Real Time Pulsed Eddy Current Detection of Cracks in F/A-18 Inner Wing Spar Using Discriminant Separation of Modified Principal Components Analysis Scores

The aluminum inner wing spars of F/A-18 Hornet aircraft may undergo stress corrosion cracking along the spar between the fasteners that secure carbon-fiber/epoxy composite skin to the wing. Inspection of the spar through the wing skin is required to avoid wing disassembly. The thickness of the skin varies between 9 and 21 mm (0.36-0.83 in) and fasteners may be either titanium or ferrous. Pulsed eddy currents (PECs), generated by a probe centered over the fastener, demonstrate the capability of detecting simulated cracks within spars with the wing skin present. Comparison of signals from separate pick-up coils, mounted to either side of the excitation coil, is used to detect differences in induced eddy current fields, which arise in the presence of cracks. To overcome variability in PEC signal response due to variation in: 1) skin thickness; 2) fastener material and size; and 3) centering over fasteners, a large calibration data set is acquired. Multidimensional scores from a modified principal components analysis (PCA) of the data are reduced to 1-D using a discriminant analysis method. Under inspection conditions, calibration PCA scores combined with discriminant analysis permit rapid real time go/no-go PEC detection of cracks in F/A-18 inner wing spar.