Transcript Stress and Strain Lab - Gateway Engineering Education Coalition

Engineering H192 - Computer Programming
Stress and Strain
Lab 5
Winter Quarter
Gateway Engineering Education Coalition
Lab 5
P. 1
Engineering H192 - Computer Programming
Objectives
• To understand how
engineers compare
different materials.
• To understand how a
strain gage works.
• To understand important
issues for data acquisition.
• To understand the need for
experimental data.
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• To understand some basic
structural features of a
bicycle.
• To make measurements on
a statically loaded bicycle.
• To understand why
different materials are
used to make bicycles.
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P. 2
Engineering H192 - Computer Programming
Stress vs. Strain
Strain:

Hooke’s law:

Stress:   F
l
A
  E
Where:
E = Modulus of Elasticity


= stress
= strain
For the bike fork material E = 29.0 x 106 psi.
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P. 3
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Measuring Load on a Bicycle Fork
Loads applied by rider
STRAIN GAGE
A sensor that
measures strain
When the bicycle is loaded (a person is
riding), the fork is loaded. Due to the load,
the fork deforms. The STRAIN GAGE is used
to measure the strain in the fork.
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Loads on a Bicycle Fork
Compression
Loads applied by rider
Bending
Loads applied by the road
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Shearing
The fork is loaded
by a combination
of compression
shear and bending
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Data Acquisition
System
Strain
Gage
Transducer
Converts
measured
quantity to
measurable
electrical
quantity
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Wheatstone Bridge
+ Amplifier
Conditioning
Circuits
Converts
transducer
output to
electrical
signal to be
amplified
Data
Processing
Recorder
Records
amplifier
output.
Recorder may
be analog or
digital
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Computer
analysis,
display
graphs,
tables
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Transducer: Strain Gage
•
The strain gage is a resistor.
Its resistance changes if its
length changes.
•
Measured Quantity: strain
•
Measurable Electrical
Quantity: resistance.
•
The strain gage is cemented
to the bicycle fork. When the
bicycle is loaded the fork is
strained and the strain gage
resistance changes.
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Backing Film
Grid
(electrical resistor)
Copper-plated
Solder tabs
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Strain Gages
• Resistance is Proportional to Length

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R
R
or
S g 
R
R
Where: Sg is the “gage factor”
and  is the strain.
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P. 8
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Conditioning Module: Wheatstone Bridge
• The strain gages are connected as the four
resistors in the Wheatstone Bridge.
• The bridge converts the change in the strain gage
resistance to an output voltage (Vout) that is
proportional to the strain in the fork due to the
forces acting on the fork. The output voltage is
fed to an amplifier.
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Conditioning Module: Wheatstone Bridge
The change in the output voltage from the
Wheatstone Bridge is related to the strain of the
strain gage by:
Vout  Vin  A  S g  
Where:
Vin = 5.0 volts
 is the strain
A = 500 (amplification)
Vout is the change in the voltage
Sg = 2.085 (gage factor)
The equation can be solved to give the strain as a
function of the output voltage.
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Lab Experience: Part 1 – Static Test
•
The goal in this part of the lab is to find a correlation between the weight
of the bicycle rider and the stress/strain due to loading of the front fork.
•
Set up the data logger as described in the quick reference guide. Record
each rider’s weight.
•
With the bike trainer attached to the bicycle, collect data with the data
logger for each member of your group. Use the following sequence to
collect one set of data (all times are approximate). Do not stop the data
logger between each step or between each rider.
– 5 seconds for an unloaded bike.
– 5-10 seconds for rider 1 in riding position (no pedaling).
– 15 seconds for rider 1 pedaling.
– Repeat for remaining team members.
•
Upload your data from the data logger to the PC. Export your data as an
ASCII file (consult the quick reference guide) and save it on a diskette.
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Lab Experience: Part 2 – Dynamic Test
•
•
•
•
•
•
•
•
The goal in this part of the lab is to observe the loading conditions in the
front fork of a bicycle while being ridden, and to compute the maximum
and minimum stress values.
Select the lightest and heaviest person in your team. These two will ride
the bike.
Set up the data logger as described in the Quick Reference Guide. Use the
“real-time data acquisition” mode to view the data collected by the data
logger.
Make sure that the initial output signal without a rider is near two volts.
Record the obtained value.
With each rider sitting still, record the output signal (in volts). Record the
weights of each rider.
Reset the data logger to have an acquisition rate of 50 Hz.
Take the bicycle outside following the instructor’s direction and have both
riders ride the bike for about 45 seconds each. Leave the bike sit unloaded
between riders for a few seconds so you know when you switched riders.
Bring the bike back to the lab. Upload your data from the data logger to the
PC. Export your data as an ASCII file (consult the Quick Reference Guide)
and save it on a diskette.
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Lab Report Guidelines
•
•
•
Cover page including name, class, team #, date and instructor.
Use the sample lab report format given.
Lab Experience:
– PART 1: Static Test
– Plot the raw voltage data vs. time and identify each event on
the graph, i.e. unloaded bike, rider 1 gets on, rider 1 sits on
bike, rider 1 pedals bike, etc.
– Calculate the strain and the stress at each data point.
– Create a graph of stress vs. time. Label your graph.
– Using average value of stress for each rider in the riding
position while pedaling, create a plot of stress vs. weight of
each rider. Create a linear trend line for the data and show the
equation of the line on the graph.
– Include sample calculations (calculating strain from voltage,
calculating stress from strain)
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Lab Report Guidelines
– PART 2: Dynamic Test
– Plot raw voltage data vs. time.
– Modify your assignment E13 to determine the maximum
and minimum peak stresses and the corresponding
elapsed times when they occur for your data. Document
your program and tabulate the obtained results.
– For each rider, compare the maximum and minimum
stress values to the ones obtained from the static test.
What is the ratio of the two quantities?
• Analysis of Plots/Results.
• Conclusions
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Equations
Recall that
where:
vout = Vin . A . Sg . 
Vin = supply voltage (5.0)
Vout = change in voltage
Sg = gage factor (2.085)
A = amplification (500)
 = strain.
  E
also,
where: E = Modulus of Elasticity


= stress
= strain
For the bike fork, E = 29.0 x 106 psi.
Winter Quarter
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Lab 5
P. 15