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Stress and Strain. Lab 4. 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. To understand some basic structural features of a bicycle.

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stress and strain

Stress and Strain

Lab 4

Winter Quarter

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.

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.

Objectives

Winter Quarter

stress vs strain

Strain:

Stress:

Hooke’s law:

Where:

E = Modulus of Elasticity

s = stress

e = strain

For the bike fork material E = 29.0 x 106 psi.

Stress vs. Strain

Winter Quarter

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.

Measuring Load on a Bicycle Fork

Winter Quarter

loads on a bicycle fork

Loads applied by rider

Compression

Shearing

Bending

The fork is loaded by a combination of compression shear and bending

Loads applied by the road

Loads on a Bicycle Fork

Winter Quarter

slide6

Strain

Gage

Data Acquisition System

Wheatstone Bridge

+ Amplifier

Data

Processing

Conditioning

Circuits

Transducer

Recorder

Converts measured

quantity to measurable electrical

quantity

Records amplifier output. Recorder may be analog or digital

Converts transducer output to electrical signal to be amplified

Computer analysis, display graphs, tables

Winter Quarter

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.

Backing Film

Grid

(electrical resistor)

Copper-plated

Solder tabs

Transducer: Strain Gage

Winter Quarter

strain gages

Where: Sg is the “gage factor”

and e is the strain.

or

Strain Gages
  • Resistance is Proportional to Length

Winter Quarter

conditioning module wheatstone bridge
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.

Winter Quarter

conditioning module wheatstone bridge10

The change in the output voltage from the Wheatstone Bridge is related to the strain of the strain gage by:

Where:

Vin = 5.0 volts

A = 500 (amplification)

Sg = 2.085 (gage factor)

The equation can be solved to give the strain as a function of the output voltage.

Conditioning Module: Wheatstone Bridge

Winter Quarter

lab experience part 1 static test
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.

Winter Quarter

lab experience part 2 dynamic test
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.

Winter Quarter

lab report guidelines
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)

Winter Quarter

lab report guidelines14
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

Winter Quarter

equations

Recall that Dvout = Vin .A . Sg . e

where:

Vin= supply voltage (5.0)

DVout= change in voltage

Sg= gage factor (2.085)

A= amplification (500)

e= strain.

also,

where:

E = Modulus of Elasticity

s= stress

e = strain

For the bike fork, E = 29.0 x 106 psi.

Equations

Winter Quarter