Deep flaw detection with giant magnetoresistive (GMR) based self-nulling probe

Giant magneto-resistive sensors provide a high sensitivity to low frequency magnetic fields. This, combined with an inherent small size, low cost, and low power consumption, make GMR sensors a good candidate for low frequency eddy current probe development. Previous research has shown that the incorporation of a GMR sensor in the NASA LaRC developed Self-Nulling Eddy Current Probe greatly enhances the low frequency detection capabilities of the device. Experimental results have shown the detection of a 1.4 cm long notch under up to 9 mm of unflawed aluminum. A limiting factor in this work appeared to be the increased background level with decreasing frequency. In order to improve the signal-to-noise ratio several techniques have been incorporated to reduce the background level and increase the signal-to-noise ratio. The incorporation of a gradiometric sensor, active feedback, and improved shielding have led to an enhanced detectability. In addition, image processing techniques have been explored to filter...