Pulsed Eddy Current Detection of Cracks in Multilayer Aluminum Lap Joints

Aging aircraft is susceptible to cyclic fatigue cracks around ferrous fasteners in multilayer aluminum wing lap joints, as found on Canadian Military aircraft CP-140 Aurora and CC-130 Hercules. Current methods of crack detection utilize bolt hole eddy current, requiring fastener removal, a time consuming process that also has the potential to result in collateral damage. In this paper, pulsed eddy currents (PECs) are induced in the aluminum wing structure by a probe centered over a ferrous fastener. The response signals are analyzed using a modified principal components analysis (PCA) and cluster analysis to show separation of groups of PCA scores from fasteners with and without notches in the bore hole. The Mahalanobis distance (MD), a cluster analysis method, is used to obtain a relative distance measurement between scores associated with cracks from those without cracks. The MD evaluation provides confidence intervals for detection of cracks, which range in extent from fastener holes from 0.9 to 5.5 mm, and a means of potentially sizing larger (>3 mm) second layer cracks. In this paper, 100% of the cracks in the sample were detectable. The results demonstrate PEC as a viable technique for second layer crack detection with ferrous fasteners present.