The Application of Time-Frequency Transform in Mode Identification of Lamb Waves

This paper is focused on the problem of mode identification of Lamb waves in ultrasonic testing, and the principle and process of two time-frequency transform method, smooth pseudo Wigner–Ville distribution and two-dimensional Fourier transform, were discussed. The experimental work were carried out in aluminium plates, and the single and multimode Lamb waves were excited by 5MHz oblique incidence longitudinal wave in 0.82mm and 1.32mm thick plates respectively. The experimental results show that these two time-frequency transform methods can identify not only single Lamb wave mode but also multimode effectively, and furthermore the first method can provide position information of different Lamb wave modes in time domain. Comparing these two methods, smooth pseudo Wigner-Vile distribution is desired to be used in the case where there is no real time required for its long time of calculation, while two-dimensional Fourier transform method is suit for automatic testing since more points of sampling were required.