5: 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.

Steel is a general term for iron alloyed with carbon and other elements like chromium, manganese, and nickel.

Variations in the composition and processing methods can tailor its properties for construction of cars, bridges, and skyscrapers, to name only a few of the nearly infinite possible uses.

Understanding steel's response to load is important when designing safe buildings and structures. One fundamental tool for modeling material characteristics is the stress-strain curve.

We will use the uniaxial tensile test to study the elastic and inelastic behavior of a mild hot-rolled steel and a hard cold-rolled steel, which represent low and high limits respectively of tensile strengths in civil engineering applications.

Stress is defined as the force divided by the area over which it is applied. Strain is the change in length divided by the initial length. Stress-strain curves describe the elastic and inelastic properties of materials by showing how a material like steel responds to applied force.

The uniaxial tensile test is typically used for studying stress and strain. In this test, a machine slowly pulls the ends of a sample with greater and greater force and measures the resulting elongation. The metal tension test is described by ASTM E8, which defines the type and size of the specimen, the type of equipment, and the data to be reported.

The stress-strain curve reveals many properties of the material under test. Among them, elastic modulus (the slope of the initial linear region, where deformation is proportional to load), modulus of resilience (the area beneath the linear region, which measures a material's capacity to absorb energy without permanent deformation), proportional limit (the stress at the point the curve deviates from linearity), yield points (where stress versus strain suddenly decreases or changes), and yield plateau (where deformation increases rapidly without increasing stress).

Steel is a ductile material. Ductility is defined as the change in length at failure divided by the initial length. Toughness is the ability of a material to absorb energy before it fractures.

Now that we understand some of the basic characteristics of materials, let's look at a method to measure stress and strain in the laboratory and investigate the relationship between these two quantities.

Obtain cylindrical test specimens for two types of steel, one mild and hot-rolled, such as A36, and one hard and cold-rolled, such as C1018.

Use a caliper to measure the diameter at several locations near the middle of the specimen. Make these measurements to the nearest 2000th of an inch.

Next, hold the specimen firmly. Scribe a gauge length of approximately two inches. Make the mark clear but very shallow to avoid creating a stress concentration that can lead to fracture. Measure the actual marked gauge length to the nearest 2000th of an inch.

Finally, install a strain gauge. The specimen is now ready for testing.

We will be using a universal testing machine, or UTM, to measure the tensile properties of the specimens. Turn on the testing machine and initialize the software. Set up appropriate graphing and data acquisition parameters, then select a test procedure that is compatible with the ASTM E8 protocol.

Set strain rates for the low strains zero to 5% and for high strain ranges greater than 5% respectively. These should be close to 0.05 inches per minute for the initial loading and 0.5 inches per minute after 5% strain. Then set any additional actions in the software, such as stopping the machine at 5% strain in the extensometer to remove it before specimen failure.

Manually raise the crosshead so the full length of the specimen fits easily between the top and bottom grips. Carefully insert the specimen into the top grip to about 80% of the grip depth. Align the specimen inside the top grip and tighten slightly to prevent the specimen from falling. Slowly lower the top crosshead. Once the specimen is within about 80% of the bottom grip depth, start specimen alignment within the bottom grips. The specimen should float in the center of the fully opened bottom grip. Apply lateral pressure to the specimen through the grips to ensure that no slipping occurs during testing. Note the tightening process introduces a small axial force on the specimen.

Use the software to impose a preload to compensate for this force and record its value. Attach the electronic extensometers securely to the specimen according to the manufacturer's instruction. The blades of the extensometer should be approximately centered on the specimen. If a strain gauge is being used, connect it.

Begin the test by applying tensile load to the specimen. Observe the live reading of applied load on the computer display. To confirm the specimen is not slipping through the grips, make sure the measured load is increasing linearly. Sometime before sample failure, the software will automatically pause the test. Leave the sample in the test machine and remove the extensometer. Resume applying tensile load until failure. Upon reaching the maximum load, the measured loads begin to decrease. At this point, the specimen starts to neck. Final fracture should occur in this necked region through ductile tearing.

After the test has ended, raise the crosshead, loosen the top grip, and remove the broken piece of specimen from it. Loosen the bottom grip and remove the other half of the specimen. Record the value at the maximum tensile load. Save the recorded data and the stress-strain curve.

Carefully fit the ends of the fractured specimen together and measure the distance between the gauge marks to the nearest 2000th of an inch. Record the final gauge length. Finally, measure the diameter of the specimen at the smallest cross section to the nearest 2000th of an inch.

To determine material properties, first take a look at the data for the A36 mild hot-rolled steel and the data for C1018 hard cold-rolled steel, respectively.

Now calculate the percent elongation for each specimen, knowing the final gauge and the initial gauge length. Calculate the reduction of area for each specimen, using the final diameter and the initial diameter of the specimen. Record these values in a results table.

Next, calculate other material parameters using the experimental stress-strain curves. A quick comparison of these curves for the two specimens shows their very different elastic and inelastic behaviors. From the much greater strain at lower levels of stress, the A36 steel is softer and far more ductile than the C1018 steel.

For the A36 steel, the stress at failure is about 58.6 kilopounds per square inch, substantially above the nominal value of 36.0 kilopounds per square inch. Maximum stress is about 86.6 kilopounds per square inch at a strain of about 20%.

This magnified plot shows an upward yield point at about 58.6 kilopounds per square inch and a lower yield point at about 56.8 kilopounds per square inch. The beginning of the yield plateau is also visible here. Strain gauge data reveals a linear elastic region for the A36 steel with a slope defined as Young's Modulus of about 29,393 kilopounds per square inch. This result is very close to the nominal value of 29,000 kilopounds per square inch.

At the point where the data deviates from linearity, we can determine the proportional limit is about 55.58 kilopounds per square inch. For comparison, due to the nonlinearity of its stress-strain curve, the C1018 steel has a very low proportional limit.

Results from the extensometer covers strain up to 5%. Data for the A36 steel shows the plastic plateau and the beginning of strain hardening where the curve rises again at a strain of about 2.7%. In contrast, the C1018 has no clear yield plateau.

Finish the data analysis by summarizing the test results for the two steel samples in the following table.

The elongation of a mild hot-rolled steel is in the range of 25 to 40%. In contrast, the elongation of a hard cold-rolled steel is only half this amount. The percent elongation is an average value for the length of material between the gauge marks, but almost all the deformation is localized to a small region around the fracture point. Consequently, the local strain could be much greater than the average.

Physical examination of the two specimens show large differences in the way they fail, corresponding to differences in their stress-strain curves.

The A36 steel has a failure surface with material drawn out at the rim during gradual final deformation and greater elongation at lower stresses, indicating a very mild but ductile metal.

In contrast, the C1018 steel has a flat failure surface, corresponding to sudden fracture and much less elongation at much higher stresses, characteristics of high strength but low ductility.

Let's look at some common applications of steel from the perspective of the relationship between stress and strain.

Civil engineers analyze structural collapses in bridges and buildings in order to improve future structural designs. This process has led to steel components like rolled I-beams for multi-story buildings, welded deep-plate I-girders for bridges, and high-strength bolts and fasteners. Each requires different types of steel with specified strengths and ductilities, often first understood through examination of their stress-strain curves.

Engineers use the stress-strain characteristics of materials to make safer automobiles. Knowing the strength and ductility of the frame and how it deforms in response to impact forces, engineers can design an automobile's body to absorb energy during collision and increase the chance of surviving a crash.

You've just watched JoVE's Introduction to Stress-Strain Characteristics of Steel.

You should now know how to perform a uniaxial tensile test to determine the tensile properties of metallic materials and how to analyze stress-strain curves for typical steels.

Thanks for watching!