Magnetostrictive Flexural Bar Transducers

Abstract

Although the use of magnetostriction to excite flexural vibrations has had scarcely any practical applications to date, very promising transducers can be made by exploitation of this principle. A basic resonator of this type consists of two flexural bars joined together by common end supports. The bars are necked down where they join the end supports to provide a flexible section, by means of which supported-end boundary conditions are approximated. The resonant frequency of this structure is very close to that of the ideal end-supported bar. To provide for magnetostrictive excitation of flexure, each bar is slotted lengthwise, and windings are placed on each half of the bar formed by the slot. Subsequently, the slot is filled with a hard plastic. An equation for the electromechanical coupling coefficient of this structure has been derived, which includes the effect of slot width. If the ideal end-support condition is realized, the coupling coefficient will be 80% of the coupling coefficient of the magnetostrictive material. A coupling coefficient of 0.25 has been achieved with laminations of nickel-cobalt alloy. Calculations indicate that a surface intensity of 1 w/cm2 in an underwater transducer should be achievable without exceeding the fatigue limit.