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.