Detection of deep hidden cracks under fasteners (CUF) poses a major challenge in traditional eddy current testing (ECT) techniques because the weak eddy current signal due to skin depth or penetration depth of the fields. Reliable subsurface crack detection in multilayer structures using ECT solution has been investigated. Giant magnetore sistive (GMR) sensors are finding increasing applications in directly measuring weak magnetic fields associated with induced eddy currents. Pulsed eddy current testing (PECT) with GMR sensors system has been developed to identify the existing of subsurface cracks around fastener sites inaircraft wing frame. As an improved technique of ECT, the applied PEC-GMR testing generates 2D rasterscan image一C-scan and distinguishes crack and no crack conditions utilizing the asymmetric GMR response observed for crack feature. The built PEC-GMR system has shown the feasibility in detecting sub surface cracks at aluminum fastener sites, where the measured normal component Bz of induced magnetic field isused to identify defective fastener sites. The asymmetry information in the GMR signals along Bz has been exploited. However, detecting cracks under steel fasteners remains a problem since a subsurface crack is dominated by the strong signal from the steel fastener. This paper presents the feasible study of detectingc racks under steel fasteners using the component Bz and additional information contained in the tangentialcomponents By and Bx of induced magnetic fields in conjunction with image processing to detect and estimate subsurface corrosions of multilayer airframe structures. The applied PEC-GMR technique has presented the reliable inspection of the CUFs.
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