Mechanisms of electromagnetic generation and detection of ultrasonic Lamb waves in iron-nickel alloy polycrystals

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Abstract
Measurements of the efficiencies of electromagnetic generation and detection of ultrasonic Lamb waves in Armco iron, 1018 steel, nickel, and Invar are presented as a function of both the static and dynamic magnetic field strengths in a geometry in which both are parallel to the surface. Three distinct coupling mechanisms are suggested by the data, and this is confirmed by a quantitative comparison of the amplitude and phase of the generated waves to theoretical predictions. In the iron, 1018 steel, and nickel, Lorentz forces are the primary mechanisms of transduction for static fields above about 2 kOe, whereas Joule magnetostriction is dominant at lower fields. In Invar, a forced volume magnetostriction mechanism is observed in the 1–10‐kOe range that is greater in strength than the Lorentz force mechanism. The data also demonstrate that the amplitudes of waves generated by the Lorentz force mechanism are proportional to the macroscopic magnetic field H rather than the magnetic flux density B.