An important issue in the maintenance of aging aircraft is the detection of small cracks and corrosion embedded in the multi-layered structures of airframes. Sensitive methods of nondestructive evaluation are required to detect these deep-lying defects, particularly since they commonly occur near fasteners which themselves yield a strong signal and can mask a smaller signal from a defect nearby. A highly-sensitive measurement system has been developed by combining pulsed eddy-current technology with the unsurpassed magnetic-field sensitivity of superconductive probes. The performance of pulsed, superconductive sensors in detecting signals expected from embedded cracks has been assessed both experimentally and theoretically. The theoretical model describes the response of a superconductive probe to a crack embedded at an arbitrary depth in a layered conductor. The formulation of the field problems leads to a time-dependent integral equation which is solved by a boundary-element method and the solution used to determine probe signals. Key aspects of the theoretical model are described in this paper and the results of numerical calculations compared with experiments.
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