Novel nondestructive evaluation transducer for imaging of low-impedance targets

Piezoelectric materials, such as lead zirconate titanate, display high electromechanical coupling, low internal losses, and exceptional environmental durability making them the material of choice for acoustic imaging transducers. Traditionally, piezoelectric transducers are actuated in the thickness-extension mode and they often make use of “matching” films to improve the signal transmissibility into lower acoustic impedance targets. In this article, we propose the use of transducers coupled to a structure in a direction orthogonal to the resonating plane. An analytical solution for the electromechanical behavior of a piezoelectric plate coupled transversely is derived. The analytical model shows that when resonated transversely, a transducer could display significantly lower effective acoustic impedance compared to transducers actuated in the thickness-extension mode. This feature eliminates the need for the matching films, hence helping reducing manufacturing costs. General guidelines on how to optimize the design of the proposed transducer for impedance imaging are also given. Finally, this article shows one example of how using a lead zirconate titanate transducer resonated transversely can reliably detect matrix cracking in fiberglass composite plates.