On trailing waves rather than lamb waves being generated by buried in-plane dipoles in thick plates

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The potential for trailing waves (TWs) rather than Lamb waves to be observed in the out-of-plane displacement signals from acoustic emission (AE) sources in thick plates was examined. Finite element modeling (FEM) was done for buried in-plane dipole (IPD) sources in steel plates with thicknesses up to 125 mm and propagation distances from 250 mm to 1500 mm. The domain was large enough so that significant reflections were not observed during the primary signal arrivals. The run times extended to 1 ms. “Ring-type” sources were used so that axisymmetric modeling could be done. The rise time of the IPDs was typically 1 μs with a cosine-bell time dependence, and the two 1 N forces were spaced by 200 μm. The IPDs were located at several depths from very near the top surface of the plates to the midplane. The displacements as a function of time were obtained on both the top and bottom surfaces. The analysis primarily considered a bandpass of 80 kHz to 500 kHz. In addition, fast Fourier transform results and time/frequency analysis were used. The typical character of TWs consisted of a train of short wave-packets (each 10 to 25 microseconds). The significant-amplitude TW packets arrived after an arrival time corresponding to the Rayleigh wave velocity. The number of packets in the train increased with increasing propagation distance. The likelihood of the signals exhibiting TWs increased with increases in plate thickness, increases in the distance of the IPD below the surface of interest. The presence of the TWs complicates the determination of fixed velocity arrival times necessary for source location determination. Also, since TWs rather than Rayleigh waves appear for IPD depths at some distance below the surface used to sense AE signals, there are potential issues when the AE approach depends on a Rayleigh wave being present.