Surface Crack Detection by Laser Excitation-unidirectional EMAT Receiving

Surface cracks are one of the most common types of material surface defects, which have a significant impact on material properties and service life. To meet the demand of noncontact detection of surface cracks, an inspection method using a pulsed laser to excite ultrasonic waves and a unidirectional electromagnetic ultrasonic transducer (unidirectional EMAT) to receive the signals is proposed. A finite element model under the ablation mechanism was established to simulate and analyze the phenomenon of reflection of surface waves in the workpiece after encountering surface cracks. The time history curve of the coil voltage is extracted to evaluate the defect location according to the time of flight (TOF) of the defect reflection signal, and the evaluation curve of the crack depth is established based on the peak-to-peak value of the transmission signal. The results show that the non-contact measurement of surface cracks can be realized by using the method of laser excitation-unidirectional EMAT receiving method, and the relative localization error of surface cracks is within $4 \%$, and the depth evaluation error is within 5%.