Influence of aperture angles and design focal depths on the performance of point-focusing shear vertical wave electromagnetic acoustic transducers

Electromagnetic acoustic transducers (EMATs) with concentric meander-line (CML) coils possess the capability of producing point-focusing shear vertical (PFSV) waves. For the CML coil, an intercepted circular arc of the concentric circle with a continuously changing spacing, the aperture angle is a key factor effecting the point-focusing performance. Thus, the influence of aperture angles of CML coils on the point-focusing behavior of PFSV-EMATs is analyzed using the established finite element modeling in detail, which also considers the effect of lift-off distance on the signal amplitude. The beam directional pattern is characterized quantitatively by the half-power beam width, indicating the relationship between the aperture angle and beam width. In addition, since the focus offset is sensitive in depth direction, the effect of design focal depths on focal offset is also carefully studied. Three CML coils with an aperture angle of 30°, 90°, and 150° are fabricated by printed circuit board technique and experimentally analyzed; the experimental results are well consistent with the simulation results. These findings can be used to design optimized PFSV-EMATs with good point-focusing performance.