STRUCTURAL HEALTH MONITORING USING LINEAR AND NONLINEAR ULTRASONIC GUIDED WAVES

The dissertation addresses Structural Health Monitoring (SHM) using linear and nonlinear ultrasonic guided waves, with an emphasis on the development of analytical and numerical models of guided wave propagation and interaction with linear and nonlinear structural damage. An analytical model was developed based on the exact Lamb wave solution for the simulation of Lamb wave propagation and interaction with damage. The damage effects were inserted into the model using complex valued wave damage interaction coefficients (WDICs). The analytical procedure was coded as a framework into a MATLAB Graphical User Interface (GUI), and the software WaveFormRevealer (WFR) was developed as a general description of wave generation, propagation, interaction with damage, and detection. The WDICs were extracted from the harmonic analysis of small-size local finite element models (FEM) with non-reflective boundaries (NRB). By joining the analytical framework and the local FEM, a Combined Analytical/FEM Approach (CAFA) was developed for efficient simulation of Lamb wave propagation and interaction with damage. To model guided wave propagation in composite structures, the semi-analytical finite element (SAFE) procedure was investigated and coded with MATLAB, and the software SAFE-DISPERSION was developed to generate guided wave dispersion curves, mode shapes, and directivity plots for composites. Nonlinear ultrasonic SHM techniques were also examined because they have been reported to have remarkable sensitivity to incipient damage. An analytical model was developed to capture the nonlinear higher harmonic generation phenomena for localized damage. Also investigated was the nonlinear interaction between Lamb waves and breathing cracks using FEM through two specific simulation techniques: element activation/deactivation method and contact analysis. A damage index based on the nonlinear features of the sensing signal was proposed in order to identify the presence and severity of a breathing crack. Nonlinear ultrasonic SHM was also investigated for monitoring bolt tightness status of an aerospace bolted lap joint. The 3-D contact FEM simulation was carried out and compared with experiments using Scanning Laser Doppler Vibrometer (SLDV). The dissertation finishes with a summary of contributions followed by conclusions, and suggestions for future work