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

Abstract Laser-excited ultrasound is characterized by high ultrasonic conversion efficiency compared to electromagnetic ultrasonic transducers. Compared with the optical reception method, the electromagnetic ultrasonic transducer receives ultrasonic waves with low requirements on the surface condition of the workpiece. Thus, the combination of these two technologies can form a non-contact ultrasonic inspection system. During the inspection process, a pulsed laser is used to stimulate ultrasonic waves in the workpiece based on the ablation mechanism, and a unidirectional electromagnetic ultrasonic transducer is used to receive the surface echo signals. Due to the size of the electromagnetic ultrasonic transducer, the ultrasonic stimulation and reception positions cannot coincide. To address this problem, the acoustic range compensation method to make the ultrasonic excitation and reception position coincide to meet the conditions of the phase migration algorithm to achieve fast imaging based on the frequency domain. For the inspection data of defective specimens, conventional B-scan imaging, synthetic aperture imaging in the time domain, and phase migration frequency domain imaging were carried out, and the defect location, defect transverse size, and imaging time were analyzed in the above three imaging results, which verified the reliability of the method of using acoustic range compensation to achieve ultrasonic excitation and reception at the same location.