Defect imaging of electromagnetic acoustic transducer dead-zone with oblique incidence and full skip SAFT method

As EMAT generate vibration and ultrasonic directly on the surface of testing object, the dead-zone that caused by transducer near filed and instrument system initial impulse cannot be eliminated with time-delay wedge block like piezoelectric transducer. Thus detecting the defects in near surface dead-zone is a key problem for EMAT non-destructive testing. To address the problem this paper proposed a new Full-skip Synthetic aperture focusing technique (FS-SAFT), and optimized oblique incidence meander-line coil EMAT for this imaging algorithm implement. Finite element model was developed to analyze the propagation characteristics of EMAT transmitted acoustic field with different line space, frequency and line number. Also, the interaction of secondary reflected waves with defects. Experimental verification was carried out on aluminum alloys and 20# test blocks with through-hole and grooved crack defects. The simulation and experimental results show that the new imaging method and optimized EMAT can imaging the near surface field and testing defects in dead-zone under full transverse wave mode. At a high temperature of 300°, the transducer can detect near-surface defects in 20# steel by using this algorithm, and the test sensitivity at high temperature can reach a 5 mm diameter through hole. This new EMAT imaging method lays a foundation for detecting and imaging near-surface defects in metal equipment components especially on elevated temperature.