In-situ structural health monitoring with piezoelectric wafer active sensor guided-wave phased arrays

Health monitoring of structures is a major concern in the engineering community. Multi-site fatigue damage, hidden cracks in hard-to-reach locations and corrosion are among the major flaws encountered in today's extensive diagnosis and/or prognosis of civil and military vehicles and structures. One way to achieve structural interrogation is through ultrasonic waves by introducing a high frequency stress pulse or "wave packet" into a material and observing the subsequent propagation and reflection of this energy. Among, ultrasonic waves, the Lamb waves are particularly advantageous because they can propagate at large distances as guided waves in plates and shells. The Lamb waves travel into the structure and are reflected or diffracted by the structural boundaries, discontinuities, and damage. By studying their propagation and reflection, many fundamental properties of the structure can be determined. Our Lamb wave-based active SHM method uses piezoelectric wafer active sensors (PWAS) to transmit and receive Lamb waves in a thin-wall structure. PWAS are lighter, smaller, and less expensive than the conventional ultrasonic transducers, and can be permanently attached to the structure. Thus, they permit on-demand structural interrogation. The PWAS phased arrays consist of a group of PWAS located at distinct spatial locations and with distinct phase delays in the signal pattern. A PWAS phased array may be used to point towards a fixed radiation pattern, or to scan the structure rapidly by varying the relative phases of the respective signals feeding the PWAS. The effective radiation patterns of the array are determined by the relative amplitudes of the signals radiated by the individual array element and the configuration of the array.;This dissertation is focused on using PWAS to construct guided Lamb-wave phased arrays for in-situ structural health monitoring. An unique feature of our method, which makes it essentially different from the traditional piezoelectric phased arrays, is that the PWAS phased array performs virtual scanning by steering the beam through a signal post processing procedure. The PhD dissertation will first address the background of using PWAS transducer and guided Lamb waves for damage detection. Then, it will intensively focus on the development of mathematical fundamentals and practical implementations of PWAS phased arrays in 1-D and 2-D patterns, followed by the use of advanced signal processing techniques to improve the phased array performance and detection capability. Mathematical formulation, numerical simulation, and experimental validation of the application of PWAS arrays for damage detection and structural health monitoring of plate and shell-type thin-wall structures will be presented.