Abstract: To overcome the limitations of a single non-destructive testing method, a sensing structure that integrates magnetic flux leakage（MFL） detection and electromagnetic acoustic transducer（EMAT） detection methods is proposed to simultaneously detect surface and internal defects in steel specimens. The proposed hybrid sensor structure makes full use of the magnetic circuit characteristics of the MFL and EMAT. The combination of the yoke and permanent magnets creates a horizontal magnetic field inside the sample to detect defects on and near the surface of the sample. Secondly, the local vertical magnetic field provided by the magnetization structure of the MFL interacts with the EMAT excitation coil to produce ultrasonic waves in the specimen to identify internal flaws in the specimen. The excitation source for MFL is quasi-static and the ultrasonic signal is a pulsed excitation source at 2 MHz. The significant frequency difference between the two signals is helpful in reducing mutual interference. Simulations and experiments show that the proposed sensing structure can overcome the blind zone caused by near-field emission in EMAT and the depth limitation of MFL, and is able to simultaneously detect and classify both surface defects and internal blind hole defects in large thickness ferromagnetic sample.

Key words: magnetic flux leakage-electromagnetic acoustic transducer, multiphysics fusion, surface and internal defects, synchronous detection