Development of a Dual-Mode Electromagnet-Based EMAT With Halbach Array Configuration

Abstract

Electromagnetic acoustic transducers (EMATs) enable noncontact ultrasonic inspection without couplants, but bulk-wave excitation and modal selectivity depend strongly on the bias magnetic-field distribution. This article presents a dual-mode electromagnet-based EMAT with a Halbach-type bias configuration (DMHB-EMAT) for selective excitation of longitudinal and shear bulk waves. Independently energized longitudinal-wave and shear-wave bias-coil sets, combined with a dual-layer butterfly excitation coil, switch the required bias-field orientation without structural reconfiguration. The electromagnetic bias coils were optimized using an orthogonal design method, and the magnetic flux density in the effective excitation region reached approximately 0.20 T. Compared with the center-only bias-coil configuration, the full Halbach-type configuration increased the target longitudinal-wave and shear-wave displacement amplitudes by factors of approximately 1.89 and 1.63, while reducing the corresponding modal interference ratios from 0.38 to 0.21 and from 0.071 to 0.057. Magnetic-field measurements and through-transmission ultrasonic experiments were conducted. The dominant received wave packet changed with the active bias-coil set. The voltage-based apparent modal interference ratios were 0.20 and 0.09 for longitudinal-wave and shear-wave excitation, corresponding to target-to-undesired voltage-amplitude ratios of approximately 5.0 and 11.1, or amplitude differences of approximately 14.0 and 20.9 dB, respectively. The results show that the proposed DMHB-EMAT can selectively excite longitudinal and shear bulk waves.