Ratcheting strain and its effect on low cycle fatigue behavior of Al 7075-T6 alloy

The aim of this investigation was to examine the role of accumulation of ratcheting strain on low cycle fatigue behavior of aluminum 7075-T6 alloy. For this purpose, first ratcheting behavior of the alloy was studied using asymmetric stress-controlled low cycle fatigue tests up to 2000 cycles. Post-ratcheting stain-controlled low cycle fatigue tests were done on ratcheted specimens up to failure. Further, stain-controlled low cycle fatigue tests without any previous ratcheting were also conducted up to failure in order to compare the behavior with and without previous ratcheting. A few ratcheted specimens were studied using transmission electron microscope to assess the dislocation features generated due to ratcheting. The results indicated that increase of either stress amplitude or mean stress increased the ratcheting strain accumulated in the material. Ladder-like dislocation structure prevailed in the specimens deformed at lower stress levels while dislocation tangles were predominantly present in specimens deformed at higher stresses. Analysis of post-ratcheting low cycle fatigue behavior showed that fatigue life of the alloy decreased with the increase of previously imposed ratcheting strain. Comparison of low cycle fatigue behavior of the investigated alloy with and without ratcheting indicated that ratcheting strain was detrimental to fatigue life of aluminum 7075-T6 alloy.