Dynamic Shear Characterization in a Magnetostrictive Rare Earth - Iron Alloy
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ABSTRACT A previous study about the power limits of the magnetostrictive alloy called Terfenol-D, using a device biased by coils, showed the intrinsic superiority of this material compared to piezoelectric ceramics when used in high power sonar transducers. This fact was confirmed by another study where giant dynamic peak-to-peak deformations of 3500 ppm were measured with a transducer biased by permanent magnets. Experience shows that the longitudinal functioning of magnetostrictive transducers is sometimes disrupted by interference caused by 'flexure' in the magnetostrictive rods. In fact, due to the shape of the rods, the flexure effects are strongly coupled to shear. Characterization of shear in Terfenol-D under conditions of magnetic bias and prestress is then necessary to control those effects. This was made possible by using a simple and original device that was designed totally with Computer Aided Design programs. INTRODUCTION A previous study about the power limits of the magnetostrictive [1] alloy called Terfenol-D [2] using a characterization device in which the magnetic bias was created by coils, showed the intrinsic superiority of this material compared to piezoelectric ceramics when used in high power sonar transducers [3,4]. Another study showed, with measured dynamic peak-to-peak deformations of 3500 ppm [5], that high power is obtainable even with a bias created by permanent magnets. Experience shows that the functioning of transducers is sometimes disrupted by interference caused by flexure and shear of the magnetostrictive rods, which is also a cause of mechanical failure of the transducer. In earlier studies, it was established that the flexure modes are fully determined by the longitudinal characteristics of Terfenol-D, and that they are generated by non symmetrical magnetic excitation fields or mechanical structures. However, these results on flexure are not directly applicable to sonar transducers because in such devices the Terfenol-D rod length is rarely more than ten times its diameter, so that flexure effects are strongly coupled to shear. Thus, to calculate flexure/shear modes, both longitudinal and shear characteristics must be taken into account. As the longitudinal characteristics are well known [3,6], the purpose of the study was the characterization of shear in Terfenol-D. The first step was to study, through calculations done with a Computer Aided Design program, how it is possible to make pure shear deformation modes appear in Terfenol-D with a simple device. This led to the design and construction of a Shear Characterization Device, called DCC. This device is a totally original concept, and it was designed to reproduce shear deformations in Terfenol-D identical to those found when it is used in sonar transducers; that is, with prestress and magnetic bias. The objective was to reproduce shear deformation modes as pure and as well coupled electrically as possible, in order to facilitate the shear deformation measurements and increase their accuracy. In the final stage of the study,
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