Mutual Nonlinear Interaction Of Ultrasonic Guided Waves In Plate Applications For Nde

Nondestructive Evaluation (NDE) and Structural Health Monitoring (SHM) of materials are investigated through nonlinear ultrasonics. The material nonlinearity associated with microstructural features distorts the ultrasonic wave packet, altering the frequency spectrum. Since microstructural changes precede macroscale damage, nonlinear ultrasonics provides the unique opportunity to diagnose material degradation early in its progression. Thus, the unique sensitivity of nonlinear ultrasonic waves to microstructural damage is the motivation for the current study to develop more reliable methods to detect early damage in materials. Ultrasonic guided waves, which travel in a bounded medium, are of great interest due to particular advantages such as rapid scanning of plates. The focus of this dissertation is on the mutual interaction of guided waves in a plate, which occurs when multiple waves propagating in the plate mix together. The wave vectors are considered to form the nonlinear stress fields associated with the interaction of two different guided waves with arbitrary mixing angle. By solving the differential equations, the conditions that generate the strong and cumulative nonlinear perturbations are found. Based on internal resonance conditions, the wave triplets (two primary waves and the secondary wave) determined form the basis for experimental studies and numerical simulations. An analytical model is given which simulates the interaction of guided waves with finite wave packets. The rest of the dissertation is on demonstrating the proposed experimental techniques and then exploring the results. Codirectional shear horizontal guided wave mixing is considered, where limited cumulative behavior of out-of-plane displacements is used for early damage characterization. In addition, interaction counter-propagating shear horizontal guided waves is studied. The results indicate the strong generation of a secondary wave field that travels independently from primary wave packets.