Impact Response and Damage Evaluation of Grid Stiffened Composite Panels

Transcription

1 Impact Response and Damage Evaluation of Grid Stiffened Composite Panels Prakash Jadhav, Ph.D Candidate, Dept. of Mechanical Engineering, University of Mississippi University, MS P. Raju Mantena, Professor, Dept. of Mechanical Engineering, Composite Structures and Nano- Engineering Research, Carrier 201-D, University of Mississippi University, MS ABSTRACT The energy absorption characteristics of E-glass/PP grid stiffened composite panels under high velocity transverse impact loading has been investigated. Experimental results were validated using finite element analysis for both the skin and rib side loading. The results of test and simulations show that grid stiffened composite panels absorb lot of energy under dynamic impact load without catastrophic failure. The panels absorbed more energy when loaded on the skin-side than on the rib-side. The specific energy absorbed under dynamic impact loading was far greater than that for quasi-static loading. Vibration testing was used to analyze the cause of vibratory component in the impact response load-time history. Low velocity induced progressive impact damage in grid stiffened composite plates was also evaluated in a separate study using the vibration response measurement technique. This technique appears to be partially successful in detecting the various levels of induced impact damage in grid stiffened composites. INTRODUTION The transverse loading of grid-stiffened composite panels has significance in automotive crashworthy applications where optimizing energy absorption is stipulated along with weight-critical restrictions. For example, the results of a previous study demonstrate that the placement of a grid-stiffened composite panel inside a car door with the ribs facing the side-impact event would result in greater energy absorption, thus offering superior protection to the occupants for the same panel weight [1-4]. The phenomenon of failure and energy absorption in composites is complicated, and this research is just a beginning for understanding the behavior of grid-stiffened composites under dynamic load conditions. This will help in recommending isogrid composite panels for use in applications where energy absorption is important, such as in the side doors of automobiles. Energy absorption characteristics of grid-stiffened E-glass/PP composite plates (shown in Figure 1a) under transverse quasistatic loading were presented by Gan and Gibson, both experimentally and analytically for the short range of displacements [1]. The authors have been further investigating the energy absorption in grid-stiffened composite panels (shown in Figure 1b) under transverse quasi-static loading for the longer range of displacements [2-4]. The authors have performed quasi-static tests on a grid-stiffened composite panel under transverse quasi-static loading using loading and boundary conditions similar to those used by Gan. The observations of various modes of failure of isogrid panels during the quasi-static test event were discussed in the authors previous publications [2-4]. Experimental results show that the grid-stiffened composite panels failed in a gradual and forgiving way in a sequence of relatively small load drops. No catastrophic load drops were observed in the grid structures over the range of displacements investigated here. The specific energy absorbed, (SEA, given by the nonrecoverable energy divided by mass of isogrid panel between supports) computed from the area under load displacement curve to failure for rib-side loading is more than for skin-side loading. However, the maximum peak load was more for skin-side loading than for rib-side loading, and the energy absorption occurred over a larger displacement range for loading on the ribside. The experimental results obtained by the authors and Gan were validated and compared using finite element analysis. A user-defined program was utilized in Ansys to simulate composite damage enabling validation of the experimental results. All of this previous research on grid-stiffened composites has mainly focused on design, manufacturing, static-loading and other factors. Very little work has been done, however, on the energy absorption of grid-stiffened composites under dynamic impact load. The main focus of this paper is to investigate the impact energy absorption of these E-glass/PP isogrid composite panels at high velocities (Federal Safety Standards FMVSS214 [5] recommended velocity to simulate side impact in automotive vehicles-14.8 m/s) both experimentally and analytically. Another independent study of the induced progressive impact damage assessment in an isogrid plate was performed using modal analysis technique. In this case, baseline frequencies and mode shapes of the isogrid plate were obtained initially, and at subsequent stages of induced damage.

2 EXPERIMENTS One of the popular thermoplastic matrix composite materials, E-Glass/polypropylene (PP) made by Vetrotex, Inc. was used for making isogrid plates consisting of co-mingled E-glass and Polypropylene fibers. Four laboratory scale 300 x 250 x 7.5 mm composite isogrid plates (Figure 1a) were fabricated using co-mingled E-glass/polypropylene yarn (Twintex by Vetrotex) in a thermoplastic stamping process [1]. The ribs are made of 30 unidirectional Twintex E-Glass/PP rovings, and the skin is made of four woven Twintex E-Glass/PP fabrics. One of the isogrid composite plates was used for the induced impact damage evaluation using modal analysis techniques. As discussed by authors in their previous publication [2-4], the other three fullsized plates were separated into two parts of 125 x 300 x 7.5 mm each retaining the same span as used in quasi-static tests, to facilitate impact loading inside the test machine. Figure 1b shows an example of two symmetric half-size panels and a central longitudinal beam obtained by machining along a full-size isogrid plate. Out of these three pairs of newly created halfsize isogrid panels, a pair was used for quasi-static tests [2-4], and the remaining two pairs were used to investigate the dynamic impact response. By reducing the length of the loading cross-rod to 125 mm it was possible to integrate it with the impact system s load cell. A fixture was fabricated that could fit at the base of the impact machine, with the half-size isogrid panels mounted freely for performing dynamic impact tests. This fixture was designed in such a way that it could be used for both quasi-static and dynamic testing and would provide a three-point-bend boundary condition. A. Dynamic impact test The DYNATUP Model 8250 Instrumented Impact Test Machine was used for characterizing the crashworthiness of gridstiffened composite panels. The test system consists of a drop weight tower coupled with signal conditioning/processing units and software (GRC 930-I) for data acquisition and analysis. The outputs of the test machine include a trigger signal used for starting data acquisition and determining impact velocity, as well as, recording the load signal generated by the tup. The area under the load-displacement curve is computed at regular intervals of time/displacement to determine the energy absorbed by the specimen. Numerical values of the impact energy and velocity; the maximum load; energy, time and displacement to maximum load; and total energy data are available in both graphical and ASCI format for exporting to a spreadsheet program (MS-EXCEL). This particular model is capable of generating 0.6 to 303 Joules of impact energy within a velocity range of 0.61 to 3.66 m/s with different combinations of drop-weights in the gravity mode of operation. In the pneumatic-assist mode, impact energy levels of 16.2 to 840 Joules can be generated with an upper velocity range of 3.66 to m/s, when standard impact fixtures (izod and hemi-spherical) are used. This system has been used extensively by the authors for characterizing the impact properties of different resins, composites and foams [6-7], and was used for characterizing the half-size isogrid panels. One of the objectives of dynamic impact testing was to simulate similar boundary conditions used for the quasi-static tests of full-size plates [1-4], to facilitate comparison. As seen in Figure 2, the same fixture used in the quasi-static test, was mounted at the impact machine base with the half-size isogrid panel resting freely on it. The 125 mm cross-rod along with a 6.53 kg crosshead weight integrated with a 44 kn load cell can also be seen. Dynamic impact tests of the half-size isogrid composite panels were performed at a velocity of 9 m/s using the Dynatup 8250 Instrumented Impact Machine in the pneumatic mode using the fixture and set up described earlier. Because of the machine and air pressure limitations, FMVSS214 suggested velocity of 14.8 m/s could not be obtained. For the dynamic impact tests, since only four half-size panels were available, two were used for the skin-side and two for rib-side impact at a velocity of 9 m/s. B. Induced Impact damage evaluation using modal analysis Vibration response monitoring techniques were previously used by the authors for damage assessment in wooden structures [8]. The same technique has been applied here as an independent study to assess induced impact damage in isogrid plates. It should be noted that, three of the four supplied isogrid plates were used for quasi-static and dynamic tests and the remaining fourth plate has been used for damage assessment study using modal analysis technique (set up shown in Figure 3), in which baseline frequencies and mode shapes were obtained initially and at subsequent stages of induced damage. Modal data acquisition was performed using the Model DSPT SigLab from Spectral Dynamics Corporation. The accelerometer is a PCB Piezotronic Model #352C43 uni-axial accelerometer with a frequency range between 1 Hz to 8 khz and a sensitivity of 9.52 mv/g. Model #086B01 PCB Piezotronic Impulse Force Hammer with a force range between 0 to 450 N with plastic tip was used for exciting the plate. The SigLab system was controlled through interfacing with a laptop computer via a PCMCIA card connector. The FRF data was then exported from SigLab to the ME scopeves TM vibro-acoustic software for postprocessing. The isogrid composite plate was discretized into 81 equally spaced grid points and placed on a foam pad simulating free-free boundary conditions. The impact hammer was used to tap each of the 81 grid points sequentially. Modal parameter estimation, curve fitting and animation of mode shapes were performed using the Vibrant Technology ME scopeves TM software. Two levels of damage were then induced on this plate, each level at a different location, using a Dynatup Model 8250 instrumented impact test system. A 12.5 mm dia. hemispherical sheet penetration impactor was used for inducing damage and the load response measured using a 45 kn load cell. The total mass of the setup, which consists of the crosshead weight, tup, tup bolt, extension rod and penetration fixture is 6.78 kg. The composite plate was placed on a fixture and held tightly using clamp vices. The 1 st level and 2 nd level damage were induced at a point on the skin and at another point on rib joint respectively. 1 st level damage was induced by dropping the impactor head (in gravity mode) from a height of meter with impact energy of Joules. The 2 nd level damage was induced by dropping the impactor head from a height of 1 meter with impact energy of Joules. The load deflection curve and other impact parameters like the impact energy and impact

3 velocity were obtained from the GRC Dynatup 930 software [10]. Figure 4 shows a rib side picture of the isogrid plate after impact where the 2 nd level damage is quite noticeable. For each level of damage i.e. baseline, 1 st level damage and 2 nd level damage, a complete vibration modal analysis was performed. FINITE ELEMENT ANALYSIS Finite element analysis was used to validate the experimental results and simulate other grid geometry variations. For performing dynamic impact simulation, a model of the half-size isogrid panel was created in Ansys/LS-Dyna using explicit elements, shell 163 for the skin and solid 164 for the ribs (6 elements along the thickness) as seen in Figure 5. The models (truncated cylinders for ease of brick type meshing) of a loading bar and two support bars (assumed to be made of steel) are also created using solid 164 elements to simulate the same boundary condition as experiments. Separate nodal components were created for the loading bar, support bars and the isogrid panel. Automatic node to surface contact (which is supposed to be most efficient when a smaller surface comes in contact with a larger one) was defined for pairs of components, a loading bar-isogrid panel and a support bars-isogrid panel. Because of large difference in stiffness of contacting materials, large penalty factor in the range of 2 to 3 was used in the analysis which accommodates for sliding at contact surfaces. The support bars were constrained to move in all the directions, however, the loading bar was allowed to move freely only in the direction of transverse loading. The material properties used, including orthotropic elastic properties and failure strengths, were obtained from from tests [1] and from the manufacturer s website [9] as shown in Table 1. Chang and Chang s Progressive Damage Material Model for Composites (LS-Dyna) was used here to simulate the impact test event [11-12]. The bulk modulus was approximately calculated using fundamental equations that relate it to other known properties. Shear factor was assumed to be around 0.5. An initial velocity of 9 m/s and approximate acceleration levels of 100 m/s 2 (in transverse direction), were assigned to the loading bar to perform the impact analysis. The solution time of 10 milliseconds (similar to experimental time) was set for performing the explicit dynamic analysis. RESULTS AND DISCUSSIONS A. Dynamic impact test results A typical load-displacement plot obtained from a dynamic impact test of an isogrid panel when loaded on the skin-side is shown in Figure 6, and on the rib-side shown in Figure 8. The load-displacement responses for impact tests are quite different from quasi-static test curves. The stiffness (slope of initial part of load-displacement curve) of panels obtained under dynamic impact load is almost about 20 times more than that under quasi-static loading. The peak load seems to be very high (almost 3 times) compared to that in quasi-static and there appears to be a vibratory component in the impact test curves. Two- Hundred-Points-Moving-Average Method was used to smooth out these load fluctuations. The half-size isogrid composite panels did not fail abruptly or break into two separate parts in the impact test. When panels were impacted on the skin-side, an impactor rebounding, skin buckling, rib joint cracking and fiber separation in the ribs were observed without catastrophic failure. For panels impacted on the rib-side, the similar impactor rebounding and rib joint cracking were observed without catastrophic failure, but the skin remained intact and skin-rib separation was observed at the central rib region. The total specific energy absorption (recoverable and non-recoverable energy/mass of the specimen between supports) of the half-size isogrid panels under quasi-static and dynamic loading conditions is tabulated in Table 2. The total SEA values are observed to be higher for skin-side loading compared to that of rib-side loading under dynamic impact loading conditions (both up to 52 mm transverse displacement and up to final failure). The total SEA values are also higher in all the cases of dynamic loading compared to that in corresponding quasi-static load conditions. B. Oscillations in impact load-time response Large oscillations were observed in the load-time response when the half-size isogrid panels were impacted at a velocity of 9 m/s. A study was conducted to analyze the possible causes of these oscillations, which may be due to the simply supported boundary condition and excitation of some of the structural resonances of the impactor, isogrid panel or both combined. Fast Fourier Transform Analysis of the load-time history was performed using Matlab software to obtain the frequency content. Because the impact event was finished in only about 9 milliseconds, the FFT analysis provided high frequency contents up to 250,000 Hz. Due to data acquisition limitations of the impact test system, frequency resolution was observed to be coarse in the zoomed power spectra over the desired Hz range. Some of the frequency peaks identified from the power spectra were approximately at 200, 450, 650, 1000, 1250, 1600, 1800 Hz (shown in column 1 of Table 3). To determine natural frequencies of the isogrid panels, impactor or both combined, they were excited with a PCB hammer, and the response was picked up using a PCB miniature accelerometer. The boundary conditions were similar to that of the impact test event. Siglab multi-channel data acquisition software was used to acquire the flexural and extensional vibration response [8]. This study showed that the observed load-time response oscillations under dynamic impact may be attributed to any of the structural resonances of the isogrid panel, impactor or both combined. As shown in Table 3, some of the structural resonances (highlighted in bold face) were found to be close to these oscillating frequencies. C. FEA Results Ansys/LS-Dyna was used for simulating the transverse impact test on the half-size isogrid panels and validating the outcome with the experimental results as described earlier. The software runs took a long time (approximately 9 to 11 hours for each run) to converge, but provided satisfactory results. Figures 7 and 9 show sample contact force-displacement plots obtained for the skin-side and rib-side loaded half-size isogrid panels, respectively. The fluctuations observed in the FEA loaddisplacement plots are similar to those obtained experimentally. This indicates that the simply supported boundary condition

4 and sample resonances may be the reason for appearance of fluctuations in the load-displacement response. The same fluctuations are observed in the rib-side loaded panels too. The total energy absorbed was calculated by computing the area under the curve for both skin and rib-side loading. The total specific energy absorption was also computed and compared with that of the experimental data in Table 2. The FEA and experimental results in terms of the total SEA (up to 52 mm displacement) compare well within reasonable limits. The slight differences in the results may be attributed to factors such as non-uniformity in the skin and rib dimensions generated during the fabrication, defects introduced during the machining of the half-size panels from full-size plates, and approximations (assumptions) made in the finite element analysis. Due to machine limitations experimental results were obtained only up to 52 mm displacement, however, the FEA results show energy absorption occurred for a longer range of displacement under dynamic impact load for both skin and rib-side loading. The total SEA values at final failure show the same trend, that is, the total SEA under the skin-side is higher than that under rib-side. The total SEA values at final failure are far greater than that under quasi-static. Figures 10(a), (b) and (c) show representative stress distribution patterns (skin side loading) observed in FEA for half-size panels at the initial failure point (around 4 mm deflection) under dynamic impact loading. The approximate location of initial failure point (where the load reaches its peak level) was found by performing a series of runs at different solution times in the lower displacement range and observing corresponding stress levels. Figure 10(a) shows panel edge regions where longitudinal compressive stress reached its maximum limit at the rib/skin interface, and Figure 10(b) shows the edge regions areas where transverse tensile stresses reached maximum limit. The transverse tensile stress limit was observed to have reached slightly earlier than the longitudinal compressive stress limit. Figure 10(c) shows the in-plane shear stress distributions in the cross-ribs at this 4 mm deflection. At increased deflections, the failed elements showed lower stress levels while elements in some other areas reached their stress level limits. Figures 11(a), (b) and (c) show representative stress distribution patterns observed in FEA for half-size panels (skin-side loading) at a deflection of 52 mm, which was also the experimentally obtained maximum deflection under dynamic impact loading. The high stress levels observed in rib joints near the central region might have caused localized shearing and microbuckling, as observed in the failed specimens. High in-plane shear stress levels accompanied by compressive stresses in the longitudinal rib could have caused skin-rib separations at the peak load. At this displacement, high longitudinal strain levels generated in the central rib region caused tensile fiber failure at the bottom surface of the central rib, as indicated by fiber separation in the failed specimens. Skin buckling in central region may be explained by the presence of high longitudinal compressive stresses under skin-side loading. A similar analysis was performed on half-size isogrid panels loaded on the rib-side and verified with experimental results at 52 mm deflection [10]. The behavior of grid-stiffened composite panels can thus be predicted effectively under dynamic impact conditions using finite element analysis. D. Evaluation of induced impact damage Modal frequencies were obtained after post processing the acquired data for each damage level using ME scopeves TM software. Table 4 shows progressive drops in the first 10 modal frequencies of the isogrid plate due to induced localized impact damage. However, comparison of the mode shapes of isogrid plates for different levels of damage did not show much change. The drop in frequencies can at best be correlated to some loss of structural integrity due to the induced impact damage. Quantitative techniques described by Khoo et al [8] can be further used to locate the damaged region in the isogrid plates, by monitoring the residues and damping data obtained from modal analysis. SUMMARY Grid-stiffened (isogrid) E-glass/PP composite panels were tested under dynamic impact conditions, and the results were validated in finite element analysis. The panels absorbed more energy when loaded on the skin-side than on the rib-side. The total SEA values under dynamic impact loading are far greater than that for quasi-static loading. Vibration tests were also performed, and it was concluded that the oscillations observed in the impact load-time response could be attributed to the structural resonances of the impactor, panel or both. Experimental modal analysis was also used for evaluating induced impact damage in isogrid plates by monitoring changes in modal frequencies. Parametric study (FEA) of optimizing the grid geometry of isogrid composite panels to maximize specific energy absorption under transverse quasi-static and dynamic impact load is underway. ACKNOWLEDGEMENT The authors would like to thank Professor Ronald Gibson at Wayne State University for providing the isogrid composite plates and technical input during these investigations. REFERENCES: 1. Changsheng Gan, Ronald Gibson, Golam Newaz. Analytical/Experimental Investigation of Energy Absorption in Grid- Stiffened Composite Structures under Transverse Loading. Experimental Mechanics. Vol. 44(2), pp April, Prakash Jadhav, Raju Mantena, Ronald Gibson. Energy Absorption and Damage Evaluation of Grid-stiffened Composite Panels under Transverse Loading. Composite: Part B Engineering. (In Press).

5 3. Prakash Jadhav, Raju Mantena, Ronald Gibson. Analytical and Experimental Investigations of the Impact Response of Grid-stiffened E-Glass/Polypropylene (PP) Composite Panels. American Society of Composites-Annual Technical Conference, Atlanta, GA, Paper IE-2, Published on CDROM. October P.Raju Mantena, Prakash Jadhav. Impact and Dynamic Analysis of Grid-stiffened Composite Panels. 11 th Annual International Conference on Composites/Nano-Engineering- Hilton Head, South Carolina, Published on CDROM. Aug Federal Motor Vehicle Safety Standards for Side Impact Protection (FMVSS214) - websitehttp:// 6. P. Raju Mantena, Richa Mann, Chadrashekar Nori. Low-Velocity Impact Response and Dynamic Characteristics of Glass-Resin Composites. Journal of Reinforced Plastics. Vol. 20(6), pp P. Raju Mantena, Richa Mann. Impact and Dynamic Response of High Density Structural Foams used as Fillers inside Circular Steel Tube. Composite Structures. Vol. 61, pp ; 8. Lay Menn Khoo, P.Raju Mantena and Prakash Jadhav, Structural Damage Assessment Using Vibration Modal Analysis, Journal of Structural Health Monitoring, vol. 3(2), pp Vetrotex web site: URL= 10. Jadhav PK. Analytical/Experimental Investigation of the Impact Response of Grid-stiffened E-glass/Polypropylene Composite Panels. Ph.D Dissertation (in Progress), Department of Mechanical Engineering, University of Mississippi. 11. Ansys/LS-Dyna Software Reference Manual 12. Chang FK, Chang KY. A Progressive Damage Model for Laminated Composite Containing Stress Concentrations. Journal of Composite Materials. Vol. 21, pp September 1987 (a) (b) Figure 1: (a) Full-size (300 x 250 x 7.5mm) laboratory scale E-Glass/PP isogrid composite plate; and (b) Two half-size panels and a central rib/skin beam machined from one full-size isogrid plate

6 Figure 2: Half-size isogrid panel at base of impact machine under dynamic loading on rib-side Figure 3: Experimental set up for flexural vibration testing

7 1 st level (on skin) 2 nd level (on rib joint) Figure 4: Location of induced damage on isogrid plate Figure 5: Ansys FEA dynamic model of half size isogrid panel

8 3 Dynamic Impact Experimental Result - Skin-side loading load load (200 moving avg) Energy Load kn Energy Nm Displacement mm 0 Figure 6: Experimental impact test result of half-size isogrid panel loaded on skin-side 3 Dynamic Impact FEA Result- Skin-side loading Load kn Displacement mm Figure 7: FEA impact simulation of half-size isogrid panel loaded on skin-side

9 Load kn Dynamic Impact Experimental Result- Rib-side loading load load (200 moving avg) Energy Displacement mm Energy Nm Figure 8: Experimental impact test result of half-size isogrid panel loaded on rib-side 3 Dynamic Impact FEA Result-Rib-side loading Load kn Displacement mm Figure 9: FEA impact simulation of half-size isogrid panel loaded on rib-side

10 (a) (b) (c) Figure 10: FEA impact simulation at initial failure point of around 4 mm deflection showing (a) longitudinal compressive stress concentrations in rib/skin interface region at the edges, (b) transverse tensile stress concentrations in rib/skin interface region at the edges, and (c) in-plane shear stress concentrations in the cross ribs.

11 (a) (b) (c) Figure 11: FEA impact simulation at 52 mm deflection showing (a) longitudinal stress concentrations in central region and near the rib joints, (b) in-plane shear stress concentrations in the cross ribs, and (c) total longitudinal strain concentrations along the central longitudinal rib and at rib joints.

12 Table 1: Material properties of E-glass/PP Composite Panels used in FEA Material Unidirectional Twintex E-glass/PP Woven Twintex E-glass/PP E1 (GPa) E2 (GPa) E3 (GPa) G12 (GPa) Material Properties G13 (GPa) G23 (GPa) ν12 ν13 ν23 ρ kg/m v f Intrinsic Strength Properties Longitudinal Direction Transverse Direction Shear Tensile SL+ (MPa) Compressive SL - (MPa) Tensile ST+ (MPa) Compressive ST- (MPa) SLT+ (MPa) Unidirectional Twintex E-glass/PP Woven Twintex E-glass/PP Table 2: Total SEA of isogrid panels under quasi-static and dynamic impact loading Total Specific Energy Absorption (Nm/kg) Type of loading Quasi-static* Experimental At 52 mm transverse displacement Dynamic Impact Experimental Dynamic Impact FEA Quasi-static* Experimental Up to final failure Dynamic Impact - FEA Skin-side Rib-side *Reference 2

13 Table 3: Vibration response data for characterizing impact load-time oscillations Resonant Frequencies (Hz) Power Spectra (from impact Impactor Isogrid Panel Impactor and Panel Joint System load-time history Flexure Extensional Flexure Panel Impactor Flexure Extensional Flexure Table 4: Drop in frequencies (Hz) for isogrid plate with induced damage Modes Baseline Induced damage 1 st level 2 nd Level