Eddy current detection of fatigue damage in Al-SiC composites

Eddy currents have been applied in a novel way to the detection of "naturally-generated" fatigue damage in an advanced Al-SiC composite. The strategy pursued in this work involved tracking small changes in surface material properties through corresponding changes in eddy current probe impedance. The probe was used to examine regions containing many distributed microcracks tens of micrometers in length, and its induced loop passed simultaneously through several such cracks when they were present. This damage summing approach was tried because of the difficulty in directly detecting individual cracks of a very small size. Smooth fatigue specimens of a 2xxx Al ahoy reinforced with 15 vol.% SIC particles were used for these experiments. The results indicate that extremely small changes in the probe impedance (< 2%) could be reproducibly detected and monitored. These observed changes in eddy current probe impedance, however, were apparently not directly correlated in a simple manner to measurable microcrack densities as determined from replicas for the same specimens. Nevertheless, the detected phenomena offer the potential for new insights into the more basic mechanisms of fatigue damage and the possibility for evaluating the mechanical state of advanced metal matrix composites by NDE. The source of the observed increases in probe impedance, and thus the specimen resistivity, is most likely the result of several processes occurring during fatigue, of which microcrack formation is only one. These other processes may be related to even more elementary fatigue damage mechanisms than the microcracks followed in this study. Examples of possible mechanisms, which include cracks smaller than the observable size in the replicas, dislocations, point defect complexes, slip bands, and changes in the solute structure or concentration, are discussed.