Generation of horizontally polarized shear waves in ferromagnetic materials using magnetostrictively coupled meander-coil electromagnetic transducers

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Abstract
A new electromagnetic transducer configuration is described for generating horizontally polarized shear (SH) waves in ferromagnetic materials. The transducer consists of a meander coil and static bias magnetic field parallel to the coil elements. This configuration generates no ultrasonic waves in a nonmagnetic metal since the induced eddy currents are parallel to the bias field and the driving Lorentz forces vanish. However, the configuration provides coupling to SH waves in ferromagnetic materials through magnetostrictive effects. Experimental measurements of the variation of transduction efficiency with bias field in nickel and 4130 steel plate are presented and compared to the efficiency obtained with the same meander coils when the bias is rotated 90° in the plane of the plate so that antisymmetric Lamb waves are generated. Peak efficiencies occur at considerably different bias fields for the two configurations. This result, as well as other features in the data, are interpreted in terms of a simple model.