Fast imaging of surface defects based on acoustic path compensation for laser excitation-unidirectional surface wave EMAT reception

Abstract Laser-excited ultrasound (LEU) exhibits superior ultrasonic conversion efficiency compared to the conventional electromagnetic acoustic transducer (EMAT). While optical reception method requires workpieces with highly reflective surface conditions, EMAT retains reliable ultrasonic reception performance even on rough surfaces. In this study, LEU based on the ablative mechanism is used to generate stronger ultrasonic waves, and a unidirectional surface wave EMAT is employed to receive signals, forming an efficient system for surface defect detection. However, due to the physical dimensions of the EMAT, the positions of ultrasonic excitation and reception cannot perfectly coincide. To address this issue, an acoustic path compensation (APC) method is proposed to align the excitation and reception positions, thereby satisfying the requirements of the phase migration algorithm and enabling rapid imaging in the frequency domain. Experimental investigations on specimens encompassed three using three imaging techniques: conventional B-scan imaging, time-domain synthetic aperture focusing technique, and frequency-domain phase shift migration (PSM) imaging. The imaging results were analyzed in terms of defect location, lateral defect dimensions, and imaging time. Then the reliability of the PSM imaging based on APC was validated in the hybrid LEU-EMAT inspection system.