Mode separation of Lamb waves based on dispersion compensation method

Abstract

Ultrasonic Lamb modes typically propagate as a combination of multiple dispersive wave packets. Frequency components of each mode distribute widely in time domain due to dispersion and it is very challenging to separate individual modes by traditional signal processing methods. In the present study, a method of dispersion compensation is proposed for the purpose of mode separation. This numerical method compensates, i.e., compresses, the individual dispersive waveforms into temporal pulses, which thereby become nearly un-overlapped in time and frequency and can thus be extracted individually by rectangular time windows. It was further illustrated that the dispersion compensation also provided a method for predicting the plate thickness. Finally, based on reversibility of the numerical compensation method, an artificial dispersion technique was used to restore the original waveform of each mode from the separated compensated pulse. Performances of the compensation separation techniques were evaluated by processing synthetic and experimental signals which consisted of multiple Lamb modes with high dispersion. Individual modes were extracted with good accordance with the original waveforms and theoretical predictions.

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“…The ability to isolate 97 the guided modes of interest is the key for a successful analysis 98 of ultrasound data. Post-acquisition signal processing techniques 99 such as singular value decomposition [11], s À p transform [12], 100 group velocity filtering [13], dispersion compensation [14], and 101 the joint approximate diagonalization of eigen-matrices algorithm 102 (JADE) [15] are viable methods to separate wavefields. Guided 103 modes can also be selectively excited by using angle beam.…”

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