This paper investigates the Lamb wave imaging method combining time reversal for health monitoring of a metal-lic plate structure.The temporal focusing effect of the time reversal Lamb waves is investigated theoretically.It demonstrates that the focusing effect is related to the frequency dependency of the time reversal operation.Numerical simulations are conducted to study the time reversal behaviour of Lamb wave modes under broadband and narrowband excitations.The results show that the reconstructed time reversed wave exhibits close similarity to the reversed nar-rowband tone burst signal validating the theoretical model.To enhance the similarity,the cycle number of the excited signal should be increased.Experiments combining finite element model are then conducted to study the imaging method in the presence of damage like hole in the plate structure.In this work,the time reversal technique is used for the recompression of Lamb wave signals.Damage imaging results with time reversal using broadband and narrowband excitations are compared to those without time reversal.It suggests that the narrowband excitation combined time reversal can locate and determine the size of structural damage more precisely,but the cycle number of the excited signal should be chosen reasonably.
Travel time Lamb wave tomography has been shown to be an effective nondestructive evaluation (NDE) technique for plate-like structures.The methods used previously to extract arrival times of the fastest or multi Lamb wave modes are mostly based on various time-frequency methods such as Wigner-Ville distribution,short-time Fourier transform,and recently explored wavelet transform(WT).Frankly speaking,uses of these signal processing methods improve the accuracy of the arrival time extraction to a great extent relative to directly extract arrival times in time-domain from Lamb waveforms.Hilbert-Huang transform(HHT) is also an efficient way for analyzing and processing non-stationary signals.The resolving power of time and frequency is restricted from Heisenberg principle in wavelet analysis,while in HHT,the time resolving power is precise and steady,and frequency resolving power is adaptive according to signal intrinsic characteristics.Conclusion can be made that the HHT method is more adaptive than WT analysis in analyzing non-stationary signals.Based on the above,HHT method is attempted to extract arrival times from Lamb waveforms in this paper.The Lamb wave tomography images generated with arrival times from HHT method were compared with those of WT.The results show that the new method improves the quality of tomography image,which demonstrates the applicability of HHT method in extracting arrival times of Lamb waves.
