Chirp-based dispersion pre-compensation for high resolution Lamb wave inspection

Highlight
• Dispersion worsens resolution and makes Lamb wave signals hard to interpret.
• A scheme for dispersion pre-compensation is established.
• Closely-distributed defects can be recognized after dispersion compensation.
• The resolution of received wave packets is quantitatively evaluated.

The dispersion of ultrasonic guided waves causes the energy of a signal to spread out in space and time as it propagates, which decreases the performance for damage detection significantly. A lot of signal processing methods have been proposed on how to obtain each mode under serious dispersion for this reason. Based on the chirp technique, a scheme is established to reduce the effect of dispersion by performing dispersion pre-compensation on the original narrowband excitation signals, and thus the time duration of received wave packet can be compressed during the extracting process. Furthermore, benefits from the broadband chirp excitation, information of multiple distinct frequency ranges can be acquired simultaneously, and thus responses to a few narrowband excitations could be readily extracted. This makes the optimal design of the excitation waveform much more efficiently. Numerical simulation and experiment are carried on aluminum specimens to investigate the behavior of the proposed method and the strategy for parameters selection. By using the proposed method, the closely distributed structural features can be recognized with ease in time domain.