GEORGIA INSTITUTE OF TECHNOLOGY
School of Aerospace Engineering
AE3145 Laboratory
Strain Gage Installation and Tensile Coupon Test
Purpose:
This is a two-part laboratory and will take place over two successive weeks. The purpose of this
lab is to familiarize you with some of the basic characteristics of bonded electrical resistance
strain gages and then to use these gages in a simple tensile coupon test to determine the elastic
modulus of a sample. You will also learn how to apply analytical techniques (strain
transformation and Mohr’s Circle) to analyze the resulting strain gage data from single and
rosette strain gages.
Procedures: Week 1 - Strain Gage Application
Lab Work
The first lab session will be devoted to learning to apply metal foil strain gages to a flat specimen
using cyanoacrylate adhesive. You will see a demonstration and then you will apply one or more
strain gages to a tensile test coupon specimen (“dogbone” specimen). The general procedures
are as follows:
The lab class will be broken into smaller groups and each group will be given a “dogbone”
specimen of a particular metallic material. You will then complete the following:
1 Measure the cross section dimensions of the “dogbone” specimen using a micrometer.
2 Lay out the location for the strain gage(s).
3 Prepare the surface to receive the gage(s).
4 Glue the gage(s) to the surface.
5 Solder jumper wires and leadwires to the gage(s).
6 Test the electrical integrity of the gage(s).
Supplementary slides are available on the web page explaining the above procedures.
Strain Transformation Exercises
In preparation for the next lab and for other labs involving the reduction of strain gage data from
rosette gages, you are expected to review the material from AE2751 and AE3120 (or the quarterequivalent courses) on 2D stress and strain and Mohr’s Circle.. Supplementary notes are
provided to review strain transformation equations and Mohr’s Circle.
A set of pre-lab homework problems in also included in these supplementary notes. These
problems are to be worked and turned in at the start of this Strain Gage Application Lab
This will complete the first lab session. No formal lab report is required but you are
expected to turn in the pre-lab homework instead.
AE3145 Strain Gage and Tension Coupon Test Lab (S00)
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Procedures: Week 2 - Tensile Coupon Testing
The purpose of this next lab experiment is to investigate the properties of a material under
tension (a tensile test) in the elastic region and from the elastic limit to fracture. The scope of this
lab will include the effects of yielding on the stress-strain curve and the relation ship between
strain hardening and the elastic modulus. The elastic-plastic relations will be investigated up to
the fracture. The specimen will be an aluminum or steel “dogbone” shaped coupon (the TA’s
will provide the material type). This will be the same specimen prepared by your group in the
previous lab session.
Procedure
1 Determine the nominal yield strength for your specimen from handbook data. Express the
yield strength in terms of applied load for the specimen.
2 Place the specimen in the Instron Test Machine and attach a digital strain indicator to the
strain gage(s).
3 Each loading cycle is divided into three stages. Stage I should reach 85% of the yield load
while Stage II should reach 110% of the yield load. Stage III unloads to zero. Take at least
six data points in Stage I, eight data points in Stage II and six data points in Stage III.(unless
otherwise specified by TA's) Complete three full cycles before final cycle to failure.
Data Analysis:
Your analysis of the results must consider the following specific items along with any other
points that are raised.
1
Plot the stress- strain relations and determine the Elastic Modulus of the specimen within the
elastic limit in Stage I.(region prior to reaching yielding where stress is proportional to
strain). Do this for every full cycle completed.
2 Determine the yield strength from your experimental results and compare it the theoretical
result determined in step 1 of the procedures.
3 Determine the Elastic Modulus of the specimen during unloading(Stage III) after the yield
strength has been achieved, and compare this value to that of #1 above; discuss the results.
Do this for every full cycle completed.
4 Determine the plastic deformation after every cycle. Discuss the results.
5 Report the engineering failure (fracture) stress for this specimen.
6 What is the effective length of the “dogbone” when the strain is taken to be dL/L. Report this
length for strains 1000 µstrain, 2000 µstrain, ..., gage failure. Does the gage length remain
constant throughout the range of deformation?
7 Determine the strain-hardening exponent for this loading. Fit a curve through the stress-strain
curve at the strain hardening region.
σ = σ 0 (ε 0 + ε p ) n
Lab Report
You are required to submit a formal lab report for this tensile coupon testing portion of the lab
assignment. Use the format provided in the first lab.
AE3145 Strain Gage and Tension Coupon Test Lab (S00)
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