Experiment # 2 Tension Testing of Metals

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Nov 29, 2013 (4 years and 1 month ago)

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CEEN 043 – Behavior and Properties of Materials Laboratory Manual and Exercises
Tension Testing of Metals


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Experiment # 2
Tension Testing of Metals


OBJECTIVES OF LABORATORY
To give the student an opportunity to conduct tensile testing for grade A36 steel and 6061-T6 aluminum to
determine the behavior of these common structural metals when subjected to tensile stress and to
experimentally determine mechanical properties of these materials suitable for design. The student will also
be given the opportunity to apply experimental data for design and analysis.

OUTCOMES OF LABORATORY EXERCISE
Upon completion of the laboratory exercise, the student will be able to describe the pertinent material
properties obtained via tension testing. He or she will be able to conduct a tension test using a universal
testing machine and will be able to describe the unique components of the testing apparatus (e.g.
extensometer, cross-head, grips, etc…). The student will be able to determine the elastic limit, the yield
strength, and the modulus of elasticity for metallic materials. The student will be able to describe the
meaning of set (permanent) deformation within the context of tension testing and will be able to describe the
unloading behavior of metals after the elastic limit has been surpassed. The student will be able to apply the
0.2% offset technique to determine the modulus of elasticity of metals without a well-defined yield point.
The student will be able to apply experimentally determined material properties to design and analyze
structural components.

INTRODUCTION
The monotonic static tension test is the most common testing methodology for determining mechanical
properties of metals. In general, tension tests provide information related to the strength and ductility of
metals under direct tension stress. It is a very common testing procedure for quality control and specification
validation.

One should be very careful to recognize that a static tension test may not reflect the actual in-service behavior
of the material. For example, the tension test is conducted with controlled loading rates, high-tolerance
machining of specimens, controlled temperature conditions, and other environmental conditions unique to the
laboratory. However, the data obtained from tension testing is very useful for design purposes.

Tension testing of metallic materials is covered under the Standard Test Methods for Tension Testing of
Metallic Materials: ASTM E8 – 96a, and AASHTO No. T 68. Several other ASTM standards referenced by
E8-96a are: A356/A356M; A370; A557; E4; E6; E8M; E29; E83; E345; E691; and E1012.

EQUIPMENT
The testing apparatus most often used for tension testing is the universal testing machine. A Riehle universal
tension-compression testing machine is housed in the Structural Engineering Laboratory (EN151). The
loading magnitude supplied by the testing machine has been calibrated (perhaps some time ago) to ensure
accurate measurement of loading. The loading is read from a load cell inside the machine and strain is
recorded using an extensometer with two-inch gauge length.

There are several specimen varieties that satisfy ASTM E8-96a. Our experimental setup utilizes round
specimens with a segment of reduced cross-sectional area to initiate yielding within a predetermined segment
of the specimen. The specimen is held in the testing machine crossheads using wedge-type grips. The
laboratory instructor will go over all the important elements of the testing apparatus at the time of the testing.

CEEN 043 – Behavior and Properties of Materials Laboratory Manual and Exercises
Tension Testing of Metals


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EXPERIMENTAL PROCEDURE
The ASTM E8-96a standard testing method has been tailored for our course. For example, the E8-96a
requires that upper- and lower-yield points of the material be determined. Due to equipment limitations (at
present) we will not be able to do this in our laboratory. Other modifications have been made for our
purposes, however, the spirit and main requirements of the ASTM standard have been preserved for our
laboratory exercise.

The following procedure should be used when conducting the laboratory.

1.) Measure and record the diameter of the reduced section of the steel tensile specimen.

2.) Place two marks, approximately 2 inches apart, centered on the reduced portion of the specimen
and record this dimension as the gauge length. It should be noted that when the extensometer is
“attached” it might leave “dimples” in the steel/aluminum. This can be used to help establish the
gauge length. Mount the tensile specimen within the testing machine and affix the extensometer
within the reduced portion of the tensile specimen. Zero all readings (turn the dials as required).

3.) Place a piece of graph paper on the X-Y plotter and adjust the pen position to a lower grid origin
location. Establish the maximum load range as 20 kips and re-zero the pen position.

4.) Apply the tensile load at a strain rate between 0.05 and 0.50 in/in per minute. One can use the
digital readout on the extensometer to adjust the strain rate. As the tensile loading is applied to the
specimen, the Poisson effect will cause it to contract. Therefore, continue to tighten the set-screws
on the extensometer during loading application.

5.) Monitor the LOAD during the initial portion of the test to determine the Y-axis scale values.
Reverse load during the initial linear portion of stress-strain curve before a tensile load of 2 kips is
reached and again after the yield point has been exceeded but before 1.0% strain has been observed.
Continue loading until 2 % strain has been observed. Remove extensometer and continue loading
until failure. Place the load cell on the Peak Hold setting so that one can record the ultimate tensile
stress obtained by the specimen.

6.) After specimen failure, fit the fractured ends together carefully and measure both the distance
between the gauge marks and the reduced diameter of the specimen to the nearest 0.01 in. If the
reduced shape is elliptical, the reduced area may be calculated by
1 2
4d d, where
1
d and
2
d are
the major and minor diameters, respectively.

7.) Repeat steps 1 through 6 for each remaining tensile specimens.

Upon completion of the testing, we naturally would like to have the results for later use. Please record all
data contained on the Tension Test Data Sheet and give to the lab instructor at the end of the laboratory. He
will collect all laboratory group data and coalesce them into a single spreadsheet for downloading from the
web page(s). All groups can then use this data for their lab write up and exercises.

CEEN 043 – Behavior and Properties of Materials Laboratory Manual and Exercises
Tension Testing of Metals


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CALCULATIONS AND EXERCISES
This laboratory contains a set of computations that need to be performed using the data collected from the
laboratory experiments. In addition, this data will be used to solve several mechanics problems.

Part A
:
Collect all experimental data for the A36 steel and the 6061-T6 aluminum specimens. Using a spreadsheet,
compute the average and standard deviation for the following quantities:

1.) The tensile strength of the materials:
u
ut
o
P
A
 
2.) The percentage elongation in the specimens:
100
f o
o
L L
L

 
 
 

3.) The percentage reduction in area for the specimens:
100
f o
o
A A
A
 
 
 

4.) The modulus of elasticity for the materials:
offset o
o
offset
P A
P A
E
 
 
 
 
 
 
. Use the appropriate stress and
strain levels that are consistent with the ease in which the yield point can be seen in the experimental
data.
5.) The tensile yield strength of the materials:
y
yt
o
P
A
 


Part B
:
Consider the rigid bar and tension rod system shown below.


20 k
5 ft.
5 ft.2 ft.
A B C
D
Rigid Bar



One would like to design rod BD for the given system. The objective of the design is to determine the cross-
sectional area needed for a 6061-T6 aluminum rod design and an A36 steel rod design. Two criteria should
be considered in your designs: (a) the stress in rod BD should be less than or equal to the average material
yield stress minus one standard deviation; (b) the deflection at point C on the rigid bar should not exceed
0.15 inches (use the average modulus of elasticity for deflection estimates).