EXPERIMENT A1
Mechanical Testing of Materials
Summary:
Welcome to the ME-242 laboratory! In this laboratory you will perform hands-on
experiments with the Instron tensile test machine and conduct analyses that will allow you to determine
the materials properties of several test articles. These materials properties will include:



yield strength
tensile strength
elongation
Before testing, you will learn to calibrate the load cell and extensometer and select appropriate
operating conditions. After testing you will need the graphs from your chart recorders along with the
various measurements of sample geometry to calculate sample properties. Your samples will include
some or all of the following:




a hot worked steel
a cold worked steel
an aluminum alloy
and several plastics with various properties
Instructions:
Your key to success in this lab is to come prepared!

Before arriving at the lab, read through this lab module so that you will understand what the lab
procedure is and how the lab equipment is used.

Each group should answer all the questions on the preliminary question sheet to be turned in at
the beginning of the lab.

Each group will write one report. General guidelines for writing this report may be found in the
section on weekly laboratory reports contained in the lab manual.

Timing: This lab takes the majority of one afternoon (approximately 4-5 hours).
What’s an “Instron?”
The term Instron is frequently bandied about test labs and in industry. An
Instron is a universal test machine. But be careful! Instron is a brand name
and there are many brands of universal test machines including MTS and
Tinius-Olsen, among others. The labs in the Department of Mechanical
Engineering feature both Instron and MTS test machines.
Instron was established in 1946 in Boston, Massachusetts by Harold
Hindman and George Burr. Mr. Hindman was working on a project to
determine the properties of new materials to be used in parachutes. Since
test machines available at that time did not have the necessary
performance criteria to adequately evaluate these new materials, Mr.
Hindman teamed up with Mr. Burr to design a material testing machine
based on strain gauge load cells and servo control systems.
The resulting prototype was so successful that Mr. Hindman and Mr. Burr
formed Instron Engineering Corporation. The name was derived from the
‘ins’ in the word instruments and the ‘tron’ in the word electronics.
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Example of a Universal
Test Machine
Background:
The background for this lab can be found in most introductory materials science texts such as
Materials Science and Engineering, by Callister.
Engineering stress is the force per unit (original) area.
Engineering strain is the elongation per unit (original) length.
following symbols:
Engineering Stress, S 
Ao
lo
F
l
Where
F
Ao
and Engineering Strain, e 
They are represented by the
l
lo
=
original cross sectional area of specimen
=
=
=
original length of the gauge section
applied force
change in length
For a linear elastic material, these parameters are related by Hooke's law,
S  Ee
where E is Young's modulus. It is implicit here that only axial stresses and strains are of interest.
Otherwise, Hooke's Law is significantly more complex since stress is also dependent on the strain in
other directions. Note, it is assumed S  0 when e  0 so that S  E e represents a line that passes
through the origin with E as the slope.
True stress and true strain differ from engineering stress and strain by referring to the
instantaneous areas and gauge lengths respectively. The symbols for these values are the Greek letters
 and  :
True stress,
where
li
Ai

F
Ai
and
True Strain,
d 
dli
li
= instantaneous length of gauge section
= instantaneous cross-sectional area.
The strain has a natural logarithm dependence because it is determined from the instantaneous
gauge length. To show this, we can integrate the instantaneous true strain increment d

li
dl
l
lo
 d  
0
to obtain
l 
lo 
  ln  i .
Note that
so that when
l  lo ,
1
1
1
ln 1  x   x  x 2  x 3  x 4   ,
2
3
4
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l  l 
 ln 1  e  e .
 lo 
  ln  o
For strains of about 1%, the "error" is of order of  or 10 -4. Consequently, there is no significant
difference in the engineering and true strains when all measurements are of small strains. The true stress
and strain are also related by the modulus E ,   E  since the modulus is established at a small strain
level where Ai is approximately equal to Ao and li is approximately equal to lo .
2
For large strains in plastic deformation, the volume of specimens is approximately conserved. Because of
this, the instantaneous area Ai can be calculated from the true strain. Assuming volume is conserved,
Volume =
Aolo  Aili . Or, rearranging and taking the natural logarithm, we obtain


Ai 
lo 
A 
l 
  ln  o   ln  i .
Thus, Ai  Ao exp( ) . Note that a tensile true strain followed by an equal compressive true strain
reproduces that initial length of the specimen. This is not true for engineering strain.
Tensile Test Specimens
During a tension test, it is desirable to apply forces to the specimen
large enough to break it. Hence, some test engineers spend their
careers breaking things for a living!
In order to collect useful data in a tension test the grip region of the
test specimen must have a large enough area to transmit the forces
without significant deformation or slipping. Consequently, most
specimens have a reduced gauge length and enlarged grip regions.
Example of a tensile test
specimen
Grip Area
While most material properties are supposed to be specimen
geometry and grip independent, there are some weak
dependencies. Thus, the American Society for Testing Materials
(ASTM) has specified standard specimen geometries.
Gauge Length
ASTM has also prescribed test methods so that data reported for
design purposes is obtained in a standardized way. The specimen
geometry is usually reported as part of the test results.
More info on the ASTM may be found at: www.astm.org
Returning to our discussion of the properties, the data we will record is the load elongation curve. Since
many materials are rate sensitive, the rate of elongation is controlled during the tensile test by moving
one of the grips at a fixed displacement rate relative to the other. Usual testing rates correspond to
 3 1

s
engineering strain rates of about e  10
where the  represents differentiation with respect
to time. For example, if the specimen had one inch gauge length, the displacement of the machine is 10-3
inches per sec. and the load is recorded on a strip chart traveling at constant speed, say 1/10 inch per
-3 -1
second, then it is clear that the 10 s strain rate will produce 10-3 inch displacement in 1/10 inch of chart
or 1% strain in one inch of chart. Chart length and strain are then parametric variables, both dependent
on time. This is the simplest way of measuring the load-elongation curve and is the most common.
However, the elongation determined in this way also included the elongation of the grips, the ends of
specimen, the load measuring transducer (load cell) and the deflection of all the test frame. Typically, at
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the yield strength of a steel, the other elongation outside the gauge length is about 5 times larger than the
elongation inside the gauge length.
Consequently, we cannot measure the elastic modulus from the slope of the load vs. elongation curve
determined in this way. To circumvent this problem and make direct measurements, an extensometer is
installed on the specimen that measures displacement within the gauge length. This transducer is
designed to produce a linear voltage output with respect to displacement. Since the initial gauge length is
fixed, the output is then proportional to the engineering strain. If the load signal (voltage which is
proportional to the applied force) and the extensometer signals are plotted using an X-Y plotter, the initial
slope is then the elastic modulus.
For stability, the load must increase all the time. The tensile deformation is unstable and strain is no
longer uniform when the load reaches a maximum. Deformation stability is achieved when the specimen
hardens during deformation. The result is uniform elongation. If the hardening rate is too low, a runaway
situation called necking develops. To avoid neck formation, the hardening rate must be faster than the
decrease in cross sectional area
d


dA
.
A
Now if the volume remains constant
or
V  Al
The strain can be written in terms of the change in area as
d 
dV  0  A dl  l dA ,
dl
dA

.
l
A
Substituting, we obtain the requirement for stability
d
.
d
When
d
  , then dF  0 and the sample is unstable. This can be shown as follows. By definition
d

F
A
F   A.
or
Differentiate this equation to obtain
dF  A d   dA.
When the load is maximum,
dF  0 and
A d   dA  0
or
d
.
d
This is the critical value for the work hardening rate. As a result the specimen may neck down and begin
local deformation. This occurs at the peak load. To determine the true stress strain behavior beyond the
peak load requires knowledge of the non-uniform geometry of the neck in both the calculation of strain
and the stress distribution. In certain materials, the true stress at fracture can be several times the
engineering stress.
Most data you will be exposed to are engineering stress and strain unless otherwise specified. If there is
a yield point, namely, a sharp transition between elastic and plastic deformation, yield stress is defined as
the stress at the yield point. If there is a yield drop, the maximum stress is the upper yield point and the
minimum stress is the lower yield point. If the curve is smooth, yield stress is defined at a specific amount
of plastic strain. Usually 0.2% permanent strain is used to define the yield stress. Then the yield stress is
so identified as S. The proportional limit is the stress where the flow curve first deviates from
linearity. This is intrinsically difficult to measure because it is related to the sensitivity of your instruments.
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Try to estimate the proportional limit when you analyze your data. The ultimate tensile strength is the
largest engineering stress achieved during the test to failure.
The elongation to failure is the permanent engineering strain at fracture determined at zero load. It does
not include elastic strain but does include both the uniform strain and the localized, necking, strain. The
elongation to failure is usually stated as percent strain over a given gauge length. The reduction in area
is also a measure of ductility. The true strain at fracture is determined by measuring the areas of the
fractured specimen at the fracture site. Recall using the constant volume approximation that

Ao
.
Ai
The area under the engineering stress-strain curve is a measure of the energy needed to fracture the
specimen. It has units of energy per unit volume of the gauge length and it is sometimes referred to as a
measure of a material's "toughness." However, the term fracture toughness more commonly refers to the
energy required to propagate a crack per unit area increase of crack size.
Advanced Test Applications
The Instron you will be using today can apply a
load…either tensile or compressive…in one axis.
In industry, test engineers might want to apply
multiple loads across a variety of axes in order to
determine ultimate performance of a product or
device.
One example of this is the structural tests that were
performed on the Chandra Space Telescope’s
optical metering bench. The Chandra Telescope is
one of NASA’s Great Observatories and it’s optical
metering bench was designed, fabricated, and
tested by the Eastman Kodak Company in
Rochester, NY.
In order to perform a structural test a special 3story test frame was constructed in which dozens
of actuators were mounted. The actuators were
attached to the Chandra optical bench and various
loads were applied that simulated conditions that
the structure might experience during it’s launch
into orbit. In function these actuators are similar to
the Instron’s in that they apply a controlled load in a
specific direction.
This 26 ft-long by 100in. diameter optical bench
was designed and built by Kodak for NASA's
Chandra X-ray observatory. Weighing 675 lbs., the
honeycomb structure is the largest composite
metering device ever built for use in space.
For more information: http://chandra.harvard.edu/
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Apparatus:
In this experiment we will use an Instron machine designed to do tensile tests of specimens. The
machine has a 5000 lb. capacity. It consists of a large heavy duty test frame with a fixed beam at the top,
a moving beam (referred to as the crosshead) and a gearbox and very large motor located in its base.
The specimen is mounted between two grips, one attached to the fixed top beam and the other attached
to the moving crosshead. The fixed beam at the top contains a load cell (which works on the principle of
strain gauges). It measures the applied force on the tensile specimen. The movement of the crosshead
relative to the fixed beam generates the strain within the specimen and consequently the corresponding
load. The gearbox below selects high and low speed ranges for movement of the crosshead.
Fixed Beam
Specimen
Grips
crosshea
d
Start / Stop
and
Speed control
Console
Load Cell
Bridge control
and
Chart Recorder
Console
Figures 1 Instron Tensile Tester
Load Cell
bridge gain
Up / Down /
Stop
Buttons
Load Cell
Shunt
Circuit
Calibration
Speed
Selector
Figures 2 & 3: Control Consoles for Instron Tensile Tester
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Next to the test frame is the associated electronics consoles. They contain the main start/stop controls
for testing and the adjustments for the sensitivity of the strain gauge load cell (a strain gauge bridge) as
well as a chart recorder to read the output of the load cell bridge. The electronics consoles also contains
the gear speed selection box for the gearbox (allows us to select the various strain rates) and the main
on/off switches for the instrument, one to turn the instrument on directly and the other to turn the amplifier
for the gearbox motor on/off (called the Amplidyne switch).
In order to enhance the accuracy of our measurements of Young's Modulus we will add an extensometer
directly to the sample to measure the actual elongation between two given points on the sample to record
the load vs. elongation curve for the elastic region of the sample only.
Finally, a data acquisition system utilizing a PC, a National Instruments data acquisition card, and
LabVIEW software will be provided to collect data directly from the extensometer and load cell. Data will
be gathered from this system and post processed with a spread sheet program such as Excel.
Figure 4: Instron Tensile Tester virtual Instrument.
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Experimental procedure:
1. Equipment Calibration:
Calibration coefficients for the Instron load cell and extensometer must be generated in order to
convert the voltage data acquired during a tensile test to real data.
Since the extensometer (an LVDT) and the load cell are linear in our testing regions, two point
static calibrations will be sufficient for each device.
Prior to running calibration please switch the “Operation Mode” switch to CAL and wait for the
yellow LED to activate.
a. LVDT Calibration:
i. Run the LabVIEW program “Instron Tensile Tester Fall 2005.vi”
ii. Attach the extensometer to the provided calibration stand ensuring the
extensometer is fully closed.
iii. Measure and record the gage length of the extensometer with vernier calipers.
iv. While monitoring the voltage output of the load cell on the LabVIEW program
,zero the voltage output of the extensometer with the “Zero” control on the LVDT
Conditioner.
v. Collect 5 to 10 seconds of data with the Instron Tensile Tester Program
vi. Extend the calibration stand 0.010” and collect a second set of data.
vii. Using Excel, determine the calibration coefficient for the extensometer in
(volts/in).
NOTE: Although we are measuring strain (which is unit-less) with the LVDT the
calibration coefficient will be necessary to determine the gage length of the test
specimen in volts for the software to function correctly.
b. Load Cell Setup and Calibration:
i. Set the Full Scale Load selector to 20.
ii. While monitoring the voltage output of the load cell on the LabVIEW program,
zero the load cell voltage with the balance knobs on the Instron left control
console.
iii. Collect 5 to 10 seconds of data with the Instron Tensile Tester Program
iv. Depress the “Calibration” button on the Instron Console and collect a second set
of data while holding the button down. Pressing this calibration button applies a
resistance change to the load cell bridge equivalent to that caused by hanging a
5000 lb. weight on the load cell. The preliminary calculation that you have done
in the preparatory questions should confirm that for the steel samples we should
use the 5000 lb. full scale range for measurement.
v. Using Excel, determine the calibration coefficient for the load cell in (lbs/volt).
NOTE: For a sanity check of the load cell calibration, place a hanging weight of
30lbs on the load cell and check to see if your calibration coefficient works for the
applied load.
After completing calibration please switch the “Operation Mode” switch to Test and wait for the
green LED to re-activate.
2. Measure and record the diameter and lengths of all the samples.
3. Install the first specimen in the grips. Be careful to follow the recommended installation
procedures as given by the instructor so that no damage occurs to you or the test equipment. Be
careful to avoid placing any part of your body at a pinch point. The final coupling should be
performed by trial and error by slipping the pin in by hand with the machine stopped. Move the
crosshead up and down at a very slow speed until you can do this manually.
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4. Install the calibrated extensometer on the specimen. Be sure that it is centered and straight and
that it is fully closed. Rezero the extensometer with the zero control on the LVDT Conditioner.
Any deviation will be an offset error that will need to be addressed when scaling your results.
Figure 5: Test Specimen in the Grips with Extensometer Attached
5. Select the appropriate crosshead speed for the material being tested by the table provided. The
Instron speed scale is in cm/min, and the table gives speeds in in/min. Be sure to do the
conversion before continuing!
In the material specs tab on the Instron Tensile Tester.vi, input the following:
a. Crosshead Speed in (in/min)
b. Gage Length (in volts)
c. Strain for LVDT Release (extensometer release) (typically 2% strain is sufficient)
6. Depress the Strain units button under the Real Time Stacked Plots of Load and Strain tab in the
VI (Virtual Instrument) this will enable the conversion of voltage data from the extenosometer to
strain values. The gage length must be entered correctly for this feature to work properly.
7. Input a data file name in the VI.
8. Start Saving data, and select the Load – Strain Graph to view the data acquisition
(Note, the Load Strain Graph has auto scaling axis, the noisy looking data will transform as the
test begins.)
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9. Double check the following:
a. Your test specimen is properly loaded in the grips of the tensile tester
b. The lower grip pin is in place
c. The correct crosshead speed is selected.
d. The material specs are input into the LabVIEW VI.
e. The Strain units button is depressed (in the LabVIEW VI).
f. You are saving data.
10. Start the test by pressing the down button on the Instron control console.
11. Observe the specimen. Do not get too close because fracture of the specimen liberates all the
stored elastic energy in the specimen. Do you see bands propagating along the steel specimen?
These are Lu ders bands indicating the multiplication and motion of dislocations. They will not be
visible unless the specimen is highly polished.
12. Be sure to record both load vs. time and load vs. strain for the initial portion of the test. Remove
the extensometer when the LabVIEW VI displays the “REMOVE EXTENSOMETER” on the
Load – Strain Graph and continue the test recording the load vs. time curve until fracture.
Observe the neck formation. Note that it occurs right after the maximum load.
13. Do this for all of you specimens. (You will not use the extensometer on the 0.5 in diameter plastic
specimen.) Use the conditions given in the chart in the appendix for each of these samples.
Reduced area observed
Figure 6: As the specimen approaches ultimate stress the reduction in
area becomes clearly visible. This is referred to as “necking.”
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Discussion:
Report the following data for each of the samples if it exists:
Young's modulus
Proportional limit
Upper and lower yield stress
0.2% yield strength
Ultimate strength
% Elongation at fracture
% Reduction in area
Compare your results of Young’s modulus, yield stress, and ultimate strength with published values.
Explain any discrepancies.
In your discussion please address the following
1. Determine the error that would result if you calculated Young's modulus from the load
displacement curve without the extensometer clipped on the specimen. Explain the cause of this error.
Consider an in-series spring representing the machine stiffness (that would also include the grips and the
part of the specimen outside of the gage length). Determine and compare the values of the machine
spring constant calculated from the data for each specimen. Why are these values different?
2. Plot a true-stress versus true-strain curve for the cold rolled steel specimen (Hint: use a
constant volume approximation) and compare to the engineering stress versus engineering strain plot.
Plot only for the region where the calculation is valuable. What is the limit of the calculation and why?
3. The stress-strain curves for plastics are very different for those of metals (e.g. aluminum and
steel). Explain in terms of the differences in atomic or molecular deformation mechanisms.
4. The cold worked steel specimen does not show a yield point, the hot worked steel does.
Why? After plastically deforming the sample, would either of these samples show a yield point upon
reloading? Why?
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APPENDIX
Recommended chart speed, crosshead speed and full scale
Materials
Color
Chart speed
Crosshead
Speed
Full Scale
(A) 1018 cold-worked
(B) ASTM A-36 hot-worked
(C) 2024-T351
(D) Nylon-101
(E) Polyethylene-Hi-density
(F) PVC
white-blue
blue-blue
Red
Blue
Green
Gray
0.5 in/min
0.5 in/min
0.5 in/min
1.0 in/min
0.5 in/min
0.5 in/min
0.02 in/min
0.05 in/min
0.05 in/min
0.2 in/min
1.0 in/min
0.1 in/min
5,000 lb
5,000 lb
5,000 lb
5,000 lb
2,000 lb
2,000 lb
Caution:
Changing the chart speed requires replacement of gears. The chart motor has considerable inertia and
requires several seconds to stop. Do not touch the gears while they are moving.
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EXPERIMENT A1 MECHANICAL TESTING
PRELIMINARY QUESTIONS
Group number (names):_____________________________
1.
Date:______________
What are potential safety concerns for this experiment?
2.
How does a1030 steel differ from a1040 steel? Be specific. Address what the numbers mean as
well as how the properties differ.
3.
How does a cold-worked steel differ from a hot-worked steel? Be specific.
4.
For a 1020 steel sample with a length of 2.25" and a diameter of .235" calculate the maximum
load you would expect to have to apply to fracture the sample. Based on this value, what load cell range
would you choose and why? Also, estimate the maximum elongation a 2 inch sample would experience
before plastic deformation (estimate this value assuming yield occurs at 0.2% strain). Based on this
value, what crosshead rate would you choose for your experiment and why? At this crosshead rate, how
long would you predict it would take to fracture the specimen?
5.
Assume that the load cell being used is set to a 2000 lb. full scale and has an accuracy of 2% full
scale.
(a)
What will be the accuracy in reading a 1000 lb. load (in terms of a % of the
actual load)?
(b)
What will be the accuracy in reading a 200 lb. load (in terms of a % of the
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actual load)?
6.
Why do we put an extensometer on the sample rather than just use the extension of the frame of
the Instron? Is use of the extensometer important in measuring the elastic modulus? Is the use of an
extensometer valuable for measuring the ultimate strength?
7.
Consider the Instron machine (with stiffness k m ) and the sample (with stiffness k s ) as springs in
series with total stiffness k t. What is the relationship between these three stiffnesses? During the test, you
must keep track of the scales on each of your charts and label them appropriately. If your computer gives
a plot of force versus crosshead position and another plot gives the force versus sample elongation from
the extensometer clipped on the specimen, what stiffness would be given by the slopes of each of these
plots?
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