Development of a low-frequency high purity A0 mode transducer for SHM applications.

The reliability of structural health monitoring (SHM) systems based on guided ultrasonic waves is improved if pure modes are generated by the transducers. A piezoelectric- based transducer generating high purity A0 mode guided waves at low frequencies (around 20 kHz) was developed. The through-thickness resonance of a piezoelectric element was lowered to the frequency region of interest by use of backing masses and low-stiffness front layers. Soft front layers also reduced significantly the transmission of in-plane displacements caused by Poisson's ratio effects in the piezoelectric element to the structure. Parametric studies were undertaken by varying the backing mass length, the transducer diameter, and the thickness of the front layer. The thickness of the plates on which the transducers were operated was found to be a critical issue and these effects were evaluated. Results obtained by finite-element analysis were validated by experimental measurements and showed that signals with A0/S0 energy ratios substantially above 40 dB can be obtained.