| Structural Health Monitoring (SHM) of critical structural parts plays a very important role for preventing structural failure and loss of human lives. Current defect detection methods require bulky, expensive transducers and equipments. These limitations make them not suitable for in-situ, online SHM. In response to this need, the use of piezoelectric wafer active sensors (PWAS) and Lamb wave propagation for structural health monitoring has been proposed.; The goal of the research was to develop the scientific and engineering basis for the use of PWAS and the Lamb wave propagation method in structural health monitoring. The fundamental theory of Lamb wave phenomenon was reviewed. The derivation of the characteristic equations and wave speed dispersion curve of Lamb waves are provided. The complex behavior of the Lamb wave modes at different frequency ranges was explained. PWAS were studied from both theoretical and practical aspects. Based on these theories, the interaction of thin plates and PWAS was studied.; In the experimental study, PWAS were bonded on thin aluminum alloy plates to evaluate its capability in Lamb wave generation and detection. The effects of excitation signal modulation and signal processing techniques are discussed. Signal propagation speed was calculated from the experimental data, which agree with the theoretical prediction. Operations similar to those used in conventional ultrasonic testing, e.g. pitch-catch and pulse-echo methods, were tested.; A PWAS phased array was proposed to perform 2-D scanning of large areas. The algorithm for generating a virtual scanning beam and offline signal focusing were derived. A prototyping PWAS array was constructed on aluminum alloy plates with simulated fatigue cracks. With the proposed phased array algorithm, cracks were detected successfully. |
