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Project: Cycle Advancements for Rugged Terrain Project Record: Strain Gage Field Testing Version 1 October 24, 2016 Adam Peris and Jonathan Bright 1 | Page Version History Version Date Description/Notes 1 9/29/2016 Goal, Recording Data, Calibration 2 | Page Table of Contents Abstract Goal Recording Data Calibration Testing Conclusion Appendix I 3 | Page 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 4 | Page 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 5 | Page 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 6 | Page 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 7 | Page Figure 7: Location of Strain Gage 1 Figure 8: Location of Strain Gage 2 8 | Page 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. 9 | Page 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. 10 | Page Appendix I 11 | Page 12 | Page 13 | Page