Nonlinear frequency mixing of counter-propagating collinear A0-S0 mode Lamb waves for delamination detection in CFRP composite plate

Delamination is unavoidable during the fabrication or utilisation of carbon fibre reinforced polymer (CFRP) components. Due to the high sensitivity of the A0 mode for detecting delamination, nonlinear frequency mixing of counter-propagating A0-S0 mode Lamb waves was numerically and experimentally investigated in the CFRP composite plate. Considering the nonlinear effect caused by the heterogeneous structure of CFRP composite, the counter-propagation of primary Lamb waves can minimise the nonlinear interaction induced by material nonlinearity compared to the one-way propagation. Numerically, the nonlinear interaction between primary Lamb waves and delamination was investigated. Moreover, delaminations with different sizes, depths, and interface gaps were explored. It was found that the acoustic nonlinear parameter grows with expanding delamination size and diminishes with increasing delamination depth and interface gap. Furthermore, the nonlinear frequency mixing of counter-propagating A0-S0 mode Lamb wave was experimentally performed. The nonlinear frequency mixing effect induced by the delamination can be maximally isolated from the material nonlinearity. Consequently, the technique based on the nonlinear frequency mixing of counter-propagating A0-S0 mode Lamb waves provides a potential and effective method to detect defects in the composite plate with heterogeneous structures.