A broadband Terfenol-D driven flextensional projector

Abstract

Since 1994 Lockheed Martin has developed a number of Terfenol-D driven Class VII inverse flextensional underwater projectors, known as ‘‘Terfenol Dogbone Flextensional Projectors.’’ The underlying motivation for these projectors has been to match the high-strain, high-energy density, and relatively high-compliance characteristics of magnetostrictive Terfenol-D alloys with the high-compliance characteristics of the dogbone flextensional shell. The resulting projector is reliable, rugged, and has a smaller size, lower-frequency and a broader bandwidth relative to conventional ceramic driven flextensional projectors. The Class VII shell, by virtue of its shape, is designed to naturally accumulate axial compressive stress on the drive assembly with depth. Hence, the dogbone shaped shell is dynamically more compliant to better match the lower modulus of the Terfenol driver. Additionally, both the fundamental bending mode and the second mode of vibration provide dominantly in-phase radiation and well behaved beam patterns. Recently, a small permanent magnet biased projector demonstrated a mechanical quality factor of unity and high source level in its fundamental mode of vibration. A 3-dB band from 2.9 to 6.4 kHz (fundamental mode) and 10 to 18 kHz (second mode) where achieved. The merits, challenge, and future outlook for this technology are discussed. [Work supported by ONR Code 321.]