On Lamb Modes as a Function of Acoustic Emission Source Rise Time
Marvin A. Hamstad
A study was carried out to examine Lamb-wave modal content as a function of the acoustic emission (AE) source rise time. The study used a validated finite element code to model the source operation and subsequent wave propagation up to a distance of 480 mm in a 4.7 mm thick aluminum plate with large transverse dimensions. To obtain the large propagation distances with sufficient transverse dimensions so that plate edge reflections did not arrive during the significant part of the direct AE signal, an axi-symmetric code was used. The buried dipole AE sources were located at three different depths below the top surface of the plate, where the pseudo AE sensors were located. These sensors provided the out-of-plane displacement as a function of time. The rise times for the different finite element runs varied from 0.5 µs to 15 µs. The resulting data was high-pass filtered at 40 kHz and re-sampled with a time step of 0.1 µs. The intense portions of the Lamb modes were determined by use of a Choi-Williams Transform (CWT) for the range of source rise times, three different source depths and the signal propagation distances. Higher Lamb modes were observed in the CWT results for the shortest rise times, but fundamental mode frequencies still dominated for all the rise time cases and different source depths. Thus, only the fundamental modes need to be considered in the determination of accurate signal arrival times.