Flaw detection by line-focusing electromagnetic acoustic transducers

This paper presents an EMAT technique to focus the shear-vertical wave (SV wave) to a line in a solid body. The EMAT consists of a permanent magnet to apply the bias magnetic field normal to the solid surface, and a meanderline coil to induce the dynamic field and eddy currents in the surface region. The meanderline spacing is continuously changed so that the excited SV waves become coherent on the focal line after traveling oblique paths from the parts of coil. The printed circuit technique makes it possible to fabricate such a meanderline coil of theoretically generated nonuniform spacing. The EMAT is used for detecting the shallow notch introduced by electric-discharge technique on the opposite surface of an aluminum block, where the focal line is located. The scattered and reflected signal at the notch wall is received by the same EMAT, which shows enough strength even for the notch of 0.05 mm deep.