A model for the electromagnetic generation of ultrasonic guided waves in ferromagnetic metal polycrystals

A model is presented which predicts the efficiency of the electromagnetic generation of ultrasonic Rayleigh or Lamb waves in ferromagnetic metal polycrystals. The practically important case in which the static magnetic field, the dynamic magnetic field, and the propagation direction are parallel is considered. Two distinct forcing mechanisms are explicitly included, magnetostrictive stresses, and Lorentz forces on induced eddy currents. The ability of the resulting formulas, which depend only upon the applied magnetic field and independently known elastic and magnetic properties of the material, to correctly predict such important quantities as the field dependence of generation efficiency (both phase and amplitude), the relative importance of magnetic permeability, magnetostrictive coefficients, and electrical resistivity in determining this efficiency, and the decrease in efficiency with distance of the transducer from the sample, is demonstrated by direct comparison to measurements in iron. Results are included in a form useful in predicting the efficiencieosf transducers in new applications.