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Review of in situ fabrication methods of piezoelectric wafer active sensor for sensing and actuation applications

Structural health monitoring (SHM) is important for reducing maintenance costs while increasing safety and reliability. Piezoelectric wafer active sensors (PWAS) used in SHM applications are able to detect structural damage using Lamb waves. PWAS are small, lightweight, unobtrusive, and inexpensive. PWAS achieve direct transduction between electric and elastic wave energies. PWAS are essential elements in the Lamb-wave SHM with pitch-catch, pulse-echo, and electromechanical impedance methods. Traditionally, structural integrity tests required attachment of sensors to the material surface. This is often a burdensome and time-consuming task, especially considering the size and magnitude of the surfaces measured (such as aircraft, bridges, structural supports, etc.). In addition, there are critical applications where the rigid piezoceramic wafers cannot conform to curved surfaces. Existing ceramic PWAS, while fairly accurate when attached correctly to the substance, may not provide the long term durability required for SHM. The bonded interface between the PWAS and the structure is often the durability weak link. Better durability may be obtained from a built-in sensor that is incorporated into the material. An in-situ fabricated smart sensor may offer better durability. This paper gives a review of the state of the art on the in-situ fabrication of PWAS using different approaches, such as piezoelectric composite approach; polyvinylidene fluoride (PVDF) approach. It will present the principal fabrication methods and results existing to date. Flexible PVDF PWAS have been studied. They were mounted on a cantilever beam and subjected to free vibration testing. The experimental results of the composite PWAS and PVDF PWAS have been compared with the conventional piezoceramic PWAS. The theoretical and experimental results in this study gave the basic demonstration of the piezoelectricity of composite PWAS and PVDF PWAS.