Modal solutions for SH guided waves radiated by an EMAT in a ferromagnetic plate

Electromagnetic acoustic transducers (EMAT) generate forces which are sources of elastic waves in a part without contact with it. Depending on the EMAT design, normal or tangential forces can be generated. Thanks to these capabilities, EMATs constitute an interesting alternative to piezoelectric devices in many configurations of non-destructive examination. In this paper, only shear horizontal (SH) guided waves radiated by an EMAT in a plate made of ferromagnetic material are modelled. These waves are particularly interesting to use for testing welded structures: SH waves may propagate in a weld without scattering and mode conversion phenomena. All the forces generated by an EMAT (Lorentz's, magnetosctrictive and magnetic forces) being exponentially decreasing with depth, they are rewritten as series of moments, then, approximated as equivalent surface stresses. Surface stresses are then taken into account as terms of source of elastic waves. Specific features of EMAT can eventually be exploited to derive simple analytic expressions of the applied stress, leading to closed-form solutions for the modal amplitude of SH guided waves. Using these solutions, which are calculated at no computing cost, it becomes easy to study the influence of typical EMAT parameters on the modal amplitude of waves generated in the plate.