Pre-Stress of Underwater Flextensional Transducers and Its Variation with Water Depth

It is important to control accurately the applied compressional pre-stress on the active elements of underwater flextensional transducers, both in assembly and in operation. This is especially the case for Terfenol rod, whose magnetostrictive properties strongly depend on the applied compressional pre-stress and magnetic bias. In addition, the magnetic bias is determined by compressional pre-stressing. The initial compressional stress derives from the displacement of the shell body during assembly. The required displacement of a Class VII flextensional transducer is calculated using the finite element method (FEM), which is validated by experiment. Compressional pre-stress as a function of water depth is also investigated, since this has the potential to exploit the high-power capability of Terfenol rod. The method described here may be adopted for other types of flextensional transducers.