An assessment of acoustic emission techniques on medium-sized pressure vessels tested to failure

Location analysis has been used to examine acoustic emission from a partial thickness slit in a test vessel during pressurization. Preliminary invesigations showed that sharp emission pulses at a source would propagate as sharp pulses to a sensor several metres distant and that intrinsic material attenuation was negligible at the 165 kHz operating frequency. Pulse arrival times at three sensors were used to locate emission sources and the requirement for near-coincidence in arrival time at all sensors reduced background noise to very low levels. The slit was clearly delineated by its emission. Emission activity moved from the centre to the ends of the slit as the stress on the slit increased, suggesting that emissions were generated during the initial yielding of the steel. This indication was strengthened by the fact that the emission count appeared to be directly proportional to the volume of plastically yielded material associated with the defect, based on a simple model for the increase in plastic zone size with pressure. Application of acoustic emission techniques to structural validation is discussed in terms of the measured amplitude spectrum of the detected acoustic emissions. A relationship is derived between the detected emission count, the threshold amplitude for detection, the sensor spacing and the created plastic volume for the steel studied. (Auth.)