Experimental Validation of Dispersion Curves in Plates for Acoustic Emission

This paper presents the findings of an investigation to determine theoretically and empirically the wave speeds and frequency content of the two primary Lamb wave modes, the symmetric (S0) and anti-symmetric (A0). A 2 mm thick steel plate measuring 700 mm by 700 mm was used to perform all measurements. A broadband pulse propagated through the plate and detected by a conical type piezoelectric receiver was used to show how the dispersive properties of the plate influenced the detected AE signals. It was shown that the two primary Lamb wave modes cover a very broad range of velocities, leading to a severe spreading of arrival times. A further investigation was completed using four acoustic emission sensors to record a pencil lead fracture, which was used as an artificial source. Reflections in the plate were shown to cause interference in the signal that can complicate the interpretation of the arrival modes. A recorded signal 400mm from the source was filtered into frequency bands. The arrival times of the wave modes were determined for each frequency band and the appropriate velocities calculated allowing a dispersion curve to be plotted experimentally. The plotted curve was shown to be a very close approximate to the calculated curve.