An EMAT consists of a coil to induce dynamic electromagnetic fields at the surface region of a conductive material, and permanent magnets (or electromagnets) to provide a biasing magnetic field. An EMAT configuration depends on the modes of elastic waves to be excited and detected. Optimum design of an EMAT requires understanding the coupling mechanism of energy transfer between the electromagnetic and elastic fields. This is a long-running topic and many studies appeared (Thompson, 1977, 1978, and 1990; Kawashima, 1976 and 1985; Il’in and Kharitonov, 1981; Wilbrand, 1983 and 1987; Ogi, 1997a; Ogi et al., 2003a). Previous studies revealed that three mechanisms contribute to the coupling: (i) Lorentz-force mechanism caused by the interaction between eddy currents and the static magnetic flux density, (ii) magnetization-force mechanism between the oscillating magnetic field and the magnetization, and (iii) magnetostriction mechanism by the piezomagnetic effect. The Lorentz-force mechanism arises in all conducting materials, while other two appear only in ferromagnetic materials. For nonmagnetic metals, therefore, the Lorentz-force mechanism explains exactly the transfer with an EMAT (Gaerttner et al., 1969). However, the coupling is rather complicated for ferromagnetic materials.
KeywordsLorentz Force Static Magnetic Field Coupling Mechanism Ferromagnetic Material Dynamic Field
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