LAB 1 TENSION TESTS

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LAB 1
TENSION TESTS



1.1 Objective
The objectives of this lab are:
• to perform tension tests on four commonly used metals to gain an appreciation of
tensile testing equipment and procedures
• to examine the resulting stress-strain curves to gain an appreciation of the tensile
behavior of the tested metals and to identify/calculate the significant mechanical
properties of those metals
• to compare the mechanical properties of the four metals to gain an appreciation of
the physical differences between the metals
• to compare the physical tensile-failure characteristics of the four metals to gain an
appreciation of the physical differences between the metals


1.2 Apparatus
• A 30 kN capacity electro-mechanically operated universal tension/compression load
frame will be used to test the tensile specimens. The applied load on the specimen is
determined indirectly from a tensile load cell.
• A caliper will be used to measure the diameter of the test specimens.
• The elongation of the loaded test specimen will be determined indirectly by using an
extensometer
• A computer data-acquisition system will be used to generate load and displacement
data


1.3 Materials
Four metals will be studied: A36 hot-rolled steel, Gray cast iron, 6061-T4 aluminum, and free-
machining brass. The specimens are turned on a lathe mill from round stock .

A36 hot-rolled Steel. This steel contains about 0.26 wt% carbon. It is a low-strength steel,
weldable, and one of the least expensive to produce. It may be used, for example, to make
automobile components, I-beams, and sheets used in bridges.

Gray 20 Cast Iron. This cast iron contains 2.5-4.5 wt% carbon and 1.0-3.0 wt% silicon. It is
low-strength, but is highly resistant to wear and provides better vibrational damping than steel. It
may be used, for example, to make engine blocks and machine frames. Of all metallic materials it
is the least expensive to produce.

Aluminum. Type 6061-T4 aluminum is actually an aluminum alloy. It is composed of 0.30 wt%
copper, 1.0 wt% magnesium, 0.6 wt% silicon, and 0.20 chromium. It has been heat treated by a
solution treatment with natural aging. It may be used, for example, in trucks, railroad cars, and
pipe.

Free-cutting brass. Brass is an alloy of copper and zinc. The addition of zinc alloy to copper
increases strength. It may be used, for example, in musical instruments, coins, and jewelry.







1.4 Experimental Procedure
1. For each test specimen, measure the mean diameter, taking the average of three
measurements. Write mean initial diameter value in Lab 1 Test Data Table
.
2. Load the test specimen into the load frame.
3. Open the appropriate program on the data-acquisition computer. Follow all computer
instructions and provide information required by the program. The gage length used in the
data-acquisition program is 1 in. Write this gage length Lab 1 Test Data Table.

4. After receiving verbal approval from the instructor, turn on the load frame.
5. Carefully observe each specimen as it is being deformed.
6. After specimen has failed, carefully observe the general features of the fracture surfaces of
each specimen. Draw these features in Lab 1 Test Data Table
.
7. Measure the mean diameter at the point of fracture. Write mean final diameter Lab 1 Test
Data Table
.
8. Using the displacement/load test data, use Excel to calculate stress/strain values.
9. Use Excel to create appropriate stress-strain curves.

1.5 Analysis of Results


1. Determine the tensile strength (
u
σ
)
for each material


On your stress-strain curves: label
u
σ
and show

how you determined its value. Write this
value in Lab 1 Results Summary Table.


2. Calculate the maximum load (Pmax) for each material.


Using the tensile strength value and the cross-sectional area, calculate P
max
for each
metal.
Write these values in
Lab 1 Results Summary Table
.
(Be sure to include in your calculation work the equations used, the values used, and the
units.)


3. Calculate the Modulus of Elasticity (E) for each specimen
.

Use Excel to create the linear-elastic portion of the stress-strain curve diagram.

a. Using a straight edge, draw a straight line through the linear portion of the stress-
strain curve. Label the new “0-strain” location on the strain axis, if there is one.
b. Choose two points on the straight line you’ve drawn that are not at either the top or
bottom of the linear portion of the curve, but are sufficiently spaced from each other.
Hand-mark these points on the linear curve
. Draw a horizontal line from these
points to the stress axis
. Draw a vertical line from these points to the strain axis
.
The stress and strain values corresponding to the upper point will be labeled as
2
σ
=
慮搠
2
ε
Ⱐ牥,p散瑩癥汹⸠ Write these values down as part of your calculation
. The
stress and strain values corresponding to the lower point will be labeled as
1
σ
慮搠
1
ε
Ⱐ牥,p散瑩癥汹⸠ Write these values down as part of your calculation
. To
calculate the Modulus of Elasticity:


12
12
εε
σ
σ


=E
.

Be sure to include equation, values used, and units in your calculation work. Write your
calculated value of the Modulus of Elasticity for each material Lab 1 Results
Summary Table
.


4. Determine/calculate the yield strength ,
Y
σ
, for each material
.

1018 CF Steel: For steel, the yield strength is typically taken to be the stress value
corresponding to the bottom of the “dip” that occurs after the linear portion of the curve.
From the stress-strain curve diagram for the 1018 steel, determine the yield strength using
this method. Write the yield strength value in Lab 1 Results Summary Table
.

6061-T651 Aluminum and Free-Cutting brass: For these materials it may prove
difficult to determine the yield strength by observing changes the stress-strain diagrams.
Therefore a method called the 0.2% offset method
will be used to determine the yield
strength.

0.2% offset method
: On your linear portion of the stress-strain curve
diagram, find the 0.2% strain
(
)
002.0%2.0 ==
ε

as measured from your “0-strain” point on the strain
axis. Starting at the 0.2% strain value, draw a line
that has a slope equal to E
upward until it crosses
the stress-strain curve. Where the line crosses the
stress-strain curve is called the yield point
. From the
diagram, determine the stress value that corresponds
to yield point. That stress value is the yield strength
of the material.

After drawing the lines on the stress-strain diagrams for the 0.2% offset method, look at
the behavior of the stress-strain curve from zero stress-strain up to the yield point. Notice
whether the stress-strain curve remains linear all the way to the yield point or not.

Turn in the diagrams with drawn 0.2% method lines as part of the lab.

Write the yield strength value in Lab 1 Results Summary Table.


Gray cast iron: On the stress-strain diagram for gray cast iron, notice whether or not you
see the same type of behavior you observed on the stress-strain curve up to the yield point
for aluminum and brass.


5. Calculate the percent reduction of area, %RA, for each material
.
The %RA is given by










=
0
0
100%
A
AA
RA
final
.
Using the initial and final diameter measurements, calculate the %RA for each metal.
Write these values in Lab 1 Results Summary Table
.


6. Sketch the fracture surfaces of each specimen in Lab 1 Results Summary Table
.




1.6 Points for Discussion
1. Compare the yield strength of the four metals. Did all four metals yield ? What is the
significance of yield strength/yielding ? How is yield strength used in engineering design?
2. Compare the tensile strength of the four metals. What is the significance of the tensile
strength? How is tensile strength used in design ?
3. Compare the Modulus of Elasticity of the four metals. What is the significance of the modulus
(i.e., does it take more or less stress to get the same strain if the modulus of one metal is
higher than the modulus of another metal) ? How would the modulus be used in engineering
design ?
4. Compare the ductility of the four metals, based on %AR. What is significant about a material
being ductile or brittle ? Why is ductility, or brittleness, important in engineering design ?
5. Compare the fracture surfaces of the four metals. What does the shape of the fracture surface
tell you about how each metal failed ?

1.7 References

Callister, W.D., Jr., Materials Science and Engineering: An Introduction. 4
th
ed. New
York:Wiley, 1997. See Sections 6.2, 6.3, 6.5, 6.6.

Phillips, James W., ed. Behavior of Engineering Materials: Laboratory Notes 1996-1997
(TAM 224/CE210). Urbana-Champaign: University of Illinois/College of Engineering,
1996. See Lab 5: The Tension Test.