Theoretical and experimental evaluation of the health status of a 1018 steel I-beam using nonlinear Rayleigh waves: Application to evaluating localized plastic damage due to impact loading

This study proposes a sensitive and baseline-free method to evaluate the health status of a 1018 steel I-beam by measuring its material nonlinearity using a new nonlinearity parameter defined for Rayleigh waves. This parameter yields a true value of material nonlinearity using the Rayleigh wave harmonics obtained from the experiments carried out at the intact and impacted states of the I-beam. Accordingly, the evaluated nonlinearities are inherent and damaged induced respectively. The results show that, for an intact state, the nonlinearity obtained using the new parameter and the experimental results for different propagation distances, consist of several peaks and the first peak reaches the true material nonlinearity. Whereas, in case of damaged state, the nonlinearity parameter at the impacted location shows a sudden increase and reaches a value higher than that of the nonlinearity evaluated at the same location for intact state. Thus, the health status can be easily tracked by comparing the nonlinearity obtained from the current state of the I-beam at its first peak with that of a physics based nonlinearity parameter evaluated at the intact state using the higher order elastic coefficients of the material. Therefore, this method is termed as baseline-free. Lastly, a novel concept of evaluating the population of dislocations formed in the material as a result of impact loading, using the new nonlinearity parameter is introduced and an equation for its estimation is given. The trend of the results given by this new equation are in accordance with those reported in the literature. In contrast, deviation between the linear parameter such as the wave velocity at the intact and impacted state remains marginal. Thus, by using the new nonlinearity parameter, it has been proven that the inspected steel specimen can be easily differentiated whether it is at the intact or damaged state.