3D EC-GMR sensor system for detection of subsurface defects at steel fastener sites

The problem of detecting deep embedded cracks under fastener heads (CUF) in multilayer structures has resulted in the development of several variations of eddy current based techniques. Eddy current excitation combined with magnetoresistive (MR) sensing has improved the effectiveness in detecting second and third layer defects around fastener heads. An eddy current-giant magnetoresistive (EC-GMR) sensor system has been developed and shown to be feasible in detecting subsurface cracks at fastener sites with aluminum fasteners. The normal component BzBz of induced magnetic field is used to identify defective fastener sites by exploiting the asymmetry information in the measured signal. However, the problem of detecting cracks under steel fasteners remains a major challenge since the dominant signals from steel fasteners can mask the contribution from small subsurface defects. This paper presents the feasibility study of detecting cracks under steel fasteners using additional information contained in the tangential components ByBy and BxBx of induced magnetic field. A model-based study exploiting information in all components using image fusion techniques is presented and feasibility of the approach for automated defect detection is shown.