Recent years have brought more attention to new damage detection approaches based on nonlinear phenomena associated with Shear Horizontal (SH) waves. Many nonlinear effects–previously observed in ultrasonic wave propagation–have been considered for structural damage detection. The major effort has been put on classical nonlinear effects, such as higher harmonic generation. More recently, nonlinear vibro-acoustic modulation and modulation transfer mechanisms have been also observed in SH wave propagation. However, these phenomena have not been used for structural damage detection. The paper attempts to fulfill this gap. The proposed method involves two excitation waves. The low-frequency pumping wave is used for damage perturbation. In addition, high-frequency SH wave is used as a probing wave. The probing wave is modulated by the pumping wave in the presence of structural damage. The method is used in the paper for fatigue crack detection in metallic structural components. The results demonstrate that the proposed approach has a potential for structural damage detection. Previous research work demonstrates that classical nonlinear effects (e.g., higher harmonic generation) observed in SH waves offer better sensitivity to material microdefects than similar effects observed in longitudinal wave propagation. Therefore, it is anticipated that non-classical nonlinear affects associated with SH wave propagation will show similar potential. However, more research work is needed to confirm this assumption.
Various classical and non-classical nonlinear effects have been observed in ultrasonic wave propagation and used for contact-type damage detection. The former relates to higher harmonic generation, whereas the latter is based nonlinear vibro-acoustic modulations effects. More recently both nonlinear effects have been observed in shear horizontal wave propagation. However, the nonlinear crack-wave interaction is still not fully understood. It is assumed that this interaction is enhanced by local nonlinear elasticity and dissipation of elastic waves. The latter effect is the major focus of the paper. Previous experimental research studies demonstrate that high-frequency ultrasonic waves propagation through crack faces that are in contact–and perturbed by low-frequency excitation–exhibit local nonlinear effects of elastic and dissipative nature. The amplitude level of these effects depends on applied stresses. Both nonlinear effects have a great potential for structural damage detection. However, more theoretical and modelling research work is needed to fully understand these non-classical nonlinear effects. Numerical simulations based on nonlinear crack-wave interaction are investigated in the paper. Three models of local nonlinearity are investigated. These are: the Coulomb friction, the nonlinear viscous damping and the hysteretic stress-strain models. Nonlinear wavefield distortions–due to crack-wave interactions–are observed and analyzed. Numerical simulations are performed using the Local Interaction Simulation Approach (LISA), implemented for shear horizontal wave propagation. Wave amplitudes corresponding to generated higher harmonics and modulated sidebands are investigated in the presented work.
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