Technology & Engineering
Tensile Testing
Tensile testing is a method used to determine the mechanical properties of materials by subjecting them to tension. It involves applying a controlled force to a sample and measuring the resulting deformation and ultimate failure. This test helps engineers and researchers understand how materials behave under tension, which is crucial for designing and manufacturing various products.
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10 Key excerpts on "Tensile Testing"
- David C. Jiles(Author)
- 2007(Publication Date)
- CRC Press(Publisher)
99 7 Mechanical Testing Methods In this chapter, we consider the various tests for determining the mechanical properties of materials. These include tensile tests using application of load on a specimen and measurement of the resulting deformation, hardness tests using surface indentation methods, crack tests using liquid penetrants and other related methods, and fracture tests using impact methods. Some of these tests are destruc-tive and some are nondestructive. 7.1 Tensile Testing Tensile tests usually apply controlled loads to samples and measure the deformation (strain) of the sample. The stress can be determined exactly by knowing the cross-sectional area of the sample, known as the true stress, but more often the approx-imate stress is calculated assuming that the cross-sectional area remains the same throughout. This is known as the engineering stress. Tensile tests are used to provide mechanical property data of materials. The tests are nondestructive up to the yield point. In this region, the stress–strain curve shows a linear relationship, and the slope of the graphs of stress against strain is the elastic modulus. Beyond the yield point, the material suffers permanent plastic deformation, and at the ultimate tensile strength the material will fail. The equipment used for tensile tests may be screw- or hydraulically-driven. An example of a screw-driven system is shown in Figure 7.1. 7.1.1 S TRESS –S TRAIN C URVE A typical stress–strain curve that would result from such a tensile test is shown in Figure 7.2. Several mechanical properties of a material can be obtained from a stress–strain curve. These include elastic modulus, yield strength, ultimate tensile strength, ductility, resilience, and toughness [1]. From Figure 7.2, it can be seen that at low stresses, below the yield strength, the variation of strain with stress is linear, meaning that the elastic modulus remains constant.
eBook - ePubArt Conservation
Mechanical Properties and Testing of Materials
- W. (Bill) Wei(Author)
- 2021(Publication Date)
- Jenny Stanford Publishing(Publisher)
Chapter 4Mechanical TestingIn Chapters 2 and 3 , we introduced the basic concepts of the static and dynamic mechanical properties of materials. In this chapter, we will introduce several methods for measuring these properties, including tensile, fatigue and vibration testing. In two exercises, a simple tensile test will be described which the reader can perform at home or in the conservation studio. We will also briefly introduce test methods for measuring special properties such as shear and peel strength, and wetting as related to adhesion strength.Examples of international standards for the mechanical testing of particular types of materials and load geometries will be introduced for several of the test methods. Many are those are published by ASTM International (formerly known as the American Society for Testing and Materials), ISO (International Organization for Standardization) and CEN (or EN—European Committee for Standardization). These are very often quite similar or even the same, but every country in the industrialized world has its own variations on these standards.It should be noted that a standard, in the general use of the term, is not necessarily the “right” way to test something. It is a compromise, often developed by commercial competitors who are forced to accept the reality that the consumer wants to be able to compare products tested in the same way. For testing purposes in conservation practice, there may be cases where one needs to perform a (mechanical) test some other way than written in a standard. The standards do, however, provide guidance to make sure that test results are scientifically valid, for example, in how test specimens are designed or loaded.4.1 Tensile Testing4.1.1 Tensile Testing EquipmentThe test used to measure force and deformation, and thus stress and strain as described earlier in Fig. 2.1 is known as a tensile test. The equipment used for a tensile test is shown schematically in Fig. 4.1 . This is a so-called screw-driven Tensile Testing system which consists of a stiff frame with a crosshead which is driven up and down using two large motor driven screws. Screw-driven testing systems are preferred over so-called hydraulic systems (see Section 4.3 and Fig. 4.9 ) for Tensile Testing. Such systems are much stiffer than hydraulic systems, a property which is especially important if the E
- C. Suryanarayana(Author)
- 2011(Publication Date)
- CRC Press(Publisher)
315 9 Tensile Testing 9.1 INTRODUCTION The. mechanical. properties. of. materials. are. very. important. in. determining. their. applications. .For.example,.one.may.be.interested.in.choosing.a.material.for.load-bearing. applications . . In. that. case,. a. material. with. high. strength. is. required . . Alternately,.if.one.is.interested.in.a.material.for.easy.fabrication,.then.the.material. should.have.high.ductility . .Thus,.for.different.applications,.an.engineer.will.have.to. select.a.material.with.an.appropriate.combination.of.properties . .Even.though.people. talk. of. materials. being. strong. or. weak,. a. quantitative. comparison. of. the. strength. properties.is.essential.for.an.intelligent.choice.of.suitable.material . The.strength.of.a.material.can.be.understood.as.its.resistance.to. plastic deforma-tion . .It.may.be.measured.by.applying.a.static.load.uniformly.over.a.surface.cross. section.and.monitoring.the.dimensional.changes.of.the.specimen . .The.load.may.be. applied.in.four.basically.different.ways— tension ,. compression ,. shear ,.and. torsion . . These.four.different.modes.are.schematically.shown.in.Figure.9 .1. .In.tension,.a.load. is.applied.in.such.a.way.that.the.load.pulls.the.specimen.apart.and.the.specimen. expands.in.the.direction.of.application.of.load . .In.compression,.the.load.is.applied.so. as.to.contract.the.specimen.in.the.direction.of.application.of.load . .In.shear,.the.load. is. applied. in. opposite. directions. on. two. parallel. faces. of. a. specimen . . In. torsion,. which.is.a.variation.of.pure.shear,.a.specimen.is.twisted.by.a.torque,.which.produces. a.rotational.motion.about.the.longitudinal.axis.of.one.end.of.the.specimen.relative.to. the.other . .Even.though.the.load.may.be.applied.in.many.different.ways,.it.is.the.shear. force.which.causes.plastic.deformation.in.a.material . .Consequently,.irrespective.of. the.way.in.which.the.load.is.applied,.it.is.the.shear.component.which.needs.to.be.
eBook - PDFStrength of Materials & Structures N5
TVET FIRST
- P du Toit, A Knott(Authors)
- 2022(Publication Date)
- Macmillan(Publisher)
c) Tensile stress testing We use a Tensile Testing machine to carry out a tensile test on test pieces that comply with standards of the South African Bureau of Standards (SABS). ductile material brittle material crack shear Figure 1.32: How ductile and brittle materials respond to a compressive force 24 TVET FIRST Module 1 The hydraulically driven tensile tester shown in Figure 1.33 uses an upper moveable jaw and a lower fixed jaw to put a load on the test piece. The load on the test piece gradually increases until it fractures. hydraulic unit test piece lower fixed jaw upper movable jaw test results Figure 1.33: A tensile tester Before testing, you have to measure the gauge length and diameter of the test piece (see Figure 1.34). Only then do you insert the test piece into the grips or jaws. During the test, the machine plots the load-extension graph. Note that once you start the test, it should be carried out without any pause. If you have Internet access, watch the following videos on Tensile Testing: ● https://www.youtube.com/watch?v=ipDZska 5VN4 | Strength of Materials: Tensile test demonstration ● https://www.youtube.com/watch?v=0mrY3f_H7mw | Tensile test diagram (Strength of materials) - Mechanical Engineering ● https://www.youtube.com/watch?v=pS2HSTwHpSw |Laboratory of Strength of Materials: Tensile Testing See it online 1.1.5 Plotting and interpreting stress and strain data from the tensile test The object of the tensile test is to obtain values of some, or all, of the following properties of a material: • Elastic limit. • Yield point. • Proof stress. • Modulus of elasticity. • Ultimate strength. • Percentage elongation (ductility). • Percentage reduction in cross- sectional area. hydraulic: relating to a liquid moving in a confined space under pressure diameter radius gauge length Figure 1.34: Before testing for tensile stress, you have to measure the test piece elongation: lengthening; becoming longer
- Donald Askeland, Wendelin Wright, Donald Askeland(Authors)
- 2020(Publication Date)
- Cengage Learning EMEA(Publisher)
6-3 The Tensile Test: Use of the Stress Strain Diagram The tensile test is popular since the properties obtained can be applied to design different components. The tensile test measures the resistance of a material to a static or slowly applied force. The strain rates in a tensile test are typically small (10 24 to 10 22 s 21 ). A test setup is shown in Figure 6-3; a typical specimen has a diameter of 12.83 mm and a gage length of 50.8 mm. The specimen is placed in the testing machine and a force F, called the load, is applied. A universal testing machine on which tensile and compressive tests can be performed often is used. A strain gage or extensometer is used to measure the amount that the specimen stretches between the gage marks when the force is applied. Thus, the change in length of the specimen (∆l) is measured with respect to the original length (l 0 ). Information concerning the strength, Young’s modulus, and ductility of a material can be obtained from such a tensile test. Typically, a tensile test is conducted on metals, alloys, and plastics. Tensile tests can be used for ceramics; however, these are not very popular because the sample may fracture while it is being aligned. The following discussion mainly applies to Tensile Testing of metals and alloys. We will briefly discuss the stress strain behavior of polymers as well. Figure 6-4 shows qualitatively the stress strain curves for a typical (a) metal, (b) thermoplastic material, (c) elastomer, and (d) ceramic (or glass) under relatively small strain rates. The scales in this figure are qualitative and different for each material. In practice, the actual magnitude of stresses and strains will be very different. The tem- perature of the plastic material is assumed to be above its glass-transition temperature (T g ). The temperature of the metal is assumed to be room temperature. Metallic and thermoplastic materials show an initial elastic region followed by a nonlinear plastic region.
eBook - PDFAn Introduction to Metallurgical Laboratory Techniques
Pergamon Series of Monographs in Laboratory Techniques
- P. G. Ormandy, K. Guy(Authors)
- 2016(Publication Date)
- Pergamon(Publisher)
Therefore Tensile Testing machines 115 Testing of Materials original gauge length (d) Young's Modulus of Elasticity (E). This is given by the slope of the initial elastic portion of the stress-strain diagram, and is defined as stress strain E = Thus percentage elongation equals extension of gauge length x 100 per cent Metallurgical Laboratory Techniques 116 usually have provision for suitable attachments to enable ma-terials to be subjected to compressive loads. The preparation of test specimens is very important. To deter-mine the general compressive properties of the material, a specimen is usually prepared in which the length is approxim-ately twice its diameter. The end faces of cylindrical specimens should be smooth and preferably polished, to eliminate to some extent friction between the face of the specimen and the face of the pressure block on the testing machine. Specimens must be carefully aligned between the pressure blocks, and should there be a tendency for the material under test to buckle under load, or where excessive deformation of the specimen may impose lateral strain to the testing machine, it is advisable to employ some form of jig to give support to the specimen. These jigs will vary according to the type and size of specimen undergoing test, being usually designed to suit individual requirements. There-fore each test should be considered prior to its commencement. The direction of the force of load is reversed in comparison to the tensile test, the majority of tests being usually carried out to obtain a stress-strain curve for a particular material which may be compared with the stress-strain curve for the same material in tension. As the load is applied to the specimen, there is an increase in the cross-sectional area of the specimen because of Poisson's ratio effect below the elastic limit, and because of plastic yield beyond the elastic limit.
eBook - PDF- William Bolton(Author)
- 2013(Publication Date)
- Newnes(Publisher)
2 Properties of materials Objectives At the end of this chapter you should be able to: • Describe the main methods used for Tensile Testing, impact testing, bend tests and hardness measurements, interpreting the results of such measurements. • Interpret thermal conductivity and electrical conductivity data. THE TENSILE TEST FOR METALS Extension Figure 2.1 The form of a typical force-extension graph for a metal, e.g. a low carbon steel In a tensile test, measurements are made of the force required to extend a standard size test piece at a constant rate, the elongation of a specified gauge length of the test piece being measured by some form of extensometer. The measurements thus obtained from the test are those of force applied and resulting extension. Figure 2.1 shows the form of a typical force-extension graph for a metal. For the part of the force-extension graph OA the extension is directly proportional to the force. Hooke's law is said to be obeyed. Point A is called the limit of proportionality. Up to a certain force the material will return to its original dimensions when the force is removed, the region of the force-extension graph for which this occurs is called the elastic region. The maximum force for which this occurs is called the elastic Umit. For many materials the elastic limit and limit of proportionality are almost identical. Beyond the elastic limit the material will not return to its original dimensions when the force is removed, the material retaining a permanent change in dimensions. This is referred to as a permanent set or plastic deformation. Beyond the limit of proportionaHty the extension is no longer proportional to the force. In some materials a situation may arise where the extension continues to increase without any increase in force; this point is known as the yield point. For the graph shown in Figure 2.1, typical of a low carbon steel, there are two distinct yield points Β and C.
eBook - PDF- T. W. Clyne, J. E. Campbell(Authors)
- 2021(Publication Date)
- Cambridge University Press(Publisher)
5 Tensile Testing The uniaxial tensile test is the most commonly used mechanical testing procedure, and indeed it is in very widespread use. However, while it is simple in principle, there are several practical challenges, as well as a number of points to be noted when examining outcomes. For example, there is the issue of converting between nominal (“engineering”) and true values of the stress and strain. While many stress–strain curves are presented, and often interpreted, only as nominal data, it is the true relationship that accurately reflects the mechanical response of the sample. Furthermore, conversion between nominal and true values is straightforward only while the stress and strain fields within the gauge length of the sample are uniform. This uniformity is lost as soon as the sample starts to deform in an inhomogeneous way within the gauge length, which most commonly takes the form of “necking.” After the onset of necking, which may be quite difficult to detect and could occur at an early stage, useful interpretation of the stress–strain curve becomes difficult. However, FEM modeling does allow various insights into the behavior in this regime, with potential for revealing information (about the fracture event) that is otherwise inaccessible. There are also several important points relating to the way that the strain is measured during a test. 5.1 Specimen Shape and Gripping 5.1.1 Testing Standards A central issue concerns the specimen size and shape. The behavior is normally monitored in a central section (the “gauge length”), in which a uniform stress is created. The grips lie outside of this section, where the sample has a larger sectional area, so that stresses are lower. If this is not done, then stress concentration effects near the grips are likely to result in premature deformation and failure in that area. Several different geometries of sample and grips are possible, with a number of associated standards and protocols.
eBook - PDFApplied Metal Forming
Including FEM Analysis
- Henry S. Valberg(Author)
- 2010(Publication Date)
- Cambridge University Press(Publisher)
Finally, the deformational behavior of the specimen in shearing is investigated with help of FEA, supported by use of experimental grid pattern techniques. This shows the interesting potential of this method, as a test that creates very high strain and strain rates in the material and at the same time allows flow stress data to be deduced from the load–stroke curve obtained in the test. 15.1 Tensile Testing The mechanics of the tensile test and use of it to acquire the flow curve of a material were thoroughly discussed in Ch. 7. It is commonly observed that the flow curves obtained in tensile and in compressive testing may differ. After the onset of necking 242 15.1 Tensile Testing 243 (a) (b) Figure 15.1. (a) Strain distributions over cross section of neck for soft-annealed aluminum alloy. (b) Stress distributions across neck as predicted in simulation. in the tensile test specimen, it is common practice to perform a Bridgman correction on the flow stress obtained, as explained in connection with Fig. 7.9. The theoretical analysis performed by Bridgman, which led to the correction procedure, was based on certain simplifying assumptions. If some of the assumptions are wrong, then the analysis is not necessarily accurate. 15.1.1 Bridgman’s Analysis of Tensile Testing When Bridgman performed his well-known theoretical analysis 1 of the stress con- ditions in the neck during the tensile test, he made some idealized assumptions. One was that the strain distribution over the cross section of the neck is even (given by eq. 7-3). Figure 15.1(a) shows that FEA of Tensile Testing does not sup- port this assumption. Instead, the effective strain at the center of the specimen is predicted to be larger than the effective strain closer to the surface of the neck. In Fig. 15.1(b), the axial stress and the effective stress distribution are also given as they are predicted in FEM simulation.
- Yuehuei H. An, Robert A. Draughn(Authors)
- 1999(Publication Date)
- CRC Press(Publisher)
Upon further deformation, the load reaches a maximum value and then drops somewhat before fracture occurs. The tensile strength (ultimate tensile stress) is obtained by dividing the maximum load during the test by the original cross-sectional area. A measure of the ductility of a material after fracture is given by the percent elongation and also by the reduction of cross-sectional area. Percent elongation after fracture is determined by dividing the change in the original gauge length by the original gauge length (multiplied by 100%). The original gauge length should always be stated in reporting the percent elongation values. The percent reduction of area after fracture is the ratio of the change in the original area determined at the smallest cross section divided by the original area of cross section (multiplied by 100%). As discussed earlier, there is a difference between intrinsic stiffness and extrinsic stiffness (rigidity). For a tensile test of bone the intrinsic stiffness is equal to the Young’s modulus ( E ), while the extrinsic stiffness is equal to EAL , where A is the cross-sectional area of the specimen and L is the gauge length of the specimen. The extrinsic stiffness is dependent not only upon elasticity, but also on specimen size. In the case of trabecular bone the specimen must be large enough that the trabecular structure can be treated as a continuum. A specimen width of at least 4 to 8 mm is recommended for trabecular bone specimens. 51,93-96 C. C OMPRESSION T ESTING Compression testing of bone specimens is a popular technique, especially for cortical bone because relatively small specimens can be used. Compressive tests, however, tend to be less accurate than tensile tests due to friction and compression-platen end effects imposed on the bone specimen during the test. Friction at the load platen–bone surface interface can be minimized using polished stainless steel platens lubricated with a coating of lightweight machine oil.
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