Eddy current detection of cracks within the bore of installed Taper Lock fastener

It is proposed to develop novel eddy current probes based on spin dependenttunneling (SDT) magnetoresistive (MR) sensors to enhance the probability ofdetection of cracks within the bore of installed Taper Lok fasteners. Preliminaryresults show that the use of shaped excitation coils, together with extremelysensitive SDT sensors properly positioned above the structure under inspection,provides a significant improvement for deep crack detection around fasteners.By using specific coil geometry, the eddy current density can be focused on thecircular edge of the fastener hole, where cracks can initiate. Consequently, theperturbation of the eddy current flow due to the presence of the crack is greatlyenhanced. The proposed rotating self-nulling probe does not require eitheradditional ferromagnetic flux focusing lenses, or compensation of the excitationfield, as in prior art. Within this phase I project, different coil geometriesand location of the sensor relative to this coil will be compared and theperformance limitations of each design will be studied. A simple manual scanningsystem will also be developed for fast inspection of holes.The novel eddy current probes will provide an effective tool for detecting smallcracks within thick wing splice joint structures, without the need to removethe Taper Lok fastener. It is envisaged that the proposed approach will outperformcurrent state-of-the-art eddy current techniques used to address this difficultproblem, in terms of both size and depth of defects that can be reliably detectedaround the fastener holes. Both military and commercial aircraft industries willdirectly benefit from the development of these novel probes.