INDIAN INSTITUTE OF TECHNOLOGY KANPUR
Department of Materials and Metallurgical Engineering
Virtual Laboratories for Thermal Processing and Characterization of Materials
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INDIAN INSTITUTE OF TECHNOLOGY KANPUR Department of Materials and Metallurgical Engineering Virtual Laboratories for Thermal Processing and Characterization of Materials |
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Mechanical Testing Laboratory |
Experiment 1
Introduction to Tensile Testing: Analysis of Stress-Strain Curves
Objectives
To develop an understanding of the methodology of tensile testing.
Obtain load-displacement curves for metallic and polymeric samples.
To calculate the stress-strain curves from the load-displacement curves.
Determine the tensile properties of the samples by analysing the stress-strain curves.
Theory
Tensile test is one of the most important mechanical property evaluation test. In this test a cylindrical or a plate shaped specimen is deformed by applying a uniaxial force as shown in the figure below. One end of the sample is fixed in a static grip while the other end of the specimen is pulled at a constant velocity. The load is continuously monitored during the test. It is usual to conduct this test until the sample fractures. During the test, the instantaneous elongation of the sample can be calculated from the velocity of deformation or can also be measured by using an extensometer.
The resulting output from such a test is recorded as load versus displacement/elongation and can be graphically displayed as a load versus elongation curve. Load versus elongation curve is then converted to engineering-stress versus engineering-strain curve to evaluate the tensile properties of materials. Very often engineering-stress versus engineering-strain curves need to be converted to true-stress versus true-strain curves.
The tensile properties that can be obtained from the stress-strain curves are yield strength, tensile strength, fracture strength, percent total elongation, uniform elongation, strain hardening exponent, modulus of resilience, and modulus of toughness.
Methodology
Note the gauge length (length between the two gauge marks on the gauge section of the sample) and the area of cross-section (for this use the values of diameter for cylindrical samples and width and thickness for plate samples) for each of the samples. The sample is then mounted on a Lloyd's Tensile Testing Machine.
Now establish a remote connection to the tensile testing machine. After establishing the connection, you will see a live image of the machine. Procedure for starting the computer program, setting the test parameters, starting the test and saving the data are given here. The test is conducted till the sample fractures. Note the final gauge length, which is length between the two gauge marks after joining the two broken pieces of the tensile sample.
The load-displacement data is saved in an excel file. Transfer the file from the remote virtual laboratory location to the local computer for further analysis.
Results
Note the values of initial gauge length, Lo, initial area of cross-section, Ao, and final gauge length, Lf.
Show plots of (i) load-displacement curves, (ii) engineering stress - engineering strain curves, and (iii) true stress - true strain curves.
From the engineering stress - engineering strain curves obtain the following tensile properties: (i) yield strength, (ii) tensile strength, (iii) percent total elongation, (iv) percent uniform elongation.
From the true stress - true strain curves obtain the following true tensile properties: (i) yield strength, (ii) tensile strength, (iii) percent total elongation, (iv) percent uniform elongation.
Discussion and Conclusions
Compare and comment on the engineering and true tensile properties.
Compare the tensile properties for the metal and the polymer samples.
List the important conclusions obtained in the experiment.
Questions