TAM 224/CEE 210 5–1

5. The Tension Test5.1. Objective The objective of this experiment is determine the tensile behavior of four common construction mate-rials: a structural steel, a cast iron, a structural aluminum, and a common polymer. Such mechanical properties as modulus of elasticity, yield strength, ultimate strength, and ductility are determined for the different materials. Also, correlations are drawn between ductility and fracture-surface morphology.

5.2. Apparatus Four Instron model 4400 load frames are available for testing the samples. The testing machines apply tensile or compressive forces by means of a moving crosshead. Digital calipers are available to measure the cross-sectional dimensions of all specimens before testing.

For the instrumented tests, each machine is fur-nished with a load cell that provides an output voltage proportional to the applied load. The load cell has a nominal capacity of 100 kN (22.5 kips).

An extensometer will be installed to measure the elongation of the specimen. The gage length of the extensometer is 25.4 mm (1 inch). The extensometer provides an output voltage that is proportional to the specimen elongation. The maximum measurable elongation is about 100%.

A computer recording system is used to construct a load–deformation curve and to prepare data files for post-processing.

5.3. Materials Four metals and one polymer are studied. All specimens are turned on a lathe, generally from 1/2-in.-dia. round stock. Nominal specimen dimen-sions are given in Fig. 1.

Steel. A 1018, 1045, or other construction-grade steel will be tested. Of these alloys, the 1045 steel, for example, contains about 0.45% carbon and is considered a medium-strength steel; it is fairly easily welded, and is amenable to heat treatment. It is used, for example, to make railway car wheels.

Gray cast iron. The lab specimens contain about 3.25% carbon and 2% silicon, the carbon being in the

form of graphite flakes. Cast iron is used in engine blocks and machine frames.

Aluminum. A 2024-T6, 6061-T6, or 7075-T6 aluminum will be used to make the samples. Of these alloys, 6061-T6 is a common-grade medium-strength alloy used to make, for example, outdoor furniture and bridge railings.

Stainless steel. Type 304 stainless steel will be used to make the samples. This is a common material for sinks, bowls, watch bracelets, and fasteners.

Polymethylmethacrylate (PMMA). This high-molecular-weight polymer, available commercially under the trade names Plexiglas and Lucite, is used as

replacement for glass, and for transparent fixtures. Although not as brittle as glass, it is brittle when compared with many other polymeric materials.

5.4. Experimental procedure 1. For each specimen, measure the mean diameter,

taking the average of three measurements. Begin filling out Table 1.

25

150

12.7

835

Fig. 1. Nominal tension specimen

dimensions (mm).

5–2 Behavior of Engineering Materials TAM 224/CEE 210

2. Set up the Instron testing machine:1

a. Install the specimen. Use the JOG buttons to position the crosshead. The load should be nearly zero.

b. Attach the extensometer (Fig. 2) while squeez-ing the gage-length setting knobs together. After installation, press BAL under STRAIN, then ENTER. Wait until the STRAIN indicator displays zero.

c. Press GL RESET to record the initial cross-head position. The extension indicator should then read zero.

1Only a brief description is given here. For more detail, see Appendix A—Instron Model 4400 Series Load Frames, at the end of this lab manual.

d. Perform a final check of the entire setup. Then press the IEEE button to allow the desktop computer to assume control of the testing machine.

3. Follow the instructions in Appendix B of this lab manual to set up the computer. LabVIEW® soft-ware will be used. Perform the test when the computer is ready.

4. During the test, carefully observe each specimen as it is being deformed, and note any changes in its shape.

5. After the specimen fails, disconnect the computer interface by pressing the IEEE button on the control console.

6. Remove the specimen pieces from the testing machine. Avoid touching the fracture surfaces together.

7. Make a detailed record of the general features of the fracture surfaces, and measure the diameter at the point of failure.

5.5. Analysis of Results 1. Construct a diagram of engineering stress vs.

strain for each of the materials tested. The tab-delimited ASCII data files contain, in order, the crosshead position, the load, the strain, and the time. (For details, see Appendix B—Instron 4400 Control Using LabVIEW®.) For ductile materials, select one strain scale such that only the elastic portion of the curve is emphasized; use the data in this range to determine modulus of elasticity. Then select a second strain scale that allows the entire curve to be included on your plot. One scale is usually sufficient for brittle materials.

2. From the stress–strain diagrams for each of the materials, compute: (a) the modulus of elasticity E, (b) the yield strength σ y , and (c) the ultimate tensile strength σ u . (Hint: for ductile materials without a clear yield point, the yield strength can be determined using a 0.2% offset strain.)

3. Compute the percent reduction of area %RA, and find the percent elongation %EL, for each material, using the definitions

%RA = %EL =A AA

l ll

f f0

0

0

0100% 100%

−×

−×, .

Fig. 2. Attaching the extensometer to the specimen.

TAM 224/CEE 210 The Tension Test 5–3

4. For the steel specimen only, superimpose a plot of the “true” stress σ t vs. “true” strain ε t up to the point of maximum load, on the same graph for the engineering stress–strain curve.2 Be careful to use natural units for strain, and not percent values, when using the true-stress and true-strain formulas.

5. Complete Table 1. Transfer data also to the appro-priate table of the Compression and Hardness Tests lab (Experiment 6 in this lab manual) and also to the appropriate table of the Bending and Torsion Tests lab (Experiment 7), for future reference.

5.6. Points for discussion Note.—Your lab instructor will indicate which of the

following questions are to be addressed in your report.

1. Compare Young’s modulus of the five materials tested.

2. Compare the yield strengths and the ultimate strengths of the five materials tested. Did all five materials yield? (Note: typical values of the mechanical properties for some materials are given in the following table.)

3. Discuss the general shape of the stress–strain curve of each material, especially the region beyond the yield point.

4. Compare the ductility of the four materials, based on %RA, %EL, and the shape changes during

2The true stress σt is based on the instantaneous area A. Thus, if it is assumed that plastic deformation occurs homogeneously at constant volume, i.e. A0l0 = Al , then

σt =PA

=PA0

⋅A0A

= σll0

= σ (1 +ε ) .

The true strain ε t is given by the integral of differential strains based on current length l, namely,

ε t =dlll0

l

∫ = lnll0

= ln(1+ ε ) .

If the strain ε is small, there is little numerical difference between true and engineering definitions of stress and strain.

Note that, once necking begins, the deformation is no longer homogeneous; therefore the stresses and strains become unknown functions of position within the necked region, and it is incorrect to refer to the true stress and the true strain.

deformation. Are the data for %RA and %EL consistent?

5. Argue, if possible, why %RA should be preferred over %EL as a measure of ductility. If %EL in your experiment is determined using a gage length of 25.4 mm, then what can you say about the value of %EL that would have been determined if the standard gage length of 50.8 mm (2 in.) had been employed?

6. Discuss the fracture surfaces observed. How do they relate to the ductility of the materials? (Sketches are helpful.)

7. Describe in detail the process by which the cup-and-cone type of failure occurs in ductile materials. In which of your materials was this mode of failure observed? Why is this mode of failure not observed in brittle materials?

8. Compare the true and engineering stress–strain curves for steel. Compute a pair of true and engineering stresses at the maximum load, and comment on the differences between the two values.

References Ashby, M. F., and D. R. H. Jones. 1981. Engineering

Materials—An Introduction to Their Properties and Applications. Oxford: Pergamon, 82-85.

Askeland, D. R. 1989. The Science and Engineering of Materials, 2nd ed. Boston: PWS-Kent. See Sections 6.1-6.6 and the tables on pp. 153, 500, 534-539, 554-555, 591, 841.

Callister Jr., W. D. 2003. Materials Science and Engi-neering—An Introduction, 6th ed. New York: Wiley, Sections 6.1–6.12, 8.1–8.4.

Flinn, R. A., and P. K. Trojan. 1990. Engineering Materials and Their Applications. Boston: Houghton Mifflin, Sections 2.1-2.3, 2.6-2.7, 2.9.

Material

Young’s modulus, E (GPa)

Yield strength, σ y (MPa)

Ultimate strength, σ u (MPa)

1045 Steel 200 600 > σ y

6061-T6 Alum. 70 300 > σ y

Gray cast iron 140 — 300 304 Stainless 190 240 > σ y

PMMA 3 — 40

5–4 Behavior of Engineering Materials TAM 224/CEE 210

Table 1—Tensile mechanical properties

Measurement or property Material

Quantity Symbol Units _____ Steel PMMA _____ Alum. Cast iron 304 S. S.

Initial data

Diameter d0 mm

Cross-sectional area A0 mm2

Strength

Approximate yield load* Py kN

Approximate max. load* Pmax kN

Shape changes during deformation (sketch)

—

—

Detail of fracture surface (sketch)

—

—

Hardness

Rockwell hardness HRB —

Ductility

Gage length l0 mm 25.4

Percent elongation %EL —

Final diameter d f mm

Final area Af mm2

Percent reduction of area %RA —

Mechanical properties derived from stress–strain diagram

Young’s modulus E GPa

Yield strength σ y MPa

Ultimate strength σ u MPa

Mechanical behavior (description)

—

—

Test date: Group: Student’s name:

*By visual inspection of load–strain plot during the test Printed 7/9/03