Development of Lamb and Rayleigh Wave-Based Nonlinearity Parameters for Estimating the Remnant Life of Fatigued Plate Structures

The present study focuses on the development of amplitude and physics-based nonlinear parameters for estimating the material nonlinearity and eventually the remnant useful life of fatigued plate structures using Lamb and Rayleigh waves. In the numerical simulations, these waves are propagated through Aluminum (Al) plate structures, and as a result of material nonlinearity, a second harmonic is generated in the response. The spectral amplitudes of the second harmonic are then subsequently used in the amplitude-based nonlinearity parameters to evaluate the nonlinearity for different distances and for different stages of fatigue. The physics-based nonlinear parameter is dependent on the sub-structural evolution parameters and higher order elastic and plastic constants. As it is independent of distance and depends only on the percent of fatigue life, it can be used to construct a theoretical nonlinearity curve (TNC). The estimation of material nonlinearity for different fatigue life through the simulation and amplitude-based parameter is found to be in close agreement with the TNC under the predefined conditions of distance and cycles in the excitation signal. Thus, the knowledge of material nonlinearity parameters evaluated for the pristine and fatigued thick plate specimens using Lamb and Rayleigh waves is shown in the present study to be useful for evaluating the remaining useful life of the fatigued specimens with fair accuracy.