Stress Monitoring of Plates by Means of Nonlinear Guided Waves

We investigate the propagation of nonlinear guided waves in plates in view of their application to the identification of the state of stress. The study is performed modelling via Finite Elements the propagation of Lamb waves in a prestressed plate, whose equations of motion are a second-order approximation accounting for both geometric and material nonlinearities. Different intial prestress conditions are investigated, parallel and orthogonal to the direction of wave propagation. A couple of physical phenomena presenting remarkable sensitivity to initial stress emerged using the resonant couple of S1-S2 modes. First, the increase of the secondary wave amplitude with propagation distance is clearly observed together with the sensitivity of the second-harmonic amplitude to the initial state of prestress. Second, a subharmonic and superharmonics are observed, due to the interaction between primary and secondary waves.