Travel time extraction of multi-mode Lamb wave with HHT

Travel time is an important feature in Lamb wave tomography.However,multiple modes and different dispersion properties make travel time extraction of single mode difficult.General feature extraction is unsuitable for Lamb waves.HHT is used to analyze the ultrasonic Lamb wave signal in this paper.Lamb waves are first decomposed into intrinsic mode functions(IMF).Each IMF is processed by Hilbert transform,and the marginal spectrum analyzed.It makes extraction of single modes easy.By comparing with FFT and wavelet transform,the result shows that HHT is an efficient and good method in showing the distribution of multiple modes with time and frequency.In contrast to wavelet transform,the Lamb wave tomography generated with the travel time data by HHT is close to the real information of defect.