Lamb Wave Generation with EMATs

Lamb waves are similar to longitudinal waves, with compression and rarefaction, but they are bounded by the sheet or plate surface causing a wave-guide effect.

Because the particles in lamb waves vibrate in elliptical orbits, they cause the largest elastic deforamations on a part's surfaces.

Electromagnetic-acoustic transducers (EMAT) designed to generate Lamb waves 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×BF=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.

Meander coil EMATs are similar to spiral coils though instead of having one pancake coil, they wire zig-zags accross the surface of the transducer. This allows for the production of L or SV waves, Rayleigh waves, or guided modes in plates.
Pictured above is a cross-sectional view of a meander coil EMAT for exciting Lamb waves in plates.