Lamb wave inspection for composite laminates using a combined method of sparse reconstruction and delay-and-sum

A narrowband tone burst excitation is usually required in most Lamb wave inspection techniques in order to reduce the dispersion effect and maintain mode purity, especially in the case that dispersion compensation is hard to implement, such as the scenarios prior knowledge of accurate dispersion curves is not available. The duration of the excitation is relatively large because of its narrow bandwidth, which degrades the resolution for inspection. Following the emerging sparse representation theory, a combined method of sparse reconstruction and delay-and-sum (DAS)for high resolution Lamb wave inspection is proposed in this paper. The scattering signals are compressed in time domain without losing the information of the arrival time of each echo. Unlike most methods that signal decomposition and reconstruction use the same dictionary, the proposed approach utilizes two different dictionaries: one for signal decomposition and another for feature extraction. The DAS imaging method combined with the feature signals is applied to damage imaging for carbon fiber reinforced plastic (CFRP)laminates. The effectiveness of the proposed approach is validated on a quasi-isotropic laminated CFRP plate. The imaging results show that the proposed approach can achieve a higher localization resolution and lower noise floor without degrading the localization accuracy.