5 May 2000 Detection of cracks near sharp edges by using giant magneto-resistance-based eddy current probe
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Abstract
A difficult problem in eddy current inspection of airplane parts is the detection of cracks near sharp edges. In particular, cracks initiate close to the edge in the case of aircraft engine turbine disks. Since the edge is a geometric discontinuity, it produces a strong eddy current response that can distort the crack signal, resulting in a degradation of the capability to reliably detect the crack. A new probe that uses giant magnetoresistive (GMR) sensors as detecting elements provides an elegant solution for this problem. The GMR sensor is placed in the center of a pancake-type excitation coil. Being a uni-directional device, the sensor detects only the field along its sensitive axis. In the measuring configuration, the sensing axis is oriented coplanar with the specimen surface, so that the sensor is insensitive to the excitation field. When the probe is scanned above a crack initiating perpendicular to an edge, orienting the sensing axis parallel to the edge, the output signal is produced only by the crack, while the usually high signal due to the edge is eliminated. A discussion of experimental observations obtained by scanning surface and subsurface edge cracks are included in the paper.
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Teodor Dogaru, Stuart T. Smith, "Detection of cracks near sharp edges by using giant magneto-resistance-based eddy current probe", Proc. SPIE 3994, Nondestructive Evaluation of Aging Aircraft, Airports, and Aerospace Hardware IV, (5 May 2000); doi: 10.1117/12.385029; https://doi.org/10.1117/12.385029
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