Bar-Wave Calibration of Acoustic Emission Sensors

This study extended a bar-wave calibration method for acoustic emission (AE) sensors. It combined laser interferometer displacement measurements and the wave propagation medium of a long bar, excited at its end with an ultrasonic transducer driven by a pulser. Receiving bar-wave sensitivities of 16 types of AE sensors were measured and compared to their receiving sensitivities to normally incident waves. The two types of the receiving sensitivity always differed for a given AE sensor. The bar-wave sensitivities of R6a sensors resembled their surface-wave sensitivities, indicating that the bar-wave sensitivities can represent the surface-wave sensitivities in typical AE applications. Some bar-wave modes were identified by comparing peaks found on observed Choi-Williams transform spectrograms with the positions on the dispersion curves for bar waves, calculated with the SAFE procedure. However, numerous bar-wave modes prevented exact identification, especially above 500 kHz. Aperture effects contributed to the sensitivity reduction at higher frequencies and to more fluctuating bar-wave receiving sensitivities even for sensors with smooth or flat receiving sensitivities to normally incident waves. Spectral dips observed in bar-wave results can be accounted for by aperture effect predictions reasonably well. For the selection of AE sensors, one needs to use the appropriate type of sensitivities considering waves to be detected.