TABLE OF CONTENTS This chapter deals with a basic aspect of the interaction between PWAS and structure during the active SHM process, i.e., the tuning between the PWAS and the Lamb waves traveling in the structure. PWAS are strain-coupled transducers that are small, lightweight, and relatively low-cost. Upon electric excitation, the PWAS transducers can generate Lamb wave in the structural material by converting the electrical energy into the acoustic energy of ultrasonic wave propagation. In the same time, the PWAS transducers are able to convert acoustic energy of the ultrasonic waves back into an electric signal. Under electric excitation, the PWAS undergo oscillatory contractions and expansions, which are transferred to the structure through the bonding layer to excite Lamb waves into the structure. In this process, several factors influence the behavior of the excited wave. We will first recall the concepts of load transferred between the PWAS actuator and structure. Then, we will model the PWAS interaction with the ultrasonic waves generated into the structure and will show that tuning opportunities exist. The tuning is especially beneficial when dealing with multimode waves, such as the Lamb waves.
The chapter adopts a gradual approach, from simple to complex. To understand the essence of the PWAS tuning mechanism, two simple cases are studied first: (a) axial waves in a bar and (b) flexural waves in a beam. It is shown that, in both cases, the tuning concept allows one to find frequencies at which the waves are strongly excited as well as frequencies...
