B Calibration of Split Hopkinson Pressure Bar Setup: Calibration is performed by comparing the wave speed of the initial pulse versus the reflected pulse and calculating the Energy Ratio. This ratio gives an indication as to the accuracy of the bar setup and efficiency of energy transfer between the incident and transmission bars. This ratio should be between 95-105% which will be seen in SurePulse after calibration shot. 1. On laptop computer, double click on the PicoScope desktop icon to open program. Double Click 2. In the PicoScope 6 software, there are a few settings that need to be changed. a. Select VIEWS from the tab headings at the top (left) of the screen, click on VIEW PROPERTIES from the drop down menu to display the PROPERTIES window. Figure 15: Pico Scope- View Properties b. Each strain gage on the bar setup is associated with a channel on the PicoScope. Change the INPUT RANGE to ±10V and the COUPLING to DC. Channel A (BLUE) is set up for the strain gage on the Incident bar and Channel B (RED) is set up for the strain gage on the Transmission bar. Figure 16: PicoScope Channel Set-Up 9
c. Set the COLLECTION TIME to 200µs/div as a good starting point, this may need to be adjusted as data collection continues. Figure 17: PicoScope Collection Time d. At the bottom left of your screen select NONE as the TRIGGER MODE. TRIGGER MODE: NONE will be used for real-time signal conditioning. TRIGGER: NONE is only for Vishay & PicoScope calibrating, not for bar calibrating. All bar calibration and test shots will use a SINGLE trigger point. Figure 18: PicoScope Trigger 3. With the Vishay Signal Conditioning Amplifier and the PicoScope software operating, it is necessary to condition the signal. a. In the PicoScope software be sure NONE is selected from the TRIGGER MODE menu and the START CAPTURE button is highlighted. This allows you to see the output signal in real-time. Figure 19: Conditioning the Signal b. Turn ON EXCITATION (Noted in Step#8.d.2) 10
c. On the front of the Vishay Signal Conditioning Amplifier, move the toggle switch for AUTO BALANCE to RESET and hold for 3 seconds. You should see the voltage jump in a positive direction on the PicoScope screen while the toggle switch is held on RESET and return to 0 (zero) when the toggle is released. Repeat for both the Incident and Transmission bars (both Channels A & B). For step by step instructions, please see Vishay Signal Conditioning Amplifier instruction manual. Figure 20: Conditioning the Signal d. To condition the signal, the calibration resistor switch must be engaged to A+ (A+ for a 120Ω Strain Gauge & B+ for a 350Ω Strain Gauge) while simultaneously using the GAIN dial to adjust the voltage to 2V as seen on the PicoScope screen. For voltage calibration detail, a more accurate adjustment, change Channel A & B on the PicoScope screen (top, left-side, menu) to ±2V. Record the voltage calibration target as this will be needed for SurePulse (in this example record: 2V). Release A+ and signal will return to 0 with no Off-Balance lights present. See Vishay Signal Conditioning Amplifier instruction manual for balancing procedures. Figure 21: Voltage Calibration Adjustment 11
e. Adjust TRIM dial to eliminate Out-of-Balance (OUTPUT) error light. See Vishay Signal Conditioning Amplifier instruction manual for balancing lights. Figure 22: Adjust TRIM 4. The system is ready to record the bar calibration shots. Calibrate the incident bar first. a. Close both the Trigger (Gas Release) Valve and the Air Supply Valve Figure 23: Trigger & Air Supply Valve b. Connect air supply to quick-connect coupling, make sure that air supply valve is CLOSED before connection of air supply. c. Position Incident bar with Momentum Trap DISENGAGED (OPEN) and slide Incident bar to 1 from the end of the barrel (just over the width of a thumb). 12
Figure 24: Engaged v. Disengaged Figure 25: Positioning Incident Bar d. For Incident Bar Calibration: Ensure that the transmission bar will not contact the incident bar during shot, leave approximately 3 between ends of bars. e. Make sure Trigger Valve is closed. With compressed air supply quick coupled to air supply valve, charge launcher by opening air supply valve to fill chamber then close valve. Adjust chamber pressure with needle valve. Fill chamber to 15psi for Calibration. When charging the launcher, listen for thud as the air valve slides into place, sealing the compression chamber. (Important: In Calibration record both: Shot Pressure and Striker Bar Speed) 13
f. Using the ram rod, ensure striker bar is butted-up (seated) to the breach pin correctly, this must be done from the muzzle end of the barrel, after air supply valve is closed and chamber is pressurized to desire setting. Figure 26: Launch Sequence g. On the PicoScope screen, set your TRIGGER MODE from NONE to SINGLE. Note: Data will not be recorded in TRIGGER MODE: NONE. For impact testing a SINGLE trigger point will be used. Figure 27: Set Trigger to SINGLE h. At the bottom of the screen, select Channel A, this will trigger the event recording off of the Incident bar strain gauge. Your trigger will show up on the screen as a small yellow diamond. Figure 28: Select Channel A i. Select the FALLING EDGE icon. In compression testing this triggers the data collection when a signal crosses the threshold in the falling direction or a compression pulse. Figure 29: Select FALLING EDGE 14
j. Input your desired THRESHOLD, which is the level that the signal must cross to trigger a capture of data. Start with a number between -100 mv and -600 mv and adjust if needed due to lab noise, etc. Figure 30: Set THRESHOLD k. Input your desired PRE-TRIGGER. This is the amount of data that is captured, before the trigger diamond, as a percentage of the total capture time (typically 10%). Adjusting PRE-TRIGGER will adjust the diamond right-to-left on the screen. Figure 31: Set PRE-TRIGGER l. Select START CAPTURE toggle in bottom left of PicoScope screen. Be sure the START CAPTURE button is highlighted Blue to record data. Figure 32: Select START CAPTURE 15
m. Open Trigger Valve quickly to take shot. Open (turn valve 90 O ) Trigger Valve in a swift and consistent motion. Caution: when firing launcher do not stand directly behind Trigger (Gas Release) Valve as some of the compressed air is released. Figure 33: Firing Accelerator/Launcher n. Taking this shot will result in a wave output displayed on the PicoScope graph on your laptop similar to graph shown below. This is your incident bar calibration shot and must be saved. Figure 34: Incident Bar Calibration Shot o. To save the results of this shot, select the FILE tab at the top of your screen, select SAVE_AS from the drop down menu. First save your results as the default (*.psdata) and then save again as a text file (*.txt); each time using same base File Name. This gives you the original strain gage readings as your Meta data, and the information in text format that can be used by the SURE-Pulse analysis software. Figure 35: Saving Incident Bar Calibration Shot 16
5. Repeat Step #B.4 to calibrate the Transmission bar, with the following change. a. This time set the Transmission bar so that it is in firm contact with the Incident bar. A good practice is to slide the two bars into contact with each other and twist them in opposing directions to make sure there is perfect contact between the bars. The following graph is a typical waveform from a transmission bar calibration shot. Again, you must record both incident bar and transmission bar signals when doing your calibration shots for both bars. C Figure 36: Transmission Bar Calibration Shot b. Save and record all results in the same way as the incident bar. Testing Specimens/Data Collection: 1. Measure and record test specimen pre-shot dimensions: Diameter and Length. 2. Grease the ends of the specimen then place specimen, centered on the impact faces between the incident and transmission bars. With specimen in place, twist both pressure bars in opposite directions about a quarter turn to seat specimen in grease. Figure 37: Specimen Impact Area 3. Charge the launcher and adjust pressure, record pressure reading. When charging the launcher, listen for thud as the air valve slides into place, sealing the compression chamber. 4. Using the ram rod, ensure striker bar is butted up to the breach pin correctly, this must be done after air valve is closed inside the launcher. 5. On the PicoScope screen, reset your TRIGGER MODE to SINGLE from the menu. Select START CAPTURE button. 17
6. Open Trigger Valve swiftly and consistently to take shot. 7. Taking this shot will result in a wave output displayed on the PicoScope graph on your laptop similar to graph shown below. This is your specimen shot and must be saved. You must record both incident bar and transmission bar signals when doing your specimen shots. Figure 38: Specimen Shot Display 8. Save your results, select the FILE tab at the top of your screen, select SAVE_AS from the drop down menu. First save your results as the default (*.psdata) and the save again as a text file (*.txt) using same base File Name. This gives you the original strain gage readings as your meta data, and the information in text format that can be used by the SURE-Pulse analysis software. 9. Measure and record test specimen post-shot dimensions: Diameter and Length 18