Shear Wave Generation
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.
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 x B
F is the body force per unit volume, J is the induced dynamic current density, and B is the static magnetic induction.
Pictured above is a cross-sectional view of a spiral coil EMAT exciting radial polarized shear waves propagating normal to the surface.
Pictured above is a cross-sectional view of a normal field EMAT for exciting plane polarized shear waves propagating normal to the surface.