Download Figure 7 : Location of Strain Gage 1

Project:
Cycle Advancements for Rugged Terrain
Project Record:
Strain Gage Field Testing
Version 1
October 24, 2016
Adam Peris and Jonathan Bright
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Version History
Version
Date
Description/Notes
1
9/29/2016
Goal, Recording Data, Calibration
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Table of Contents
Abstract
Goal
Recording Data
Calibration
Testing
Conclusion
Appendix I
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Abstract
To determine the fatigue life of the hitch, a method to determine the dynamic loads on the hitch
should be developed. To do this, strain can be found on different parts of the hitch using strain
gages placed in specific areas. From there, the loading can be determined and stresses can be
found. This record outlines the strategies used to calibrate the tools used for data collection, the
method of data collection, and a summary of the testing process.
Goal
This phase was aimed towards testing the strain experienced by the hitch throughout the duration
of a fixed testing circuit. Before testing could be completed, a method to record instantaneous
readings of strain to software needed to be developed, the strain gages box needed to be
calibrated, and a driving course for testing needed to be planned that simulated rough roads that
would be found in developing countries.
Recording Data
Strain gages were attached to the hitch in three locations that we felt would be critical. Each
strain gage location consisted of 2 gages, with one parallel to the axis of loading and the other
perpendicular to the axis of loading. This was to counteract the effects of temperature on the
output of strain gages. Because the hitch experiences dynamic loading, a way to output the strain
gage box’s instantaneous analog output needed to be recorded to a computer software. The box,
which was a Measurements Group P-3500 Digital Strain Indicator (See Figure 1), outputs an
analog signal of voltage with a range of ±2.5V. A National Instruments Hi-Speed USB Carrier
(See Figure 2) was connected to the strain gage box using a coaxial cable, and to a computer
with a USB connection. A software program, SignalExpress, reads the output given by the strain
gage box an interval of time set by the user. Initially, SignalExpress was set to record at a
sample period of 10 milliseconds, or 100 points of data each second. At this sample rate, we
noticed that when the strain data was plotted, there was only one data point at the peaks, with no
data points leading up to the peak. This raised concerns that the sample rate was not quite high
enough to capture the maximum strains that the hitch was experiencing. The sample period was
decreased to 6 milliseconds to better capture the peaks of the impacts. Shorter sample periods
than 6 milliseconds were tried, but it was found that any sample period shorter than 6
milliseconds were too small for our system to record. In the end, it was decided to use the 6
millisecond sample period, which resulted in 167 data points each second. In the future it may
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be better to record the strain gage data with an oscilloscope to achieve a shorter sample period
closer to 1 millisecond and thus a faster sample rate.
Figure 1: Digital Strain Indicator Figure 2: BNC to USB Converter
Calibration
The strain gage box’s analog voltage output is proportional to the amount of strain the gage is
experiencing. The conversion factor can be varied by the user by adjusting an amplifier on the
box. To calibrate the instrument, the balance was set to display a high, constant reading of
microstrain. Because the box displays an output of microstrain, and SignalExpress displays an
output of voltage, a ratio between the two can be used to determine the number of volts
SignalExpress reads per microstrain the strain gage box reads. After exporting the data to Excel,
this ratio is used to convert the voltage read by SignalExpress into microstrain. The conversion
factor was found by applying 4000 μє to the strain gage box and noting the output voltage of
1.9231V on SignalExpress. The conversion factor is shown in figure 3 below.
1.9231𝑉 0.000481𝑉
=
4000μє
1μє
Figure 3: Voltage-Strain Conversion Factor
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Testing
A testing course was mapped out in the gravel pathway found in the back of Messiah’s property,
behind Mellinger apartment complex. Figures 4 and 5 show the test circuit mapped out using
Google Maps:
Figure 4: Testing Course relative to Messiah College campus
Figure 5: Close up of the testing course
The circuit is intended to emulate rough conditions that would be experienced on roads found in
developing countries. The circuit was completed by the motorcycle with a loaded trailer. The
trailer was loaded with approximately 250 pounds with the weights centered over the axle of the
trailer. A laptop, converter box, and strain box were placed in a bin on the trailer to log strain
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data in real time as the test was being conducted. One strain gage was wired to the strain gage
box at a time. The trailer was then lifted off of the hitch to insure that no strain was being
applied to the hitch while zeroing, and the strain gage box reading was zeroed. The trailer was
then reattached to the hitch and the logging was started on the laptop. The motorcycle was
ridden around the test course one time until it reached the starting point and the data logging was
then stopped. This procedure was repeated twice for each strain gage location.
Figures 6 through 10 show the location of the strain gages and the setup of trailer for testing:
Figure 6: Strain gage placement on the hitch
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Figure 7: Location of Strain Gage 1
Figure 8: Location of Strain Gage 2
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Figure 9: Location of Strain Gage 3
Figure 10: Weights and logging instruments on the trailer
Once all of the tests for each strain gage were completed, the collected data was exported from
SignalExpress to Excel. The strain-voltage conversion found in the calibration stage was used to
convert from voltage to microstrain in order to plot strain over time. The largest strain that we
found was 1732 microstrain during test 1 on strain gage 3. This was the strain gage located near
the lower shock mount (see Figure 5). Graphs of the results of our testing can be found in
Appendix I.
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Conclusion
The results of the strain testing quantify the strain experienced by the hitch during dynamic
loading. Peaks and smaller, periodic data points are both distinct within the data. In the future,
the data obtained will be used to convert the strain found on different parts of the hitch into stress
to determine the loading that a peak in the data represents. These results will be applied in a
finite element model in SolidWorks, ultimately to determine the fatigue life of the hitch and
determine if our hitch design will have an acceptable life before failure.
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Appendix I
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