Fatigue damage detection in 2024 aluminum alloy by optical correlation

Abstract

The use of optical correlation techniques to monitor fatigue damage in 2024-T3 aluminum alloy is described. Topographical information from the surface of a sheet specimen is recorded holographically and compared with the actual surface by measuring correlation intensity as fatigue damage accumulates. The hologram is recorded on a thermoplastic - photoconductor device that can be developed and erasedin situ, so that repeated recordings can be made easily during a fatigue test. The results are presented as curves of correlation intensity (I _c) vs fatigue cycles(N) for unnotched specimens fatigued in tension-tension. The curves exhibit three regions, namely a decelerating loss of log I_c vs N over the first portion of the fatigue life (Region A), followed by a linear loss (RegionB), and finally by an accelerating loss over the final portion of the life (Region C). Changes in the initial surface finish of the specimen do not affect the general form of the correlation curve. Metallographic evidence indicates that the three regions correspond to fatigue-induced changes in the specimen surface, although dimensional changes in the specimen may also contribute to correlation losses. RegionA appears to correspond to a period of initial cyclic strain adjustment, and Region B to a period of crack initiation and/or early growth. RegionC corresponds to the presence of growing fatigue cracks, and the transition fromB to C occurs when the cracks exceed about 20 μm in length. Thus, the correlation intensity data provide a sensitive indication of accumulating fatigue damage and impending failure in individual specimens.