Tension Test and Hardness of Steel and Aluminum
The objectives of this experiment are:
- to determine the hardness, strength, and certain elastic and inelastic properties of a steel and
an aluminum alloy
- to observe the behavior of metals under uniaxial tensile load
- to study the failure characteristics of metals under uniaxial tensile load
- to examine how changes in the metal during failure may influence its hardness.
Essential Apparatus
A universal testing machine with grips
Hardness tester
Electronic extensometer
Caliper
Awl
Materials and Specimens
The ASTM standard flat tensile specimen of steel and aluminum alloy will be used. The
instructor will give the specific type of the materials.
Procedure
A. Tension Test
1.
Using a caliper, measure the width and thickness at several locations along the shaft of
each specimen to determine the average cross-sectional dimensions. Use the marker and
calipers to place gage marks on the specimen two inches apart. Record the average
measured width and thickness of each specimen on the data sheet.
2.
Place the specimen in the grips. Make sure that at least 80% of each end is firmly
attached to the grips, which will help ensure against slipping during the loading
operation.
3.
Insert the zeroing pin into the extensometer to establish an accurate initial gage length of
two inches on the machine. Attach the electronic extensometer securely to the specimen
(note: the extensometer gages do not need to be positioned exactly on the gage marks on
the coupon). Once the extensometer is secured, remove the zeroing pin before beginning
the test.
4.
Begin applying the tensile load to the specimen and observe the live reading of applied
load on the computer display. If the measured load does not increase, the specimen is
slipping through the grips and needs to be reattached. In this instance, the instructor will
stop the test and repeat the procedure from Step 2.
5.
Continue applying tensile load slowly, observing the shape of the computer-generated
load vs. strain graph throughout the loading. Prior to sample failure, the test will be
paused, without unloading the specimen. At this point, remove the extensometer.
6.
Resume loading the specimen until failure. Upon reaching the maximum load, the
measured loads will begin to decrease (note: the maximum load may occur before the
extensometer is removed). At this point, the specimen will begin necking and final
fracture should occur within this necked region. Record the maximum load and the load
at failure.
7.
Remove the broken specimen from the machine. Observe the location and character of
the fracture. Fit the broken parts together and measure the final gage length, thickness,
and width in the necked region. Record these values on the data sheet.
B. Hardness Test
Hardness will be measured in triplicate on two locations on the broken tensile specimens. Make
3 hardness measurements far from the region of necking, and 3 measurements in the necked
region. Below are instructions for operating the Rockwell Hardness tester:
1. Examine the printed information on the machine to determine the Rockwell scale appropriate
for the material to be tested. For most metals, scale B or C is appropriate.
2. Ensure that the indenter (diamond or ball) and load correspond to the Rockwell scale to be
used. To change the load: rotate the loadscale knob located towards the back of the machine
on the righthand side clockwise until the correct load (60, 100, or 150 kg) has been selected.
To change the indenter: see the instructor or teaching assistant.
3. Turn the anvil counterclockwise, if necessary, to lower the sample holder, allowing ample
room between the sample holder and the indenter. Place the sample flat on the holder.
4. Zero the scale on the machine face such that the long needle coincides with the B/C mark.
5. To apply the primary load, slowly turn the anvil clockwise to raise the sample toward the
indenter. When the indenter contacts the sample, the small needle in the machine face dial
will begin moving from the black dot to the red dot. After initial contact, you should turn the
anvil so that the large needle on the scale makes 3 complete revolutions, and the small
needle just reaches the red dot. The large needle on the scale should pass the B/C mark for
the 3rd time, and will be located just past B/C when you stop turning the anvil. DO NOT
CONTINUE TURNING THE ANVIL AFTER THIS POINT.
6. Re-zero the scale on the machine face such that the long needle coincides with the B/C mark.
To do this, you should have to slightly rotate the scale face counterclockwise.
7. Push the lever located on the righthand side back. Wait ~30 seconds until the scale reading
has stabilized.
8. Pull the lever forward, and allow the scale reading to stabilize again. This is the Rockwell
hardness number for your sample. If this is the first measurement made on the sample, you
should repeat the test in two more locations to get a more accurate reading.
To move or remove the sample, rotate the anvil counterclockwise, which lowers the sample
holder and the sample. If retesting the same sample, move the sample on the holder such that the
indenter will encounter a region of the sample that has not been previously tested.
Results
A. Tension Test
Plot two stress-strain diagrams for each material showing
- the initial portion of the stress-strain curve up to yield strength,
- the entire stress-strain curve.
Note that because the extensometer is removed prior to failure, the graph will need to be
extended beyond the computer-recorded strain values to include the strain at failure (measured
using the calipers).
Using the laboratory data and graphs, determine the following material properties:
(1)
Proportional Limit
(2)
Upper and Lower Yield Point
(3)
0.2% Offset Yield Strength
(4)
Maximum Tensile Strength
(5)
Rupture Strength
(6)
Percent Elongation at Failure
(7)
Modulus of Elasticity
(8)
Toughness
(9)
Type and Character of Fracture
(10) True Stress at Failure
Some of these properties may not be obtainable for some materials tested.
B. Hardness
In a table, record for each material tested the average hardness value and standard deviation in
the measurements.
n
Standard Deviation =  
Where xi
xbar
n
(x
i 1
i
 x)2
n 1
= _________
= is a measured value (strength)
= is the average value (average strength)
= number of measurements (3)
Discussion
1. Compare the measured yield strength, ultimate (or maximum) strength, elastic modulus,
ductility (or elongation at failure) and hardness for the metals with typical values.
2. Describe possible sources of error during the test and their effect on data and calculations.
3. Compare the measured properties of the two metals, including yield and ultimate strength,
elastic modulus, ductility, toughness, and hardness.
4. Discuss the difference between Engineering Stress and True Stress and whether there is a
significant difference between these values at failure.
5. Explain the necking process, and discuss how the necking of the specimen relates to the
shape of the stress-strain curve.
6. Compare the appearance of the fracture surface for each material. Discuss what the failure
surface reveals about each material’s behavior under tensile loading.
7. Compare the hardness values obtained near the necked region with those obtained further
from the necked region. Why might they be expected to differ?
UNIAXIAL TENSION TEST DATA SHEET
Material 1: ___________
Gage Length (initial):___________
Thickness (initial):_____________
Width (initial):___________
Cross Sect. Area (initial):_________
Gage Length (final):____________
Thickness (final):______________
Width (final):____________
Cross Sect. Area (final):__________
Maximum Load:_______________
Failure Load:_____________
Hardness :_____, _______, ______
Hardess in necked region: ______, _____, ______
Material 2: ___________
Gage Length (initial):___________
Thickness (initial):_____________
Width (initial):___________
Cross Sect. Area (initial):_________
Gage Length (final):____________
Thickness (final):______________
Width (final):____________
Cross Sect. Area (final):__________
Maximum Load:_______________
Failure Load:_____________
Hardness :_____, _______, ______
Hardess in necked region: ______, _____, ______
Instructor’s Signature______________________