INDIAN INSTITUTE OF TECHNOLOGY KANPUR Department of Materials and Metallurgical Engineering Virtual Laboratories for Thermal Processing and Characterization of Materials

Experiment 2

The Yield Point Phenomenon

Objectives

1. To study and understand the yield point phenomenon that is observed in some materials (and in particular exhibited in mild steels).

2. Characterization of tensile properties of mild steel.

Theory

During tensile deformation materials initially deform elastically and the yield point marks the onset of plastic deformation. For most materials the transition from elastic to plastic deformation is gradual and hence do not exhibit a sharp transition point. Such materials show engineering stress versus engineering-strain curves as shown here. However, in some cases (and more specifically in the case of mild steel) the transition is a sharp one and is exhibited in what has been termed as the yield point phenomenon.

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 the two mild steel samples of standard geometry. 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 on the first sample is conducted till fracture. Note the final gauge length, which is length between the two gauge marks after joining the two broken pieces of the tensile sample.

Now, the tensile test is started on the second sample. The test is stopped soon after the the yield drop is observed and the sample is unloaded. New gauge length and the new area of cross-section are noted and the tensile test restarted. This time the test is conducted till fracture.

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

First Specimen

1. Note the values of initial gauge length, L_o, initial area of cross-section, A_o, and final gauge length, L_f.

2. Show plots of (i) load-displacement curves, and (ii) engineering stress - engineering strain curves.

3. From the engineering stress-strain curves note the values of the lower and upper yield stress.

4. From the engineering stress - engineering strain curves also obtain the following: (i) tensile strength, (ii) fracture strength, and (iii) percent total elongation.

Second Specimen

1. Note the values of initial gauge length, L_o, initial area of cross-section, A_o, and final gauge length, L_f.

2. Show plots of (i) load-displacement curves, and (ii) engineering stress - engineering strain curves up till stopping the test.

3. Note the new values of gauge length, L₁, initial area of cross-section, A₁, and final gauge length, L_f1.

4. This time show plots of (i) load-displacement curves, and (ii) engineering stress - engineering strain curves till fracture.

Discussion and Conclusions

1. Comment on the stress-strain characteristics obtained for the first sample.

2. Compare the stress-strain curve (with that for the first sample) for the second sample obtained after the interruption.

3. List the important conclusions obtained in the experiment.

Questions

1. What is the genesis of the yield point phenomenon?
2. Do you think after interrupting the tensile test soon after the yield drop and heating the tensile sample in boiling water would make a difference in the tensile test result? If so, in what way?