Studies of Ultrasound Generated by Electromagnetic Acoustic Transducers (EMATS) Using a Modified Michelson Interferometer

Abstract

EMATs are electromagnetic acoustic transducers that generate ultrasound within the surface of a conducting or magnetic sample. They offer a cheap alternative for the generation of non-contact ultrasound and are also suitable for generating a wide variety of wave modes [1], depending on the coil geometry and magnet arrangement of the EMAT. A major drawback, however, is their much lower transduction efficiency when compared to other non-contact methods such as pulsed laser generation. It has been shown that the generation of shear horizontal (SH) and surface waves in iron using meanderline coils, in low static fields, is due to magnetostriction [2] when the field is applied in the plane of the sample surface. The case of a spiral coil has been studied on single crystal nickel [3] and on non magnetic metals [4], however, a thorough examination on engineering steels is still required. In non magnetic metals the spiral coil EMAT is designed to generate radially polarised shear waves via the Lorentz interaction between induced eddy currents and the applied magnetic field. In ferromagnets it is also possible that magnetostriction due to the coils dynamic magnetic field plays an important role in the generation process. The aim of this work was to study the ultrasound generated by such an EMAT coil and its dependence upon applied normal field. A modified Michelson interferometer was used as a detector so that the generation process could be studied in isolation from the detection mechanism, since the interferometer would simply monitor the surface displacement. The samples studied were: aluminum, nickel, Invar (64% Fe, 36% Ni), mild steel and a sample of duplex stainless steel, a two phase ferritic/austenitic (magnetic/non-magnetic) alloy.