Low-frequency eddy current imaging using MR sensor detecting tangential magnetic field components for nondestructive evaluation

An eddy-current distribution imaging system using a wide-area magnetic field exposure has been developed. This system can be employed to detect deep metal cracks using a magnetic resistive (MR) sensor. The measuring system consists of the exposure coil, MR sensors, measuring circuits, lock-in amplifier, xy stage, and a personal computer. To detect the eddy current distribution, the imaging system uses a pair or MR sensors that detect the x and y magnetic field components parallel to the metal surface. The system operates at a low-frequency range in order to obtain depth of penetration. An artificial defect consisting of a multilayer A1 substrate sample with a bottom half slit was measured to test the system. The change in the eddy current distribution image due to the presence of the flaw was obtained. In particular, it was determined that the detection of the slit depth was improved by this lower exposure frequency.