Physical Coupling Fusion Sensing of MFL-EMAT for Synchronous Surface and Internal Defects Inspection

Abstract

Non-destructive testing (NDT) techniques are widely used for inspection and evaluation of conductive materials. However, a single physical mechanism sensing cannot simultaneously satisfy multiple inspection requirements. In this paper, we propose a hybrid transducer based on magnetic flux leakage (MFL) and electromagnetic acoustic transducer (EMAT) for synchronous detection of surface and internal defects. In the proposed hybrid transducer, both MFL and EMAT share a common magnetic field. The magnetic circuit characteristics of the MFL and EMAT principles are fully exploited, and the magnetic field provided by a single permanent magnet is used to excite ultrasonic waves that can detect internal defects within the sample. Iron is used to introduce magnetic field into the sample and provide a horizontal magnetic field to detect discontinuities on the near-surface of the sample. The huge frequency difference between the MFL signal and the EMAT signal effectively suppresses interference between the two signals. Simulations and experiments have been undertaken to show that the proposed transducer can overcome the detection limitations associated with the MFL and the dead detection zone of the EMAT, while simultaneously detecting the surface defects, internal defects and bottom thinning defects in the specimen.