Experiment No. 7
Name: Strain
Gauge Trainer
Aim: To shows and compares how resistance
strain gauges work, and how they measure strains in different structures. Later
the stresses or the load could be calculated.
Theory:
The compact
Strain Gauge Trainer fits on a bench or desktop. It contains everything needed
to show how resistance strain gauges work on three different structures. The
bending system uses gauges to measure direct tensile and compression strain.
The torsion system shows the use of shear / torque strain gauges. The tension
system shows the use of two gauges at right angles in a ‘Tee’ rosette. The
strain display includes a set of high-accuracy dummy strain gauge resistors
(plugs) and controls. These allow the user to connect the strain gauges on the
structures as quarter, half or full-bridge networks.
The gauge
factor GF
Where; 🔺R
For metallic foil gauges, the gauge factor is usually a little over 2.
For a single active gauge and three dummy resistors, the output V
In order to measure strain with a bonded
resistance strain gage, it must be connected to an electric circuit that is
capable of measuring the minute changes in resistance corresponding to strain.
Strain gage transducers usually employ four strain gage elements electrically
connected to form a Wheatstone bridge circuit. A Wheatstone bridge
is a divided bridge circuit used for the measurement of static or dynamic
electrical resistance. The output voltage of the Wheatstone bridge is expressed
in millivolts output per volt input. The Wheatstone circuit is also well suited
for temperature compensation.
Technical Details
|
Item |
Details |
|
Fixed Bridge Voltage |
5 VDC |
|
Nominal Gauge Resistance |
350 Ohm |
|
Dummy Plugs Resistance |
350 Ohm |
|
Tension System Specimen |
E = 207 GPa, ν = 0.30,
Cross Section = 10 x 2 mm |
|
Bending System Beam |
E = 207 GPa, ν = 0.30,
Cross Section = 20 x 5 mm |
|
Torsion System Beam |
G = 79.6 GPa, ν = 0.30,
Dia. = 10 mm, L = 150 mm |
By the apparatus many experiments can be done through the
different bridge connections (quarter, half and full-bridge) like:
1-
Tensile stresses and strains in
different materials and comparison of Poisson’s ratio and Young’s modulus
2-
Strains and stresses in a bending system.
3-
Strains and stresses in a
torsion system.
A- Tensile Test Procedure:
1-
Make the
quarter bridge connection by using one strain gauge for longitudinal strain
with dummy strain.
2-
Fix the hook
load and set the strain gauge reading to zero reading.
3-
Increase the
load by 0.5 kg load and record the strain gauge reading for each case till the
maximum load.
4-
Draw the
relation of the calculated stress with the measured strain to get the Young’s
modulus.
5-
Repeat
procedure 1, 2 & 3 by using one strain gauge for lateral strain
measurement.
6-
Draw the
relation of the longitudinal and the lateral strains to get the Poisson’s
ratio.
7-
Make the half
bridge connection by using two strain gauges [ both longitudinal ] with two
dummy strain gauges, and repeat step 3.
8-
Make full
bridge connection and repeat step 3.
9-
Draw the gotten
results from step 7 & 8 with the applied load and get the sensitivity [
slope ] for each case. Then discuss that variation in sensitivity.
→ Make the below table for each case of the above mentioned steps of
quarter, half and full bridge of the strain gauges measurement, then complete
the required calculations.
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Measured Dimensions of the specimen =
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Used Strain Gauge: |
Quarter
Bridge |
Quarter
Bridge |
Half Bridge |
Half Bridge |
Full Bridge |
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|
Load ( kg ) |
Force ( N ) |
Calculated
Stress ( MPa ) |
Measured
Strain ( μϵ ) |
Measured
Strain ( μϵ ) |
Measured
Strain ( μϵ ) |
Measured
Strain ( μϵ ) |
Measured
Strain ( μϵ ) |
Measured
Strain ( μϵ ) |
|
0 |
0 |
0 |
0 |
0 |
0 |
0 |
0 |
0 |
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1.0 |
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2.0 |
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3.0 |
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5.5 |
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7.0 |
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8.0 |
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B-
Bending Test Procedure:
1-
Make the
quarter bridge connection by using one strain gauge for longitudinal strain at
the top surface of the cantilever with dummy strain.
2-
Fix the hook
load at specified distance from the fixed end and set the strain gauge reading
to zero reading.
3-
Increase the
load by 10 g load [ 10 pieces of the standard weight ] and record the strain
gauge reading for each case till the maximum load.
4-
Make the half
bridge connection by using two strain gauges [ both tension ] with two dummy
strain gauges, and repeat step 3.
5-
Repeat step 4
by using two strain gauges [ both compression ], and repeat step 3.
6-
Make full
bridge connection and repeat step 3.
7-
Make the below
table for each case of the above mentioned steps of quarter, half and full
bridge of the strain gauges measurement, then complete the required
calculations.
|
Measured
Dimensions of the specimen = Load Position
= |
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|
Used Strain
Gauge: |
Quarter Bridge |
Quarter Bridge |
Half Bridge |
Half Bridge |
Full Bridge |
|
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|
Load
(g) |
Force
(N) |
Bending Moment ( Nm) |
Calculated Stress (MPa) |
Calculated
Strain (μϵ) |
Measured Strain (μϵ) |
Measured Strain (μϵ) |
Measured Strain (μϵ) |
Measured Strain (μϵ) |
Measured Strain (μϵ) |
|
0 |
0 |
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0 |
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100 |
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200 |
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300 |
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400 |
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500 |
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8-
Draw the
results gained from steps 3 & 4 [measured strain with the calculated
stress] to get the modulus of elasticity of the beam and compare with the
standard value.
9-
Draw the
results gained from steps 4 & 5 [Tension strain with Compression strain] to
get the Poissons ratio of the tested beam.
10- Draw the gotten results from step 3, 4 & 5 with the applied
load and get the sensitivity [ slope ] for each case.
C- Torsion
Test Procedure:
1-
Make the half
bridge connection by using two strain gauges [ both tension ] with two dummy
strain of the torsion rod.
2-
Fit the torque
arm at the end of the torsion rod and set the strain gauge reading to zero
reading.
3-
Increase the
load by 100 g load [ 20 pieces of the standard weight ] and record the strain
gauge reading for each case till the maximum load.
4-
Make full
bridge connection and repeat step 3.
5-
Make the below
table for each case of the above mentioned steps of quarter, half and full
bridge of the strain gauges measurement, then complete the required
calculations.
|
Measured
Dimensions of the specimen: Torque Arm
Length = |
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|
Used Strain
Gauge: |
Quarter Bridge |
Half Bridge |
Half Bridge |
Full Bridge |
|||
|
Load (g) |
Force (N) |
Torque ( Nm) |
Calculated Shear Stress (MPa) |
Calculated Strain (μϵ) |
Measured Strain (μϵ) |
Measured Strain (μϵ) |
Measured Strain (μϵ) |
|
0 |
0 |
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0 |
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100 |
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200 |
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300 |
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400 |
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500 |
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6-
Draw the gotten
results from step 3 & 4 with the
applied torque and get the sensitivity [ slope ] for each case.
Discussions
1-
Compare between
the calculated modulus of elasticity and Poissons ratio of the specimens that
are used in tension and bending test.
2-
Discuss that
variation in gained sensitivity from the plotted curves for quarter, half and
full bridge for strain connections.
3-
Give practical
examples for the using of strain gauges for mechanical measurements like force,
moment, torque, pressure or other applications.
