Low-cycle fatigue behaviour of 7075-T651 aluminium alloy: Cyclic deformation and life prediction

High-strength aluminium alloys are widely used in aerospace and automotive structures, where fatigue assessments incorporating cyclic plasticity effects are often performed using strain–life approaches. The description of such effects requires advanced plasticity models capable of capturing the cyclic response and predicting damage accumulation. Nevertheless, detailed investigations into the interactions among cyclic plasticity, damage mechanisms and fatigue life prediction under variable-amplitude loading remain challenging due to limited data. Thus, this study investigates the cyclic elasto-plastic behaviour and life prediction of 7075-T651 aluminium alloy under variable-amplitude strain-controlled conditions. An Armstrong–Frederick kinematic hardening model was developed to capture the cyclic elasto-plastic response and replicate the stabilised hysteresis loops, while electron backscatter diffraction and scanning electron microscopy analyses were conducted to understand micro-structural evolution induced by cyclic loading and identify the main micro-mechanisms associated with the applied strain levels. The analysis of the results indicates that the tested alloy exhibited mixed cyclic hardening–softening behaviour, dependent on strain amplitude, and was relatively close to a Masing-type material. The cyclic stress–strain response and the associated fatigue life relationships under constant-amplitude loading, expressed via Basquin–Coffin–Manson (BCM) and Smith–Watson–Topper (SWT) parameters, showed good agreement with the literature results. Fatigue life predictions under variable-amplitude loading were successfully performed using either the BCM or SWT model, each combined with a linear damage accumulation rule. In addition, dislocations were largely confined to grain boundaries in the as-received material, whereas under fatigue loading they extended into grain interiors as well as along the boundaries, accompanied by a significant increase in the geometrically necessary dislocation density.