Influence of static prestress on the characteristics of bolt-clamped Langevin-type transducers

Bolt-clamped Langevin-type transducers (BLTs) are widely used as powerful vibration sources in high-power ultrasonic applications. In the structure of a BLT, static compressional prestress is imposed on piezoelectric ceramic elements by clamping, to compensate for their weakness to tensile stress. Nevertheless, no report has been made about the clamping effects on the characteristics of a BLT with a reasonable estimation of the prestress. Previous works on material property change of piezoelectric ceramics caused by static compressional stress have always presumed a groundless uniformity of stress distribution in the objects examined. The authors have investigated the effects by some experiments and numerical simulations with the use of the finite-element analysis system to estimate precisely the prestress. As a result of these experiments, the prestress has been found to reduce the electromechanical coupling factor of the BLT. Estimation of the prestress is considered indispensable for designing BLTs, although the microscopic mechanism of the piezoelectricity reduction is not yet clarified.