Fundamental Investigation of Fatigue Crack Growth Retardation in Aluminum Alloys

Abstract : A fundamental investigation was performed to determine the important metallurgical and environmental factors that influence retardation behavior in aluminum alloys. The retardation phenomenon was found to be very complex and dependent on loading variables as well as material parameters. The results suggest that a phenomenological model for prediction of fatigue-crack growth under variable amplitude loading can be developed by taking the various important parameters into account. Of the four alloys investigated (2024-T3, 2024-T8, 7075-T6 and 7075-T73), the alloy with the lowest yield strength, 2024- T3, was found to give maximum delay. The important material parameters were yield strength, cyclic-hardening exponent, and microstructure. Overload ratio (OLR) was the most important test-parameter. An increase in OLR always produced an increased retardation. Crack-arrest occurred at either 2.5 or 3.0 OLR depending on the alloy and the baseline stress-intensity (Kmax). For the 7075-T6 SEN specimens, no significant changes in crack-closure were detected as the cycling progressed after the overload cycle. The overload cycle manifests itself as a stretch band on the fracture surfaces of retardation specimens. The width of the stretch zone increases with increase in Kmax and decrease in thickness. For specimens with discernible striations, correlation between measured da/dN and changes in striation spacing was good. The optical interference technique was found to give a good agreement between measured plastic-zone and that calculated under plane-stress conditions.