Group velocity estimation of Lamb waves based on the wavelet transform

Lamb waves are often used in ultrasonic applications for non-destructive testing of large constructions. The difficulty in the analysis of Lamb waves is their dispersive characteristics, especially their frequency dependent phase and group velocities. This article focuses on the separation of the first symmetric mode S0 and the first asymmetric mode A0 and the reliable estimation of their group velocities. Starting from the analysis of the received ultrasonic signals at different distances from the source, three methods were implemented to estimate the group velocity. The best results were obtained by performing the wavelet transform on these signals and using the similarity of the coefficient lines for different distances between the transmitter and the receiver. The group velocity estimation was used to visualise the segment of the dispersion curve of a narrowband signal. The wavelet transform allowed taking a closer look to the particular mode of guided waves in the time and frequency space.