Detection of Hidden Flaws with GMR-Based Eddy Current Technique

Nondestructive inspections involving multi-layered structures often require a reliable NDI methodology to estimate size and location of a flaw with quantitative information about width and depth. Compared to conventional wire-wound pick-up coil based eddy current NDI methods, Giant Magneto-resistive (GMR) sensor based eddy current methods have shown a much better sensitivity (20–50 times) for a given driving current and frequency. In addition to the higher sensitivity of the GMR sensors, it has been shown that the interpretation of GMR sensor detected signals is much less complicated than the conventional eddy current signals that are normally displayed on an impedance plane. Unlike wire-wound pick up coils, solid-state GMR sensors detect magnetic fields directly, where the magnitudes of magnetic fields are displayed in terms of voltage. Flaw signals detected with GMR sensors (voltage and distance) are similar to flaw signals detected with ultrasonic transducers (voltage and time). In this study, experimental results for various hidden flaws in thick metals and in double-layered aircraft lap joints are presented. The results show that quantitative sizing of flaws can be done fast and reliably.