Fatigue Damage Detection in 2024-T3 Aluminum, Titanium, and Low Carbon Steel by Optical Correlation

Abstract

Optical correlation techniques were used to monitor fatigue damage in three metals of different strength and ductility, namely 2024-T3 aluminum alloy, commercial purity titanium, and low carbon steel. Topographical information from the specimen surface is stored holographically and compared with the actual surface as the fatigue test proceeds. Results are presented as curves of correlation intensity (CI) versus number of fatigue cycles, and data are compared for unnotched aluminum sheet specimens and notched specimens of all three materials. CI curves from both notched and unnotched 2024-T3 specimens show the same general three-stage form, and the onset of the third stage corresponds to the presence of a growing fatigue crack. The overall loss of CI over the effective life of the specimen, however, is lower for notched specimens, where fatigue damage is limited to a relatively small area of the illuminated surface. CI curves from titanium and steel specimens also develop in three stages, but for steel the initial region of the curve is protracted and the overall loss in CI is relatively large. This behavior occurs because the steel is relatively ductile and consequently the fatigue damage develops over a large fraction of the illuminated surface. Finally, the effect of the initial surface finish on the CI curve is discussed for all three materials.