Dual EMAT–MIEC Testing Method for High-Speed Surface and Backside Defect Inspection

The conventional electromagnetic acoustic transducer (EMAT) testing method has inherent limitations, particularly in its detection range. Combining EMAT with other nondestructive testing (NDT) methods can expand the detection range and improve accuracy. Most EMAT-based dual testing methods can detect surface and backside defects, but cannot be applied in high-speed inspection conditions. Therefore, this paper proposes a dual testing method that combines EMAT with motion-induced eddy current (MIEC), enabling the detection of defects on both the surface and backside of the specimen under high-speed conditions. The probe consists of a permanent magnet and a butterfly coil. First, the principles of EMAT and MIEC are analyzed, along with the interactions and interferences between the two methods. A signal separation technique is proposed based on spectral analysis of the individual EMAT and MIEC signals. A 2D simulation model was developed to assess the detection range of both methods and evaluate the impact of the MIEC effect on EMAT detection. Experimental verification confirmed the effectiveness of the method, and the sensitivity and stability of the probe were also evaluated. The results show that the dual EMAT-MIEC testing method effectively detects millimeter-scale cracks on both surfaces and backsides at a speed of up to 21 m/s.