Combination of wavelet transform with EMD to distinguish overlapped Lamb wave packets

In the non-destructive testing for plate structure with ultrasonic Lamb wave, it is hard to discriminate the overlapped scattered signal from different targets. To solve this problem, Lamb wave signal is decomposed into a set of elemental signals called “intrinsic mode functions” by empirical mode decomposition (EMD) method first, and the local maximum of the wavelet coefficients of the elemental signals is used to describe the dispersive curve of damage packets. This method can clearly find the differences of two overlapped wave packets based on time-frequency distribution map and the dispersion curve of Lamb-wave. The experiments show that this method is capable of separating the overlapped damage wave components, which is superior to the conventional Hilbert-Huang Transform (HHT) and wavelet transform methods.