LABORATORY EXPERIMENTS TESTING OF MATERIALS

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1 LABORATORY EXPERIMENTS TESTING OF MATERIALS 1. TENSION TEST: INTRODUCTION & THEORY The tension test is the most commonly used method to evaluate the mechanical properties of metals. Its main objective is the determination of properties related to the elastic design of machines and structures. Since the test is fully standardized and well established, one may state that it is a rapid way of obtaining the desired mechanical characteristics of materials. Basically, in a tension test a metallic specimen of specified dimensions according to relevant standards is pulled under the action of uniaxial forces applied at both ends until the specimen undergoes fracture. A typical tensile test specimen can be seen in Figure 1. The gage length corresponds to the effective length of the specimen over which the elongation occurs. Therefore, the initial length of the specimen is taken to be equal to the gage length L g. Turkish standards Institute (TSE) suggest a formula for the determination of gage lengths depending on the initial cross-sectional area of the specimen, A o : (where K = 11,3 for relatively long bars and K = 5,65 for relatively short bars) Figure 1. Typical Tensile Test Specimen In tensile testing of metals and polymers, an elastic deformation to a certain limit is observed first which is followed by a considerable plastic (permanent) deformation. The plastic deformation ends with the total fracture of the specimen. In contrast, in ceramics and some plastics, very little plastic deformation is observed before fracture. The first set of materials which can be plastically deformed are called ductile, where as the second set of materials are called brittle. While the applied uniaxial load is continuously increasing, the elongation in the specimen is recorded, such that at the end of the test a set of data for corresponding readings of load and displacement values is obtained. Recalling that the engineering stress is defined to be the ratio of the applied load to the initial cross sectional area, P/ AO and that the engineering strain is defined to be the ratio of the elongation to the initial length of the specimen, ENG L/ LO, one can plot the engineering stress-engineering strain curve. continuously decreasing due to the conservation of volume principle as the sample elongates during the test. Therefore the true value of stress during a tensile test should be defined as TRUE P/ A, taking A values to be instantaneous area values. Similarly, the engineering equation for strain takes it as granted that the gage length does not change, which is quite 1

2 unrealistic. A better equation for the strain values is given as, taking the change of gage length into account. The engineering values for stress and strain are most of the time appropriate for engineering purposes, which usually involve only elastic deformations, whereas the true values of stress and strain are needed to understand the behavior of materials in a better way. According to the fact that mild steel is the most common engineering material employed in structures, its stress-strain curve at a first sight turns out to be more significant. Typical low carbon steel would yield a stress-strain curve as in Figure 3, if tensile loads are applied at both ends at room temperature. ln l / O Figure 2. Stress-Strain Curve for typical low-carbon steel The engineering stress-strain curve can be best interpreted by dividing it into two parts, namely elastic and plastic portions. (i) Elastic Range As the specimen is loaded, first it behaves like a spring with a definite spring constant according to the so-called Hooke s Law: Table 1. Elastic Module of some common Engineering Materials (ii) Plastic Range Although extensive discussion of plastic deformation mechanisms is beyond the scope of this laboratory session, brief information regarding the mechanisms will be given here. As one loads the specimen beyond the yield point and then relaxes the load to zero, the material does not recover its initial dimensions completely, instead a permanent strain is observed. This 2

3 property characterizes the induced deformation as plastic and the portion of the stress-strain curve beyond the yield point is defined as the plastic range. Beyond the yield point, Hooke s Law is not applicable any more, since the stress needed to produce continued plastic deformation increases with increasing strain in the plastic region. This phenomenon is defined as strainhardening. The maximum point in the engineering stress-strain curve corresponds to the ultimate tensile strength, UTS of the material, which is at the same time the minimum necessary stress to cause the phenomenon known as necking. Necking is defined as a localized decrease in the cross-sectional area of the specimen, which results due to the imperfections which act as local stress raisers in the material. Upon application of the UTS, all further plastic deformation is concentrated in the necking region and rapid fracture follows. TASKS: 1. Measure initial and final sample dimensions such as l o, d o, l f, d f. 2. Construct both the engineering and true stress-strain curves on MS Excel. 3. Determine the following properties of the material: a. Modulus of elasticity. b. Yield strength, tensile strength, true tensile strength, breaking strength, true breaking strength. c. Discuss the mode of fracture by observing the fracture surface. 3

4 2. HARDNESS TESTING: INTRODUCTION & THEORY ROCKWELL HARDNESS TEST: This hardness test uses a direct reading instrument based on the principle of differential depth measurement. Rockwell testing differs from Brinell testing in that the Rockwell hardness number is based on an inverse relationship to the measurement of the additional depth to which an indenter is forced by a heavy (major) load beyond the depth resulting from a previously applied (minor) load. Initially a minor load is applied, and a zero datum position is established. The major load is then applied for a specified period and removed, leaving the minor load applied. The resulting Rockwell number represents the difference in depth from zero datum position as a result of the application of major load. The entire procedure requires only 5 to 10 s. Use of a minor load greatly increases the accuracy of this type of test, because it eliminates the effects of backlash in the measuring system and causes the indenter to break through slight surface roughness. The 120 sphere-conical diamond indenter is used mainly for testing hard materials such as hardened steels and cemented carbides. Hardened steel ball indenters with diameters 1/16, 1/8, 1/4, 1/2 in. are used for testing softer materials such as fully annealed steels, softer grades of cast irons, and a wide variety of nonferrous metals. In Rockwell testing, the minor load is 10 kgf, and the major load is 60, 100 or 150 kgf. In superficial Rockwell testing, the minor load is 3 kgf, and major loads are 15, 30 or 45 kgf. In both tests, the indenter may be either a diamond cone or steel ball, depending principally on the characteristics of the material being tested. Figure 3. Rockwell Hardness Testing Schematic 1. Depth of indentation under preliminary load (10 kg) 2. Increase in depth of indentation under additional load (140 kg) 3. Permanent increase of depth of indentation under preliminary load after removal of additional load, the increase being expressed in units of 0002 mm 4. Rockwell hardness HRC = 100 e 4

5 TEST LOCATION: If indentation is placed too close to the edge of specimen, the workpiece edge will bulge, and the hardness number will decrease accordingly. To ensure an accurate test, the distance from the center of the indentation to the edge of the specimen must be at least two and one-half diameters. An indentation hardness test cold works the surrounding material. If another indentation is placed within this cold worked area, the reading usually will be higher than the real value. Generally, the softer the material, the more critical the spacing of indentations becomes. However, a distance three diameters from the center of one indentation to another is sufficient for most materials. TASKS 1. Which Rockwell scale would you use for testing aluminum alloys? 2. Plot Hardness vs. Specimen (quenched, normalized, as received) data of the quenched specimens in the lab. 5

6 3. IMPACT TEST: INTRODUCTION & THEORY Two standardized tests, the Charpy and Izod, were designed and are still used to measure the impact energy, sometimes also termed notch toughness. The Charpy V-notch (CVN) technique is most commonly used impact test. For both Charpy and Izod, the specimen is in the shape of a bar of square cross section, into which a V-notch is machined (Figure 4a). The apparatus for making V-notch impact tests is illustrated schematically in Figure 4b. The load is applied as an impact blow from a weighted pendulum hammer that is released from a cocked position at a fixed height h. The specimen is positioned at the base as shown. Upon release, a knife edge mounted on the pendulum strikes and fractures the specimen at the notch, which acts as a point of stress concentration for this high velocity impact blow. The pendulum continues its swing, rising to a maximum height h, which is lower than h. The energy absorption, computed from the difference between h and h, is a measure of the impact energy. The primary difference between the Charpy and Izod techniques lies in the manner of specimen support, as illustrated in Figure 4b. Furthermore, these are termed impact tests in light of the manner of load application. Variables including specimen size and shape as well as notch configuration and depth influence the test results. Figure 4. (a) Specimen used for Charpy and Izod impact tests (b) A schematic view of an impact test apparatus The test consists of breaking by one blow from a swinging pendulum, under conditions defined by standards, a test piece notched in the middle and supported at each end. The energy absorbed is determined in joules. This absorbed energy is a measure of the impact strength of a material. 6

7 The test bar, notched in the center, is located on two supports. The hammer will fracture the test bar and the absorpted energy (in Joule) is an indication for the resistance of the material to shock loads. TASKS 1. Specify the fracture mode due to surface characteristics. 2. Describe impact energy. 3. Calculate speed of the hammer at impact. 7

8 4. HEAT TREATMENTS: INTRODUCTION & THEORY Heat treatment is a combination of timed heating and cooling operations applied to a metal or alloy in the solid state in such ways as to produce certain microstructures and desired properties. Annealing, Normalizing, Quench Hardening, Tempering, and Austempering are five of the important heat treatments often used to modify the microstructure and properties of steels. The iron-carbon (Fe-C) diagram is a map that can be used to chart the proper sequence of operations for thermomechanical and thermal treatments of a given steel. The iron-carbon diagram should be considered only a guide, however, because most steels contain other elements that modify the positions of phase boundaries. Use of the iron-carbon diagram is further limited because some heat treatments are specifically intended to produce nonequilibrium structures, whereas others barely approach equilibrium. Nevertheless, knowledge of the changes that take place in a steel as equilibrium is approached in a given phase field, or of those that result from phase transformations, provides the scientific basis for the heat treatment of steels. Figure 5 shows the Fe-C equilibrium diagram for carbon contents up to 7%. Steels are alloys of iron, carbon and other elements that contain less than 2% carbon-most frequently 1% or less. Therefore, the portion of the diagram below 2% carbon is of primary interest for steel heat treatment. This part of phase diagram is also given in Figure 6. Figure 5. The Fe-C equilibrium diagram up to 7% carbon 8

9 Figure 6. The Fe-C equilibrium diagram up to 2% carbon Time Temperature Transformation (TTT) and Continuous Cooling Transformation (CCT) Diagrams that define the transformation of austenite as a function of time at constant temperatures are referred to as time-temperature-transformation (TTT) diagrams. A TTT diagram for 1080 steel is presented in Fig. 7 in connection with the description of the nucleation and growth kinetics of pearlite formation. The TTT diagram for eutectoid steel with negligible alloy content is quite straightforward. Only pearlite forms above the nose of the IT diagram, and only bainite forms below the nose. The curves defining the beginning and end of pearlite or bainite formation are the major features of the diagram. 9

10 Figure 7. Time-temperature-transformation diagram for 1080 steel Many of the heat treatments performed on steel are carried out by continuous cooling transformation rather than by time temperature transformation, and as a result, diagrams that represent the transformation of austenite on cooling at various rates have been developed. The latter type of diagram for a given steel is referred to as a continuous cooling transformation (CCT) diagram. Generally, continuous cooling shifts the beginning of austenite transformation to lower temperatures and longer times. CCT diagram for 1080 steel is shown in Figure 8. Figure 8. Continuous cooling transformation diagram for 1080 steel 10

11 TTT and CCT diagrams generally contain 3 distinct lines: Nucleation Lines: Beginning of the Phase Transformation. 50% Completion of the Phase Transformation. ~100% Completion of the Phase Transformation. Pearlite is composed of alternating layers of Ferrite and Cementite. Pearlite layer thickness is a function of quenching temperature (and Rate): 1. Thick layers (Coarse Pearlite) are produced at a quench temperature of ~650 C. 2. Thin layers (Fine Pearlite) are produced at a quench temperature of ~500 C. 3. A particulate structure (Bainite) is produced at a quench temperature of ~350 C. Coarse Pearlite is relatively soft and ductile but is often utilized as a low-cost structural material. Its' properties depend largely on carbon content. Fine Pearlite is generally a mid-range steel with good strength and ductility characteristics; actual properties are a function of carbon content. Bainite is stronger and slightly less ductile than Fine Pearlite. Overall Bainite is relatively cheap steel which combines good strength with moderate ductility. If Austenite is quenched (cooled rapidly) at a rate such that the knee of the TTT/CCT diagram is avoided (rapid quench), a diffusionless transformation into Martensite occurs where: M S : M 50 : M 90 : Start of the Transformation. 50% Transformed into Martensite. 90% Transformed into Martensite. The Martensite transformation involves sudden reorientation of the carbon atoms in the FCC Austenite structure to a Body Centered Tetragonal (BCT) structure without allowing time for the diffusion of atoms to their lowest-energy equilibrium positions. The result is a high internal energy material that is very strong, hard, brittle and susceptible to rapid crack growth. TASKS 1. Briefly explain Normalizing, Quenching, Tempering, Austempering and Annealing processes. 2. Discuss the expected microstructures and properties for each heat treatment process. 3. Discuss the relationship between heat treatment and the resulting hardness. 11

12 REFERENCES William D. Callister, Jr., Materials science and engineering: an introduction, Wiley, (New York: 2000). George Krauss, STEELS Processing, Structure, and Performance, ASM International (2005) ASM Metals Handbook 12