| Nondestructive Testing (NDT) and Structural Health Monitoring (SHM) using ultrasonic guided waves has emerged as a powerful technique for investigating structural components for defects. Ultrasonic guided waves can travel large distances in a waveguide with little attenuation and are sensitive to small defects. This makes them ideal for interrogating long waveguide like structures. Due to reflections from the boundaries of the waveguide, these waves exhibit complex multi-modal dispersive behavior. It is necessary, therefore, to gain a better understanding of both linear and nonlinear aspects of guided wave propagation.This dissertation presents studies in the use of ultrasonic guided waves for the structural health monitoring of plates, rods, and waveguides with complex cross-sections like multi-layered composite laminates, 7-wire steel strands, and rails. Modeling of guided waves by a Semi Analytical Finite Element (SAFE) model is presented. This model assumes constant cross-section along the direction of propagation. Therefore, while SAFE can efficiently compute dispersion characteristics of complicated waveguides, it cannot be used to study scattering due to local irregularities. A Global Local (GL) model, which combines SAFE and FE, is presented. This model is used to study scattering and mode-conversion effects of local defects in Aluminum plates and composite laminates. It has been shown that the nonlinear features of guided waves are often more sensitive to small defects than the linear features. To use these nonlinear features for NDT, however, requires a thorough understanding of guided wave propagation in nonlinear medium. This dissertation presents theoretical studies in nonlinear guided wave propagation in plates and rods. This includes generation laws for plate and rod modes at nonlinear higher harmonics which underline the analogous behavior of nonlinear bulk waves and nonlinear waves in plates and rods. Experimental results and numerical simulations corroborating the theoretical proofs are presented. Finally, SAFE simulations of nonlinear waves in rail are presented. |
