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.

Design of a new optical system to generate narrowband guided waves with an application for evaluating the health status of rail material.

The Letter presents a new design of Sagnac interferometer-based optical system (SIOS) that emits a line-arrayed pattern generating narrowband high-energy waves in a specimen. The SIOS is further used to excite Rayleigh waves in a pristine rail specimen to evaluate its intrinsic nonlinearity resulting from the lattice anharmonicity and dissolved impurities. Such a nonlinearity appears in the response in the form of a second harmonic that is sensed in this Letter using a scanning laser Doppler vibrometer. In addition to this noncontact measurement, a contact measurement of the nonlinearity of rail steel using wedge transducers is also carried out to compare the performance of the SIOS. Both experimentally evaluated nonlinearities are compared with those obtained using the nonlinear elasticity equations. The close agreement with the theoretically estimated nonlinearity and higher repeatability shows that the SIOS is effective in measuring the intrinsic nonlinearity of the rail steel and, thereby, predicting the health status of the rail specimens before fixing them on the track.