The detection of defects that are located deep in thick walled ( \u3e12 mm ) aluminum plates is of interest to both the aircraft and space industries. Conventional eddy current (EC) techniques are limited to the inspection of surface and subsurface anomalies. Newly developed high sensitivity magnetic sensors, such as magnetoresistive elements and superconducting quantum interface devices (SQUIDs) have enhanced the EC technique’s capability. Such sensors can be used to detect flaws that are located deep in aluminum plates. However, inspection of a defect located 12 mm to 25 mm below the surface of an aluminum plate is beyond the ability of conventional single frequency EC techniques.
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