Lamb Wave Tuning between Piezoelectric Wafer Active Sensors and Host Structure: Experiments and Modeling

In structural health monitoring (SHM), a network of embedded sensors permanently bonded to the structure is used to monitor the presence and extent of damage. The sensors can actively interrogate the structure through ultrasonic waves. Among the ultrasonic waves, Lamb waves are quite convenient because they can propagate at large distances in plates and then interrogate a large area. Lamb waves in a plate can be produced with piezoelectric wafer active sensors (PWAS) that are small, inexpensive, unobtrusive transducers. PWAS can be surface-mounted on an existing structured or placed inside composite materials. PWAS sensors use the piezoelectric principle. An alternating voltage applied to the PWAS terminals produces an oscillatory expansion and contraction of the PWAS. An oscillatory expansion and contraction of the PWAS produces an alternating voltage at the PWAS terminals. PWAS are bonded to the structure through an adhesive layer; the coupling with the investigated structure is higher then conventional transducers. If the PWAS bonded to the structure is excited, it couples its in-plane motion with the Lamb wave particle motion on the material surfaces. In previous studies, the Lamb wave mode tuning between PWAS and isotropic plates has been observed experimentally and theoretically. Recently experiments have been performed to verify the presence of tuning between bonded PWAS and composite plates. In the present paper, it will be discussed a method, normal mode expansion (NME), for predicting the tuning frequencies of the PWAS-plate structure. This method can be used for both isotropic and non-isotropic material. Experimental values for the tuning frequencies in isotropic plates are compared with the theoretically data obtain with integral transform solution and NME. INTRODUCTION Structural health monitoring (SHM) addresses problems of aging structures, a major concern of the engineering community. Recently, damage detection through Lamb waves has proven to be an attractive option for SHM because it allows condition-based maintenance inspection instead of schedule driven inspections. Lamb waves are elastic perturbations that can propagate for long distances with very little amplitude loss in solid plate or shell structures with free surfaces. Through Lamb wave detection, fewer numbers of transducers can be used to monitor a structure and to actively investigate the health of a structure system. An embedded system of transducers is placed in the structural component and it will monitor the health of the material during its service. SHM techniques can be passive or active systems. Passive SHM are a network of sensors that “listen” to the structure to monitor whether the component is changing. Active SHM interrogate the structure health through active sensors. One way to excite Lamb waves into a structure is through strain-coupled transducers that are bonded to the structural surface or inserted between the layers of composite structures [1]. Piezoelectric wafer active sensors (PWAS) are straincoupled transducers that are small, lightweight, and