Nonlinear Lamb wave phased array for revealing micro-damage based on the second harmonic reconstruction.

It is challenging but meaningful to detect and image the micro-damage in the early stages of engineering structure failure. The nonlinear ultrasonic technique has gained considerable attention for micro-damage detection, meanwhile, the Lamb wave phased array technique has become a practical tool for macro-damage imaging. The combination of these two techniques can pave a promising way for micro-damage imaging in plate-like structures. However, the research on nonlinear Lamb wave phased array imaging is complicated and not reported. The weak nature and hard extraction of the nonlinear features, such as the second harmonic generation, limits the application of the nonlinear Lamb wave phased array. A framework of nonlinear Lamb wave phased array for micro-damage imaging in the plate-like structure is developed. To maximize the second harmonic response of the recorded Lamb waves, two kinds of lead zirconate titanate (PZT) wafers are selected to compose the nonlinear phased array. Furthermore, focusing on the nonlinear feature extraction, the sparse representation approach is introduced for reconstructing the time-domain second harmonic generated by the micro-damage, from which a wealth of waveform information corresponding to the micro-damage can be extracted. The total focusing method (TFM) is used for micro-damage imaging based on the reconstructed second harmonic. Numerical simulation and experimental validation demonstrate the capability of the proposed framework to image the local contact acoustic nonlinearity represented by the barely visible closed fatigue crack with the nonlinear Lamb wave phased array. [ABSTRACT FROM AUTHOR]