Brief Project Background
Pyrotechnic induced shock can potentially be devastating to electronic equipment. Increasing use of pyrotechnics as a means for mechanical actuation warrants increasing need to validate the effects they have on system components. These shocks were often ignored, yet further work by Moneing has shown critical failures induced by pyrotechnic shock. [1] Mathematical and computational models have difficulty with the computational resources required. In particular the FEM analysis has difficulty modeling the high frequency characteristics of pyrotechnic shock. The requirement of a large number of tests has proven to be an inefficient method of modeling these shock responses. Computational methods often yield much more conservative results due to the sacrifice in processing power. [4]
Not only is this shock difficult to recreate in a testing situation, it is also difficult to model particularly as a function of time. Irvine recommends the use of the Shock Response Spectrum, or SRS, [3] to estimate the damage potential a shock may have. The SRS facilitates the analysis of shock on the component, rather than trying to analyze the extremely short duration, transient shock in the time domain. The SRS shows peak acceleration of a pre-determined series of natural frequencies that would be imparted by a certain shock. [3]
The rapid decay, transient nature, and extreme frequencies are difficult to simulate using a shaker to induce vibrations. Mechanical shock inputs such as pneumatic and hammer blow tests can yield optimal results, yet are time consuming in their tuning. [4] Additionally, the shock imparted often cannot be subjected directly to the component in testing, but through a mounting which could have substantially different mechanical properties thereby hindering the accuracy of the results. [3] High acceleration shock loadings are more accurately created by explosives; however, this is rarely done in practice due to the obvious dangers. [4]
Works by Chu and others have noted significant sources of error in accelerometer measurements in pyrotechnic shock. Actual pyrotechnic explosions can excite piezoelectric accelerometers at their natural frequency. [5] Replicating the pyrotechnic shock mechanically, as opposed to simulating with real pyrotechnics, can potentially solve any issues encountered with accelerometer measurements.
Tests done to electronic components by Luhrs have focused mostly on using a drop test to simulate pyrotechnic shock. He notes the discrepancies between using a drop test and shaker test as opposed to identical testing on a simulated spacecraft structure with a shock induced by pyrotechnics. No equipment failures occurred, until 2500g peak acceleration was reached, where crystal oscillators began to fail. On the other hand, a simulated spacecraft structure test setup experienced no failures until upwards of 7000g peak acceleration. [5] Findings by The Harris Corporation agree with Luhrs in that the drop test was overestimating the shock accelerations. [2]
References
- [1] Wattiaux, David, Olivier Verlinden, Calogero Conti, and Christophe De Fruytier. Prediction of the Vibration Levels Generated by Pyrotechnic Shocks Using an Approach by Equivalent Mechanical Shock. Tech. no. 10.1115/1.2827985. Vol. 130. N.p.: ASME, 2007. Web. 23 Sept. 2014.
- [2] Wells, Robert. "Conference Call with Robert Wells." Telephone interview. 24 Sept. 2014.
- [3] Tom, Irvine. "AN INTRODUCTION TO THE SHOCK RESPONSE SPECTRUM." (2012): 1-3. 9 July 2012. Web. 23 Sept. 2014.
- [4] Robert, Wells. "University Capstone Development of Hammer Blow Test Device to Simulate Pyrotechnic Shock." 19 July 2013. Web. 23 Sept. 2014.
- [5] United States. The Shock and Vibration Information Center. The Under Secretaty of Defense for Research and Engineering. The Shock and Vibration Bulletin. By Anthony Chu and Henry Luhrs. Washignton, D.C.: Naval Research Laboratory, 1987. Print.