan2(2.2 omegan(i) omega,omegan(i)ˆ2-omegaˆ2);%phase sumcos=sumcos+phil(i) Amp(i) cos(phase(i));%b1 sumsin=sumsin+phil(i) Amp(i) sin(phase(i));%b2

297 Finite and Infinite (Continuous) Dimensional Systems 279 end % Ampw=sqrt(sumcosˆ2+sumsinˆ2)%Steady State Amplitude of w(l,t):b phasew=atan2(sumsin,sumcos)%steady State Phase of w(l,t) 5.4 FINITE ELEMENT ANALYSIS The number of degrees of freedom of a continuous system is infinite. For simple geometries, such as those considered in Sections 5.2 and 5.3, the governing partial differential equation of motion can be solved analytically to determine the natural frequencies, the mode shapes, and the response of the structure. However, in general, the analytical solution of a partial differential equation is not possible for a real structure, for example, a turbine blade. As a result, the structure is discretized into a finite number of elements and the solution is obtained numerically. This process is known as the finite element method, which has been successfully applied to many real engineering problems. In fact, many commercially available codes, such as ANSYS and NASTRAN, are routinely used in industries. Here, fundamental ideas behind the finite element analysis (Petyt, 199) will be illustrated via two simple examples: longitudinal vibration of a bar and transverse vibration of a beam Longitudinal Vibration of a Bar Consider the longitudinal bar shown in Figure again. This bar is divided into n elements of equal lengths (Figure 5.4.1), that is, the length of each element is l e = l n (5.4.1) Each element has two nodes associated with it. For example, the first element has nodes 1 and 2, and the last element has nodes n and

298 28 Vibration of Mechanical Systems Node 1 l e Node 2 u(x, t) Node i + 1 le K i K K n 1 n Node n + 1 Element 1 x l Element n Figure Discretization of a longitudinal bar by a finite number of elements n + 1. In general, the element i will have the node numbers i and i + 1 (Figure 5.4.2). Each node has one degree of freedom (axial displacement u ). Let u i (t) be the axial displacement of the node i. Then, a discrete displacement vector u(t) can be defined as u(t) = [ u 1 (t) u 2 (t) u n (t) u n+1 (t) ] T (5.4.2) Inside each element, the displacement is assumed to be a predefined function of the axial coordinate ξ. Here, this function will be chosen as linear. For example, inside the element i, u(ξ,t) = a 1 + a 2 ξ; ξ l e (5.4.3) Note that At ξ =, u(, t) = u i (t) (5.4.4) At ξ = l e, u(l e, t) = u i+1 (t) (5.4.5) u i u i+1 u(ξ, t) Element i ξ = l e ξ = l e Figure An element of a longitudinal bar

299 Finite and Infinite (Continuous) Dimensional Systems 281 Substituting Equations and into Equation 5.4.3, and a 1 = u i (5.4.6) a 2 = u i+1 u i (5.4.7) l e Substituting Equations and into Equation 5.4.3, ) u(ξ,t) = (1 ξle u i (t) + ξ u i+1 (t) (5.4.8) l e Differentiating Equation with respect to ξ, u ξ = 1 u i (t) + 1 u i+1 (t) (5.4.9) l e l e Equation can be expressed as u ξ = κt v i (t) (5.4.1) where [ ] u i (t) v i (t) = (5.4.11) u i+1 (t) and [ κ T = 1 ] 1 l e l e Differentiating Equation with respect to time, (1 ξle ) (5.4.12) u t = u i (t) + ξ u i+1 (t) (5.4.13) l e Equation can be expressed as u t = nt (ξ) v i (t) (5.4.14) where [ ] u i (t) v i (t) = (5.4.15) u i+1 (t)

300 282 Vibration of Mechanical Systems and ) n T (ξ) = [(1 ξle ξ l e ] (5.4.16) The kinetic energy of the element i is written as T i = l e ρa ( u t ) 2 dξ = ρa v T i where ρ is the mass density of the material and l e n(ξ)n T (ξ)dx = l e l e ( 1 ξ l e ) 2 dξ ( 1 ξ l e ) ξ l e dξ l e l e n(ξ)n T (ξ)dξ v i (5.4.17) ( 1 ξ l e ) ξ l e dξ l e ( ξ l e ) 2dξ Substituting Equation into Equation , = l e 6 [ ] (5.4.18) T i = 1 2 vt i M e v i (5.4.19) where M e is the mass matrix of the element defined as follows: M e = ρal e 3 [ ] (5.4.2) Using Equation 5.4.1, the potential energy of the element i is written as P i = EA l e = EAv T i ( ) u 2 dξ = EA ξ l e l e ( u ξ ) T ( ) u dξ ξ κκ T dξ v i (5.4.21)

301 Finite and Infinite (Continuous) Dimensional Systems 283 Column i Φi = L L 1 1 L L Column (i + 1) Figure Matrix connecting v i to u where E and A are the Young s modulus of elasticity and the crosssectional area of the element, respectively. Furthermore, l e 1 l e 1 l e dξ dξ l 2 κκ T e l 2 [ ] e dξ = l e 1 l e = (5.4.22) l 1 e 1 1 dξ dξ l 2 e Substituting Equation into Equation , l 2 e P i = 1 2 vt i K ev i (5.4.23) where K e is the stiffness matrix of the element defined as [ ] K e = 2EA 1 1 l 1 1 (5.4.24) Total Kinetic and Potential Energies of the Bar From Equation , v i (t) = i u(t) (5.4.25) where i is a 2 (n + 1) matrix defined in Figure The kinetic and potential energies of the element i are defined as T i = 1 2 ut i T M e i u(t) (5.4.26) P i = 1 2 ut T i K e i u(t) (5.4.27)

302 284 Vibration of Mechanical Systems The total kinetic energy of the bar is obtained by summing the kinetic energy of each element: T = n T i = 1 2 ut M t u(t) (5.4.28) i=1 where M t is the mass matrix defined as M t = n i T M e i (5.4.29) i=1 The total potential energy of the bar is obtained by summing the potential energy of each element: P = n P i = 1 2 ut K t u(t) (5.4.3) i=1 where K t is the stiffness matrix defined as K t = n i T K e i (5.4.31) i=1 The differential equations of motion will then be M t ü(t) + K t u(t) = (5.4.32) It should be noted that Equation refers to a free free bar, as there is no constraint imposed on any nodal displacement. Example 5.4.1: Natural Frequencies of a Free Free Bar Consider a free free steel bar with length =.4 m and cross-sectional area = m 2. Determine the first five natural frequencies using 5, 1, and 15 elements, and compare them to the theoretical frequencies obtained in Section 5.2.

303 Finite and Infinite (Continuous) Dimensional Systems 285 Solution The MATLAB program 5.3 is used. Results are as follows: First five theoretical frequencies: [ ] 1 6 rad/ sec First five natural frequencies from finite element analysis: Number of elements = 5, [ ] 1 6 rad/ sec Number of elements = 1, [ ] 1 6 rad/sec Number of elements = 15, [ ] 1 6 rad/sec As the number of elements increases, the natural frequencies converge to their theoretical values. MATLAB Program 5.3: Finite Element Analysis of a Longitudinal Bar clear all close all %Free-Free Longitudinal Bar Vibration betal2_ff=[ ]; nel=1%number of elements E=21e9;%Young s Modulus of Elasticity (N/mˆ2) rho=7.8e3;%mass density (kg./mˆ3) lt=4e-2;%length of Beam (Meter) Area=4e-4%Cross Sectional Area (mˆ2) l=lt/nel;%element Length (Meter) ndof=nel+1;%number of degrees of freedom sum_k=zeros(ndof,ndof); sum_m=zeros(ndof,ndof); P=zeros(2,ndof);

304 286 Vibration of Mechanical Systems Node 1 l e Node 2 w(x, t) Node i + 1 l e K i K K n 1 n Node n + 1 Element 1 x Element n l Figure Discretization of a beam by a finite number of elements P(1:2,1:2)=eye(2); for i=1:nel if (i>1) P=zeros(2,ndof); P(1:2,i:i+1)=eye(2); end Ke=(2 E Area/l) [1-1;-1 1]; Me=(rho Area l/3) [2 1;1 2]; sum_k=sum_k+p Ke P;% eq.(5.4.31) sum_m=sum_m+p Me P;%eq.(5.4.29) end K=sum_K; M=sum_M; % %Free-Free % ogff_th=sqrt(e/rho) pi/lt [ ]%Theoretical Natural Frequencies % [V_ff,D_ff]=eig(K,M); ogff_fem=sqrt(diag(d_ff))%natural Frequencies from FEM % Transverse Vibration of a Beam Consider the beam shown in Figure again. This beam is divided into n elements of equal lengths (Figure 5.4.4), that is, the length of

305 Finite and Infinite (Continuous) Dimensional Systems 287 w 1 (t) w 2 (t) θ1(t) w(ξ, t) θ2(t) Node i Element i Node (i + 1) ξ = l e ξ = l e Figure An element of a beam each element is l e = l n (5.4.33) Each element has two nodes associated with it. For example, the first element has nodes 1 and 2, and the last element has nodes n and n + 1. In general, the element i will have the node numbers i and i + 1 (Figure 5.4.5). Each node has two degrees of freedom (transverse displacement w and the slope w x ). Let w i(t) and θ i (t) be the transverse displacement and the slope at the node i. Then, a discrete displacement vector u(t) can be defined as u(t) = [ w 1 w 2 w n w n+1 θ 1 θ 2 θ n θ n+1 ] T (5.4.34) Inside each element, the transverse displacement is assumed to be a predefined function of the axial coordinate ξ. Here, this function will be chosen as a cubic polynomial. For example, inside the element i, w(ξ,t) = a 1 (t) + a 2 (t)ξ + a 3 (t)ξ 2 + a 4 (t)ξ 3 (5.4.35) Differentiating Equation with respect to ξ, w ξ (ξ,t) = a 2 + 2a 3 ξ + 3a 4 ξ 2 (5.4.36)

306 288 Vibration of Mechanical Systems Note that At ξ =, w(, t) = w i (t), w ξ (, t) = θ i (5.4.37) At ξ = l e, w(l e, t) = w i+1 (t), w ξ (l e, t) = θ i+1 (5.4.38) Substituting Equations and into Equations and , w(, t) = a 1 = w i (t) w ξ (, t) = a 2 = θ i (t) w(l e, t) = a 1 + a 2 l e + a 3 l 2 e + a 4l 3 e = w i+1(t) w ξ (l e, t) = a 2 + 2a 3 l e + 3a 4 l 2 e = θ i+1(t) (5.4.39a) (5.4.39b) (5.4.39c) (5.4.39d) The solutions of Equations a d yield the coefficients a 1, a 2, a 3, and a 4 : a(t) = q i (t) (5.4.4) where 1 1 = l 2 e l 2 e l e l e (5.4.41) l 3 e 2 l 3 e l 2 e l 2 e a T (t) = [ a 1 (t) a 2 (t) a 3 (t) a 4 (t) ] (5.4.42) q T i (t) = [ w i(t) w i+1 (t) θ i (t) θ i+1 (t) ] (5.4.43)

307 Finite and Infinite (Continuous) Dimensional Systems 289 From Equations , 5.4.4, and , w(ξ,t) = [ 1 ξ ξ 2 ξ 3 ] a(t) = [ 1 ξ ξ 2 ξ 3 ] q i (t) = n T (ξ)q i (t) (5.4.44) where 1 3 ξ ξ 3 l 2 e l 3 e 3 ξ 2 2 ξ 3 l 2 e l 3 e n(ξ) = ξ 2 ξ ξ 3 l e l 2 e ξ 2 + ξ 3 l e l 2 e (5.4.45) The kinetic energy of the element i is written as T i = 1 2 l e ρa ( ) w 2 dξ (5.4.46) t where ρ and A are the mass density of the material and the crosssectional area of the beam, respectively. Differentiating Equation with respect to time, w t = n T (ξ) q i (5.4.47) Therefore, ( ) w 2 = t ( ) w T w = q i T t t n(ξ)nt (ξ) q i (5.4.48) Substituting Equation into Equation , T i = 1 2 qt i M e q i (5.4.49)

308 29 Vibration of Mechanical Systems where M e is the mass matrix of the element defined as follows: l e M e = ρa n(ξ)n T (ξ)dξ (5.4.5) Using Equation , l e n(ξ)n T (ξ)dξ = l e l e 13l e l e 22l e 22l e 13l e 4l 2 e 3l 2 e (5.4.51) 13l e 22l e 3l 2 e 4l 2 e Therefore, the mass matrix of the beam element is l e 13l e M e = ρal e l e 22l e 42 22l e 13l e 4l 2 e 3l 2 e 13l e 22l e 3l 2 e 4l 2 e (5.4.52) The potential energy of the element i is written as From Equation , P i = 1 le 2 EI 2 w ξ 2 = ( 2 ) 2 w dξ (5.4.53) ξ 2 ( 2 ) T n(ξ) q i (t) (5.4.54) ξ 2 where 2 n(ξ) ξ 2 = ξ l 2 e l 3 e 6 12 ξ l 2 e l 3 e ξ l e l 2 e ξ l e l 2 e (5.4.55)

309 Finite and Infinite (Continuous) Dimensional Systems 291 From Equation , ( 2 ) 2 ( w 2 ) T ( w 2 ) w = ξ 2 ξ 2 ξ 2 Using Equation , l e ( 2 )( n(ξ) 2 ) T n(ξ) dξ = 1 ξ 2 ξ 2 = q T i (t) ( 2 n(ξ) ξ 2 Substituting Equation into Equation , l 3 e )( 2 ) T n(ξ) q ξ 2 i (t) (5.4.56) l e 6l e l e 6l e 6l e 6l e 4l 2 e 2l 2 e 6l e 6l e 2l 2 e 4l 2 e (5.4.57) P i = 1 2 qt i (t)k eq i (t) (5.4.58) where K e is the element stiffness matrix defined as K e = EI l e ( 2 )( n(ξ) 2 ) T n(ξ) dξ (5.4.59) ξ 2 ξ 2 Substituting Equation into Equation , l e 6l e K e = EI l e 6l e l 3 e 6l e 6l e 4l 2 e 2l 2 e 6l e 6l e 2l 2 e 4l 2 e Total Kinetic and Potential Energies of the Beam (5.4.6) q i (t) = Ɣ i u(t) (5.4.61) where Ɣ i is a 4 (2n + 2) matrix defined in Figure The kinetic and potential energies of the element i are defined as T i = 1 2 ut Ɣ T i M eɣ i u(t) (5.4.62) P i = 1 2 ut Ɣ T i K eɣ i u(t) (5.4.63)

310 292 Vibration of Mechanical Systems Column i Column (n i) Γi = L L L L 1 1 L L L L L L L L 1 1 L L L L Column (n + 1) Column (2n + 2) Figure Matrix connecting q i to u The total kinetic energy of the beam is obtained by summing the kinetic energy of each element: n T = T i = 1 2 ut M t u(t) (5.4.64) i=1 where M t is the mass matrix defined as M t = n Ɣi T M eɣ i (5.4.65) i=1 The total potential energy of the beam is obtained by summing the potential energy of each element: P = n P i = 1 2 ut K t u(t) (5.4.66) i=1 where K t is the stiffness matrix defined as K t = n Ɣi T K eɣ i (5.4.67) i=1 The differential equations of motion will then be M t ü(t) + K t u(t) = (5.4.68) It should be noted that Equation refers to a free free beam, as there is no constraint imposed on any nodal displacement or slope.

311 Finite and Infinite (Continuous) Dimensional Systems 293 Example 5.4.2: Natural Frequencies of a Cantilever Beam Consider a cantilever steel beam with length =.4 m, cross-sectional area = m 2, and area moment of inertia = 1 8 m 4. Determine 3 the first four natural frequencies using 5, 1, and 15 elements, and compare them to the theoretical frequencies derived in Section 5.3. Solution The MATLAB Program 5.4 is used. Results are as follows: First Four Theoretical Frequencies: [ ] 1 6 rad/ sec First Four Natural Frequencies from finite element analysis: Number of elements = 5, [ ] 1 6 rad/sec Number of elements = 1, [ ] 1 6 rad/sec Number of elements = 15, [ ] 1 6 rad/sec As the number of elements increases, the natural frequencies converge to their theoretical values. MATLAB Program 5.4: Finite Element Analysis of Beam Vibration clear all close all %Free-Free and Cantilever Beam betal2_ff=[ ].ˆ2; betal2_cant=[ ].ˆ2;

312 294 Vibration of Mechanical Systems nel=2%number of elements E=21e9;%Young s Modulus of Elasticity (N/mˆ2) rho=7.8e3;%mass density (kg./mˆ3) I=1e-8/3;%Area Moment of inertia (mˆ4) lt=4e-2;%length of Beam (Meter) Area=4e-4%Cross Sectional Area (mˆ2) l=lt/nel;%element Length (Meter) ndof=2 (nel-1)+4; sum_k=zeros(ndof,ndof); sum_m=zeros(ndof,ndof); P=zeros(4,ndof); P(1:4,1:4)=eye(4); for i=1:nel if (i>1) P=zeros(4,ndof); P(1:4,2 (i-1)+1:2 (i-1)+4)=eye(4); end Ke=(E I/(lˆ3)) [12 6 l l; 6 l 4 l l -6 l 2 l l; l 12-6 l;6 l 2 l l -6 l 4 l l]; Me=(rho Area l/42) [ l l;22 l 4 l l 13 l -3 l l; l l;-13 l -3 l l -22 l 4 l l]; sum_k=sum_k+p Ke P; sum_m=sum_m+p Me P; end K=sum_K; M=sum_M; % %Free-Free % ogff_th=sqrt((e I/(rho Area ltˆ4))) betal2_ff % [V_ff,D_ff]=eig(K,M); ogff_fem=sqrt(diag(d_ff));

313 Finite and Infinite (Continuous) Dimensional Systems 295 % %Cantilever % ogcant_th=sqrt((e I/(rho Area ltˆ4))) betal2_cant % K_cant=K(3:ndof,3:ndof); M_cant=M(3:ndof,3:ndof); [V_cant,D_cant]=eig(K_cant,M_cant); ogcant_fem=sqrt(diag(d_cant)); ogcant_fem(1:4) EXERCISE PROBLEMS P5.1 Consider the model of a bladed disk (Sinha, 1986) shown in Figure P5.1 where each blade is represented by a single mass. Furthermore, it should be noted that i + 1 = 1 when i = N and i 1 = N when i = 1, where N is the number of blades. Model parameters are as follows: m t =.114 kg, k t = 43, N/m, and K c = 45,43 N/m. x i 1 x i x i+1 K c K c K c m t m t m t k t k t k t Figure P5.1 A bladed disk model with one mass per blade sector a. Compute the natural frequencies and the mode shapes when N = 3. Examine the orthogonality of the mode shapes. b. Compute the natural frequencies and the mode shapes when N = 1. Examine the orthogonality of the mode shapes.

314 296 Vibration of Mechanical Systems P5.2 Consider the model of a turbine blade (Griffin and Hoosac, 1984) shown in Figure P5.2. Model parameters (SI units) are as follows: m 1 =.114, m 2 =.427, m 3 =.299, k 1 = 43,3, k 2 = 17,35,, and k 3 = 7,521,. a. Compute the natural frequencies and the mode shapes. Examine the orthogonality of the mode shapes. b. Let f (t) = sin ωt, where ω is the excitation frequency. Using modal decomposition, find the amplitude and the phase of each mass as a function of the excitation frequencies near the first natural frequency. Assume the modal damping ratio to be.1. x 1 f (t) m 1 k 1 x 2 m 2 k 2 x 3 m 3 k 3 Figure P5.2 A bladed disk model with three masses per blade sector P5.3 Consider the half car model in Figure P5.3. The vehicle is traveling with a velocity V on a sinusoidal road surface with an amplitude of.11 m and a wavelength of 5.3 m. Parameters of the system are as follows: l 1 = 1.35 m, l 2 = 1.5 m, I c = 1,556 kg-m 2, m 1 = 1,1 kg, m 2 = 38 kg, k 1 = 31,11 N/m, k 2 = 41,31 N/m, k 3 = 321,1 N/m, c 1 = 3,98 N-sec/m, and c 2 = 4,98 N-sec/m.

315 Finite and Infinite (Continuous) Dimensional Systems 297 x 1 m 1,I c V Center of mass, C x 2 l 1 l 2 c 1 k 1 Suspension system c 2 k 2 Suspension system x 3 x 4 m 2 m 2 Tire Tire k 3 k 3 y 1 y 2 Road surface Figure P5.3 A half-car model a. Compute the natural frequencies and the mode shapes. b. Compute the modal damping ratios. c. Find the critical speed of the vehicle. At the lowest critical speed, compute the steady-state response. P5.4 The steel wire of length.9 m and cross-sectional area of 1.3 mm is fixed at both ends in a musical instrument. The tension in the string is 22 N. A musician plucks the string while adjusting the lengths of the strings in the following sequence:.5,.7, and.9 m. Compute the sequence of the fundamental frequencies of the sound generated by the instrument. P5.5 Find the natural frequencies and the mode shapes of a fixed fixed longitudinal bar. P5.6 Find the natural frequencies and the mode shapes of a fixed fixed torsional shaft.

316 298 Vibration of Mechanical Systems P5.7 Find the natural frequencies and the mode shapes of a cantilever beam attached to a spring at its end (Figure P5.7). E, I a, ρ, A, l k Figure P5.7 A cantilever beam attached to a spring P5.8 A sinusoidal force is applied at the midpoint of a fixed fixed elastic beam. Determine the response of the system using zero initial conditions. f sin ωt E, I a, ρ, A, l Figure P5.8 A fixed fixed beam excited by a sinusoidal force P5.9 Consider a fixed fixed steel bar with the length =.1 m and the cross-sectional area = m 2. Using the finite element method, determine the first three natural frequencies and compare them to the theoretical values. P5.1 Consider a fixed fixed steel beam with the length =.1 m, the cross-sectional area = m 2 and the area moment of inertia = m 4. Using the finite element method, determine the first three natural frequencies and compare them to the theoretical values.

317 APPENDIX A EQUIVALENT STIFFNESSES (SPRING CONSTANTS) OF BEAMS, TORSIONAL SHAFT, AND LONGITUDINAL BAR In this Appendix, equivalent stiffnesses of beams, a torsional shaft, and a longitudinal bar are presented. Derivations of these stiffnesses are based on static deflection of a structure. A.1 FIXED FIXED BEAM Force F a x Deflection, δ(x) E, I a a + b = l b k eq (x) = F δ(x) = 6EI a l 3 x 2 b 2 (3al x(3a + b)) ; x a 6EI a l 3 (l x) 2 a 2 (3bl (l x)(3b + a)) ; a x l (A.1) k eq (x): Equivalent stiffness at a distance x from left end E: Young s modulus of elasticity I a : Area moment of inertia 299

318 3 Appendix A A.2 SIMPLY SUPPORTED BEAM Force F x a Deflection, δ(x) E, I a a + b = l b k eq (x) = F δ(x) = 6EI a l xb(l 2 x 2 b 2 ) ; x a 6EI a l (l x)a(2lx x 2 a 2 ) ; a x l (A.2) A.3 CANTILEVER BEAM Force F a x Deflection, δ(x) E, I a a + b = l b k eq (x) = F δ(x) = 6EI a x 2 (3a x) ; 6EI a a 2 (3x a) ; x a a x l (A.3) A.4 SHAFT UNDER TORSION l Outer dia. = d 2 Inner dia. = d 1 k eq = πg(d4 2 d4 1 ) ; G: Shear modulus of elasticity (A.4) 32l

319 Appendix A 31 A.5 ELASTIC BAR UNDER AXIAL LOAD l k eq = EA l ; A: Cross-sectional area (A.5)

320 APPENDIX B SOME MATHEMATICAL FORMULAE B.1 TRIGONOMETRIC IDENTITY sin( x) = sin x cos( x) = cos x sin(x ± y) = sin x cos y ± cos x sin y cos(x ± y) = cos x cos y sin x sin y ( ) ( ) x + y x y sin x + sin y = 2sin cos 2 2 ( ) ( ) x + y x y sin x sin y = 2 cos sin 2 2 ( ) ( ) x + y x y cos x + cos y = 2 cos cos 2 2 ( ) ( ) x + y x y cos x cos y = 2sin sin 2 2 tan(x ± y) = tan x ± tan y 1 tan x tan y e jx = cos x + j sin x; j = 1 sinh γ x = eγ x e γ x 2 cosh γ x = eγ x + e γ x 2 32

321 Appendix B 33 sin γ x = e jγ x e jγ x ; j = 1 2 j cos γ x = e jγ x + e jγ x ; j = 1 2 cosh 2 x sinh 2 x = 1 B.2 POWER SERIES EXPANSION e x = 1 + x + x2 2! + x3 3! + sin x = x x3 3! + x5 5! x7 + ; x in radians 7! cos x = 1 x2 2! + x4 4! x6 + ; x in radians 6! B.3 BINOMIAL EXPANSION (1 x) p = 1 + px p(p + 1)x2 + ; x < 1

322 APPENDIX C LAPLACE TRANSFORM TABLE f (t) f (s) δ(t); unit impulse function 1 u s (t); unit step function t n 1 (n 1)! u s(t); n = 1, 2, 3,..., e at u s (t) t n 1 e at (n 1)! u s(t); n = 1, 2, 3,..., sin(at)u s (t) cos(at)u s (t) 1 ω d e ξωnt sin ω d tu s (t) ω d = ω n 1 ξ 2 ω n e ξωnt sin(ω d t φ 1 )u s (t); ω d = ω n 1 ξ 2 ω d [ 1 ω ] n e ξωnt sin(ω d t + φ 1 ) u s (t) ω d ω d = ω n 1 ξ 2 ; φ 1 cos 1 ξ; ξ<1 1 s 1 s n 1 s + a 1 (s + a) n a s 2 + a 2 s s 2 + a 2 1 s 2 + 2ξω n s + ωn 2 s s 2 + 2ξω n s + ωn 2 ω 2 n s(s 2 + 2ξω n s + ω 2 n ) 34

323 REFERENCES Billah, K. Y. and Scanlan, R. H., 1991, Resonance, Tacoma Narrows Bridge Failure, and Undergraduate Physics Textbooks, American Journal of Physics Teachers, Vol. 59, No. 2, pp Binning, G. and Quate, C. F., 1986, Atomic Force Microscope, Physical Review Letters, Vol. 56, No. 9, pp Boyce, W. E. and DiPrima, R. C., 25, Elementary Differential Equations and Boundary Value Problems, John Wiley and Sons, Hoboken, NJ. Crandall, S. H., Dahl, N.C. and Lardner, T. J., 1999, An Introduction to Mechanics of Solids, McGraw-Hill, New York. Den Hartog, J. P., 1956, Mechanical Vibrations, Dover Publications (1984), New York. Griffin, J. H. and Hoosac, T., 1984, Model Development and Statistical Investigation of Turbine Blade Mistuning, ASME Journal of Vibration, Acoustics, and Reliability in Design, Vol. 16, No. 2, pp Griffin, J. H. and Sinha, A., 1985, The Interaction Between Mistuning and Friction in the Forced Response of Bladed Disk Assemblies, ASME Journal of Engineering for Gas Turbines and Power, Vol. 17, No. 1, pp Hutton, D. V., 1981, Applied Mechanical Vibrations, McGraw-Hill, New York. Newland, D. E., 24, Pedestrian Excitation of Bridges, Journal of Mechanical Engineering Science, Proc. Instn. Mech. Engineers., Vol. 218, Part C, pp Oh, J. E., and Yum, S. H., 1986, A Study on the Dynamic Characteristics of Tennis Racket by Modal Analysis, Bulletin of JSME, Vol. 29, No. 253, pp Petyt, M., 199, Introduction to Finite Element Vibration Analysis, Cambridge University Press, Cambridge, UK. 35

324 36 References Rao, S. S., 1995, Mechanical Vibrations, Prentice Hall, Upper Saddle River, NJ. Singer, N. C. and Seering, W. P. 199, Preshaping Command Inputs to Reduce System Vibration, ASME Journal of Dynamic Systems, Measurement and Control, Vol. 112, pp Sinha, A., 1986, Calculating the Statistics of Forced Response of a Mistuned Bladed Disk Assembly, AIAA Journal, Vol. 24, No. 11, pp Strang, G., 1988, Linear Algebra and Its Applications, Harcourt Brace Jovanovich Publishers, San Diego. Thompson, J. M. T., 1982, Instabilities and Catastrophes in Science and Engineering, John Wiley & Sons, Upper Saddle River, NJ. Thomson, W. T. and Dahleh, M. D., 1993, Theory of Vibration with Applications, 5th edn, Prentice Hall, Upper Saddle River, NJ.

325 INDEX Absorber, Absorber, optimal, Acceleration, 11, , Acceleration amplitude, 122 Accelerometer, 122, 125, , 136 Accelerometer design, Aerodynamic, Aerodynamic forces, Amplitude, 37, 44, 53 54, 55 57, 68, 72 Amplitude, maximum, 68, 112 Amplitude, peak, 113 Bandwidth, 16, 17, Base excitation, , 119, , 183 Beam, 273, , 291 Beam, fixed-fixed, Beam rigid, 4 Cantilever Beam, 3, 64, 271, 273, 275, 298, 3 Convolution Integral, , 162, 165, 17, 175, Critically Damped System, 46 49, 159 Damped spring mass system, 57, 7 Damper, 1, 22 23, 49, 129 pure rotational motion, 5 6, pure translational motion, 5, 1 11 Differential equation of motion, 3, Eigenvalue/Eigenvector, , 25, 24 Equivalent Mass Constants, Equivalent Stiffness Constants, 12, 14, 17, 19, 21, 34, 6 61, 64 65, Equivalent Damping Constants, 12 13, 23, 25, 6 61 Finite dimensional systems, Finite element analysis, 279, 295 Force Transmissibility, Fourier Series Expansion, , , 151, 179 Free vibration, 25 4 Frequency Response Function, 138, 173, Infinite dimensional systems, LaPlace Transformation, , 184 Logarithmic Decrement, 51, 53 55, 57 Longitudinal Vibration, 15, 237, 25, , 279 Mass, 5 8 Mass moment of inertia, 5 8, 2, 31, 5, 6, 65, 79, 189, , 262 Matrices, 187, , , 21 23, , , 24, 246 Modal decomposition, 227, 229, 235, 237, , 296 Mode shapes, 15, 192, 194, 196, 198, , 261, , 279, Motion planar, 6 8 pure rotation, 5 6, pure translation, 5 6,

326 38 Index Natural Frequencies, 194, 196, 239, 251, 267, 284, 293 Non-viscous energy dissipation, 72, Orthogonality, , 242, 244, Overdamped System, 47 49, 111, 119, , Particular integral, 84 85, 95, 97, 139, , 161, 21 Periodic force, 138, 149, 151, 179 Periodic function, 179 Planar motion, 7 Quality (Q) factor, 16 Rigid bar mass, 232 Rigid body, 6 Rotating unbalance, 113 Rotational motion, 5 9, 11, 19 Rotor-shaft system, 12 13, 14, , SDOF system, 2, 4, 14, 16, 21 22, 25, 37, 4, 45, 47, 57, 63, 72, 8, 82, 88, 98, 113, , , 165, 171, 186, 26, 215, 22, , 246 Simply Supported Beam, 13, , 3 31 Sinusoidal excitation, 129 Single degree of freedom systems, 1 2, 72, 138, 186 Spring, 8 1 pure rotational motion, 6 8, pure translational motion, 5 6, 1 11 Static equilibrium, 26 29, Stiffness, 6, 184, 242 Torsional Vibration, Transfer Function, 173, 175 Translational motion, Transverse Vibration, Undamped spring mass system, 82, 194 Underdamped System, 44, 51 52, 56, 158 Unit Impulse Function, , 175, 264, 276 Unit Impulse Response, , , 167, 175, 182 Vibration Absorber, Vibration Measuring Instruments, 72, Vibratory system, 5, 1 Vibrometer, , 136 design, 124 Viscous damper, 12, 22, 128, 131 equivalent, 128, 131 Wave equations,

327

328

329

330