Amplitude-dependent second harmonic Lamb waves for discriminating delamination from background nonlinearities in composite plates

Early detection of delamination in composite materials is crucial to maintaining operational safety and reducing excessive maintenance costs. Second harmonic Lamb waves have demonstrated exceptional sensitivity to micro defects in materials including breathing delamination. However, differentiating the second harmonic Lamb waves generated by delamination from other inevitable background nonlinearities, exemplified by inherent material nonlinearity in composites, poses a significant challenge for the practical implementation of the second harmonic Lamb wave-based detection methods. To address this bottle-necking issue, this study examines the characteristics of second harmonic Lamb waves generated by delamination and material nonlinearity, respectively, aiming at their differentiation based on their respective amplitude-dependent features. Results are verified through finite element analysis and experimental validations alongside the verification of the effectiveness of the proposed discrimination strategy. It is shown that the amplitude of the second harmonic waves induced by the delamination is linearly proportional to the fundamental wave amplitude, while the one by the material nonlinearity exhibits a quadratic relationship with the fundamental wave amplitude. Based on this understanding, damage indices are proposed, which prove to be effective for characterizing these two sources of nonlinearity, thereby paving the way for practical delamination detection in composite structures.