The Role of Ratcheting Deformation on Low Cycle Fatigue Behaviour of Al 7075-T6 Alloy

Abstract

The aluminium 7075-T6 alloy is being potentially used for the fabrication of aerospace and military components where during service the components experience complex kinds of cyclic loading ranging from symmetric to asymmetric in nature. Asymmetric cyclic loading induces plastic strain to a structure which is known as ratcheting. Thus, during design of these components, it is essential to investigate the influence of ratcheting on low-cycle fatigue behaviour of the material. The aim of the current research is to examine the role of previous ratcheting deformation on the low cycle fatigue behaviour of Al 7075-T6 alloy. To fulfill this objective first T6 heat treatment was done for as-received aluminium 7075 alloy. The material was then characterized for its microstructure and hardness using optical microscope and Vickers micro hardness tester respectively. Then this alloy was subjected to tensile tests as per ASTM standard E8M-08 to determine the mechanical properties and also to design the input parameters required for ratcheting and low cycle fatigue tests. Fractography analysis was done for the broken tensile samples using SEM. Ratcheting tests were conducted at uniform mean stress with different stress amplitudes and at uniform amplitudes of stress applied with varying mean stress values. Post-ratcheting low cycle fatigue tests were conducted up to the failure of the samples.
The results obtained after ratcheting tests indicates that rise of either stress amplitude or mean stress by keeping the remaining one constant, increases the ratcheting strain accumulated in the material. All the ratcheting tests were done up to 2000 cycles while strain accumulation attained a saturation level after around 250 cycles for almost all samples. An analysis of hysteresis loop area obtained after ratcheting tests reveals that material exhibits cyclic softening feature for the initial 100 cycles followed by cyclic hardening feature from 100-500 cycles and finally reaches the steady state for all the investigated mean stress and stress amplitudes. The loop area also found to increase with the rise of mean or amplitude of stress applied by keeping the remaining one constant. Analysis of post-ratcheting low cycle fatigue behaviour of the material indicates that fatigue life reduces with the rise of the previous imposed ratcheting strain.