MODAL ANALYSIS OF ACOUSTIC EMISSION SIGNALS

Abstract

This report shows the results of a continuing study to determine the type of waves present in plates of different thickness created by breaking pencil leads in-plane (IP) and out-of-plane (OOP) on the plates and by detecting the resulting stress waves with a high fidelity acoustic-emission transducer mounted on the edge and surface of the plates. The results show that the extensional velocity over the range of plate thickness from 3.13 to 12.5 mm does not change, and the flexure wave is dispersive in accordance with plate wave theory. The procedure in this work split the signal into two frequency ranges, a high frequency range with a 100 kHz high-pass filter and a low frequency range with a 20-70 kHz bandpass filter. This frequency split provided an easy method of measuring the velocity of the low frequency flexure wave. A bulk shear wave, not predicted by plate wave theory was observed along with the extensional wave in the high frequency channel. In order to confirm the presence of the shear wave, two shear plates, one with particle motion vertical to the plane of the plate and the other parallel to the plane of the plate were attached to the end of one of the bars. These transducers were used as receivers for IP and OOP pencil-lead breaks, and also as transmitters excited by a 150-V spike pulse. Both the Sv and Sh waves, created with pulsing, could be detected with a transducer mounted on the surface with petroleum jelly. The Sh wave had a much higher amplitude than the Sv wave. The results show that mode conversion of the Sv wave to a flexure wave occurs, as it propagates down the bar, which partially accounts for its loss of amplitude. From this study, it is suggested that the crack growth signal detected by most AE tests on large thin-wall structures utilizing 100 kHz high-pass filtering, resonant transducers, and large transducer spacing, is the Sh wave created by crack propagation.