Detection of Subsurface Defects in Multilayer Structures with Steel Fasteners

The inspection of deep embedded cracks under fastener heads (CUF) in multi-layer structures has contributed to the development of several variations of eddy current based techniques [1]. Although, the main difficulty is due to skin depth or penetration depth of the fields, eddy current testing methods combined with magnetoresistive (MR) sensors have improved their effectiveness in detecting 2nd and 3rd layer defects around fastener heads. An eddy currentgiant magnetoresistive (EC-GMR) sensor system has been developed and shown to be reliable in detection of subsurface cracks at Aluminum fastener sites in a multilayer structure [2]. With linear multiline excitation currents, and GMR sensor mounted on the line of symmetry the normal magnetic field associated with the induced currents can be measured In the presence of fasteners and/or cracks, the normal magnetic field (Bz) caused by induced currents is picked up by the GMR sensor and the asymmetry of the normal component signal is analyzed for crack inspection [2, 3]. However, in the presence of steel fasteners the conventional GMR signal (normal field component) due to steel fastener is dominant and completely masks the presence of defect signal.