Solid-state non-array vector-bias-field EMAT for arbitrary-direction ultrasonic guided-wave measurement and imaging

Electromagnetic acoustic transducers (EMATs) have been widely used for ultrasonic guided-wave non-destructive testing (NDT) and structural health monitoring (SHM) of plate-like structures. However, practical azimuthal imaging often relies on mechanical scanning of the EMAT or on multi-element arrays with digital beam steering, and coherent focusing methods such as the total focusing method (TFM) and the synthetic aperture focusing technique (SAFT) generally require full-matrix or synthetic-aperture measurements acquired from multiple elements and multiple spatial locations. This paper presents a fixed-position, solid-state, non-array vector-bias-field EMAT (VBF-EMAT) for arbitrary-direction Lamb wave measurement and circumferential imaging. In the proposed transducer, a multi-pole electromagnet synthesizes a continuously steerable vector bias magnetic field, and direction selectivity is physically achieved at the transducer level. Under the synthesised vector bias field, guided waves propagating along a prescribed azimuth are generated through magnetostriction. This mechanically stationary solid-state architecture enables directional control without requiring mechanical rotation. The geometries of the multi-pole electromagnet and transceiver coil were optimized to improve the uniformity of the bias field and the directional response of the transducer. Experiments on steel plate specimens demonstrate 360° circumferential S₀-mode Lamb wave imaging and accurate defect localisation without array deployment.