Non destructive evaluation of crack propagation under a composite patch repair using the eddy current method

The eddy current method is used to trace crack propagation under a composite patch repair of a cracked metallic structure, after mechanical testing in fatigue. The capability and the reliability of the eddy-current method to detect cracks under a composite obstacle of significant thickness are checked for several patch thicknesses. Notched specimens 6mm thick were fabricated using 2024-T3 Aluminum. Boron Epoxy patches bonded with film adhesive were applied to the one side of the metallic specimens, initial notches were 10mm long, while the thickness of the reinforcement was varying from 2 layers (0.25mm) to 7 layers (0.875mm) in order to represent actual structural composite patch repairs. Crack propagation from the tip of the notches was achieved by fatigue loads. The estimation of required loads to cause fatigue crack propagation was done by means of three-dimensional finite elements analysis. The eddy current method was then applied to trace the crack tip under the patch after their mechanical testing. Accuracy of the eddy-current method was verified by measuring the crack lengths on both sides of the specimen and comparing the results. The eddy-current method was found to be fully capable of tracing the crack propagation under the composite patch, requiring only proper calibration for the generator. Small differences in the crack lengths between the patched and the unpatched side of the specimen were explained by their nonsymmetric configuration, which induced different stress intensity factors at the patched and the unpatched sides, as finite elements analysis has clearly shown.