Shear Wave Generation with EMATs

Shear waves have an inherent polarization direction depending on how they are generated. Pictured below are horizontally polarized shear waves propagating along the length of a plate.

Shear waves vibrate particles of a material in the direction perpendicular to the wave propagation.

Electromagnetic-acoustic transducers (EMAT) designed to generate shear waves actually vibrate the atoms within the material being investigated. When a wire is placed near to the surface of an electrically conducting object and is driven by a current at the desired ultrasonic frequency, eddy currents will be induced in a near surface region. If a static magnetic field is also present, these currents will experience Lorentz forces of the form

F = J × B F=J\times B

F is the body force per unit volume, J is the induced dynamic current density, and B is the static magnetic induction.

Spiral coil EMATs excite radially polarized shear waves that propagate normal to the surface. This is because the North-South axis of the magnet in the transducer is normal to the surface.
Cross-sectional view of a spiral coil EMAT exciting radially polarized shear waves propagating normal to the surface.
Normal field EMATs have two magnets that each have their North-South axes normal to the surface. This excites plane polarized shear waves that propagate normal to the surface.
Cross-sectional view of a normal field EMAT for exciting plane polarized shear waves propagating normal to the surface.