Acoustic emission from depressurization to detect/evaluate significance of impact damage to graphite/epoxy pressure vessels

Six aerospace-type, filament-wound graphite/epoxy pressure vessels were studied. Four of these cylindrical vessels had each received a single, controlled impact; four had received thermal exposures including moderate heat and/or cryogenic cold. Acoustic emission (AE) was monitored during a proof test sequence (after the impact/thermal exposure), including: the final depressurization from proof pressure (i.e., the second unload cycle). A single set of wideband AE sensors was used to simultaneously record parameter-based AE and waveform-based AE on two independent AE instrumentation systems. Some slight to moderately large differences in AE activity for impacted vs. nan-impacted vessels were noted for AE from Felicity ramps and holds at pressure. However, dramatic differences were apparent in quantities of AE from the final depressurization cycle: impacted vessels experienced unload AE activity which was a minimum of an order of magnitude greater in quantity than the unload AE activity for non-impacted vessels. This difference provided a distinctive means of identifying those vessels with impact damage. A newly defined "Shelby ratio" was introduced as a means of quantitatively assessing the unload AE; correlations were obtained between vessels' residual strength and the unload "Shelby ratios." Refinements of the parameter-based data analysis approach were made possible by waveform information. Additionally, examination of the waveform data of the unload cycle revealed a significant quantity of "friction" type waveforms having a repeated, lower frequency character; similar "friction" waveforms were able to be artificially generated by rubbing a pencil lead across a composite vessel surface.