Stress-Strain Characteristics of Steels

Tension Testing of Steel Specimens

The purpose of this experiment is:

• To acquaint students with the standard laboratory test for determining the tensile properties of metallic materials in any form (ASTM E8),
• To compare the properties of commonly used engineering metallic materials (structural steel and aluminum), and
• To compare the tested properties of metals to published values.

It will be assumed that a universal testing machine (UTM) with deformation control and associated testing and data acquisition capabilities is available. Follow recommended step-by-step procedures to perform tensile tests provided by the manufacturer of the UTM, paying particular attention to the safety guidelines. Do not proceed if you are uncertain about any step, and clarify any doubts with your lab instructor, as you can seriously injure yourself or those around you if you do not follow proper precautions. Also, make sure you know all emergency stop procedures and that you are familiar with the software running the machine.

The procedure below is generic and is meant to cover most important steps; there may be significant deviations from it depending on the available equipment.

1. Prepare Specimens:

1. Obtain cylindrical test specimens for two steels, one mild and hot rolled (such as A36) and one hard and cold rolled (such as an C1018).
2. Measure the diameter of the test specimen to the nearest 0.002 in. at several locations near the middle using a caliper.
3. Hold the specimen firmly and mark, using a file, an approximate 2 in. gage length. Note: Mark the gage length carefully so that it is clearly etched, but not so deep as to become a stress concentration that can lead to fracture.
4. Measure the actual marked gage length to the nearest 0.002 in. using a caliper.
5. If possible install a strain gage as described in JoVE video on "Material Constants".
6. Collect all available information on the calibration data and resolution of all instruments being used to help assess potential experimental errors and confidence limits. These two issues are key to obtaining meaningful results but are beyond the scope of what is discussed here.

2. Test the Specimens:

1. Turn on the testing machine and initialize the software. Make sure that you have setup any appropriate graphing and data acquisition capabilities within the software. At a minimum, display the stress-strain curve and have displays for the load and strain.
2. Select an appropriate testing procedure within the software that is compatible with the ASTM E8 testing protocol. Note the strain rate being used and whether two rates, one for the elastic and one for the inelastic range, are being used. Also, set any appropriate actions in the software (e.g., for the machine to stop at 15% strain, so as to safely remove the extensometer and to record the maximum value of load that is reached.).
3. Manually raise the crosshead such that the full length of the specimen fits easily between the grips. Carefully insert the specimen into the top grips to about 80% of the grip depth; align the specimen inside the grips and tighten slightly, so as to prevent the specimen from falling. Note: DO NOT tighten the grip to its full pressure at this stage.
4. Slowly lower the top crosshead. Once the specimen is within about 80% of the bottom grip depth, make sure the specimen is properly aligned within the bottom grips (i.e. with the bottom grips in their fully opened position, the specimen should "float" in the middle of the bottom grip opening). Specimen misalignment, which will result in additional flexural and torsional stresses during testing, is one of the most common errors encountered when carrying out tension tests. If the alignment is poor, work with a technician to properly align the grips.
5. Apply appropriate lateral pressure to the specimen through the grips to ensure that no slipping occurs during testing. Note that there will be a small axial load at this point, as the tightening process introduces a preload into the specimen; the testing machines may have software adjustments to minimize this preload. Record the preload value.
6. Attach the electronic extensometer securely to the specimen as per manufacturer's specification. Note: The extensometer blades do not need to be positioned exactly on the gage marks on the specimen, but should be approximately centered on the specimen.
7. Carefully check that you have properly executed all procedures up to this point; if possible, have a supervisor verify if the specimen is ready for testing.
8. Start the loading to begin applying the tensile load to the specimen and observe the live reading of applied load on the computer display. Note: If the measured load does not increase, the specimen is slipping through the grips and needs to be reattached. If this occurs, stop the test and restart again from Step 2.3.
9. Sometime prior to sample failure, the test will be automatically paused without unloading the specimen. At this point, remove the extensometer. If the specimen breaks with the extensometer in place, you will destroy the extensometer, a very expensive piece of equipment.
10. Resume applying tensile load until failure. Upon reaching the maximum load, the measured loads will begin to decrease. At this point, the specimen will begin necking and final fracture should occur within this necked region through ductile tearing.
11. After the test is finished, raise the crosshead, loosen the top grips, and pull out the broken piece of specimen from the top grip. Once the top half of the specimen is removed, loosen the bottom grip and remove the other half of the specimen.
12. Record the value at the maximum tensile load and print a copy of the stress-strain curve. Save the data recorded digitally.
13. Carefully fit the ends of the fractured specimen together and measure the distance between the gage marks to the nearest 0.002 in. Record the final gage length.
14. Measure the diameter of the specimen at the smallest cross section to the nearest 0.002 in.
15. Document the fractured specimen with pictures and diagrams.

3. Data Analysis

1. Calculate the % elongation, and reduction of area for each type of metallic material.
   elongation = 
   reduction of area = 
2. Describe, categorize and record the predominant fracture mode for each specimen.
3. Determine material properties as described in Fig. 2 and 3. Organize the data in a spreadsheet such that the strain up to 0.004 is given by the strain gage and between 0.004 and 0.15 by the extensometer (the upper limit for the extensometer is the value of strain at which it was removed from the test; this value changes depending on the deformation capacity of the specimen).
4. Use the crosshead displacement and %elongation to estimate ultimate strain. If a strain gage is not used, be sure to correct for any initial slip of the extensometer. One can count squares in the graph to obtain the toughness (area under stress-strain curve).
5. Using a textbook or other suitable reference, determine the elastic modulus, yield strength, and ultimate strength of the materials used. Compare the published values to the test results.