Line-focusing of ultrasonic SV wave by electromagnetic acoustic transducer

An electromagnetic acoustic transducer has been developed for line-focusing the shear-vertical (SV) wave in a metal. The EMAT consists of a permanent magnet to supply the bias magnetic field normal to the surface, and a meanderline coil to induce the dynamic field and eddy currents in the surface region of the sample. The meanderline spacing is continuously changed so that the generated SV waves from all segment sources become coherent on the focal line after traveling oblique paths. The printed circuit technique enables the fabrication of such a functionally spaced meanderline coil within 1-mu m accuracy. The directivity is measured using a half-cylindrical sample of an aluminum alloy, which shows a much sharper radiation pattern of the focusing EMAT than the EMAT having a meanderline coil of constant spacings. This EMAT is then tested through detecting a shallow notch. The scattered signal from the notch is received by the same EMAT, which shows enough strength even for the notch of 0.2-mm depth. (C) 1998 Acoustical Society of America.