Eddy current measurements with magneto-resistive sensors: Third-layer flaw detection in a wing-splice structure 25 mm thick

A new eddy-current system based on low-noise magnetoresistive sensors has been developed to detect cracks and corrosion in thick, multi-layer metal structures. The new instrument has detected narrow slots as short as 6.3 mm, in the lowest layer of a stack of three aluminum plates totaling 25 mm in thickness. These flaws were detected through 19 mm of overlying aluminum, in spite of the presence of steel fasteners and the proximity of a plate edge that attenuated the eddy currents impinging on the flaw. This performance was achieved by combining the low-frequency sensitivity of magnetoresistive sensors with a special probe design that minimized background errors due to liftoff, plate-edge effects and the steel fasteners. This new system is potentially useful for inspecting thick, layered structures in aircraft, as well as ferrous metal structures such as pipelines and storage tanks, where the shortness of the skin depth has previously limited the usefulness of eddycurrent inspection.