Application of Hilbert-Huang transform to extract arrival time of ultrasonic lamb waves

Hilbert-Huang transform (HHT) is applied to multi-mode arrival time estimation from Lamb waveforms. The method consists of two parts: (1) the empirical mode decomposition (EMD), and (2) Hilbert transform. The key part of the method is the first step, the EMD, with which, the complicated Lamb waves can be decomposed into number of intrinsic mode functions (IMF), which admit well-behaved Hilbert transforms. Because the decomposition is based on the local characteristic time scale of the signal, it is applicable to arrival time extraction of Lamb waves. Comparisons with the peak envelope of the original signal, direct Hilbert transform (HT) envelope and wavelet envelope show the new method offers higher temporal resolutions in extracting arrival times of multiple Lamb wave modes.