Edge Crack Detection Using a Giant Magnetoresistance Based Eddy Current Sensor

Detection of cracks near sharp edges is a challenging problem in eddy current inspection of metal parts and structures, especially in the aircraft industry. This is because the edge itself causes a strong disruption of the eddy current flow in the crack region. With conventional probes, the large signal due to the edge can completely mask that produced by the crack. An effective solution for separating the two signals is the use of an eddy current probe comprising a giant magnetoresistive (GMR) sensor placed in the center of a pancake-type excitation coil. The sensor detects only the magnetic field along its sensitive axis and is not influenced by fields applied in perpendicular directions. By orienting the sensitive axis parallel to the edge direction, the edge signal is eliminated. Moreover, in this measuring configuration, the presence of the edge enhances the crack signal, leading to higher sensitivity and improved spatial resolution for edge cracks as compared to inside cracks. Measurement of surface edge cracks of 1 mm length and 0.5 mm depth as well as small sub-surface edge defects are presented. The enhanced sensitivity of this probe to edge cracks is explained using a qualitative interpretation of experimental results based on the schematic representation of eddy current flow in the specimen.