Full-region internal imaging of metals using multi-mode VF-SAFT and surrogate model-assisted optimization of EMAT

Oblique-incidence EMAT inspection can cover large areas and reduce blind zones, but traditional shear-vertical EMATs (TSV-EMATs) suffer from low conversion efficiency, weak directivity and severe modal interference. This work proposes an Optimized Shear-Vertical EMAT (OSV-EMAT) that uses a permanent-magnet array and a tailored FPC meander-line coil to reshape the magnetic field and enhance SV-wave excitation, purity and steering. A physics-guided semi-analytical model is built to map design parameters to SV-wave displacement and, together with a Kriging surrogate and an off-the-shelf swarm optimiser, performs multi-parameter optimisation without extensive transient finite-element simulations, achieving a 51× speed-up while retaining the optimal geometry. For imaging, a multi-mode variable-frequency synthetic aperture focusing technique (VF-SAFT) scheme steers the SV beam by frequency modulation, avoiding mechanical repositioning, and is combined with a lightweight GRDB-based fusion network to mitigate blind zones from direct and secondary modes. Experiments on a semi-circular aluminium plate show that the OSV-EMAT yields stronger, purer and more directional SV echoes than a TSV-EMAT and attains defect localisation errors below 1.52%, demonstrating an efficient solution for EMAT-based defect imaging.