Effect of strain rate on low-endurance torsional fatigue in commercially pure aluminium

Abstract

Commercially pure (99 per cent) aluminium has been subjected to high-strain low-endurance torsional fatigue under conditions of constant strain rate. (In this paper ‘strain’ and ‘strain rate’ refer to conditions at the surface of the specimen and therefore apply to the zones in which cracks are initiated.) Tests were performed at 20°C (0.315T_M) and were controlled between constant torque limits of equal but reversed magnitudes. Below a critical strain rate, ·λ_crit, the fatigue life obeys the law N_f = A·λ^m for a constant torque range. When ·λ exceeds ·λ_crit, there is no significant strain-rate effect on fatigue endurance. It is suggested that cyclically activated creep mechanisms cause significant damage at strain rates less than 2T_crit. A relaxation behaviour was observed after the first quarter cycle when torque ranges were greater than a critical level of 2 T_crit. A similar phenomenon was observed at ranges less than 2 T_crit with straining rates less than 2T_crit but only when the specimens were close to failure and the initial cyclic hardening had been removed by cyclic softening. In consequence fatigue endurances were found to be less sensitive to torque at ranges greater than 2 T_crit. Cumulative strain (cyclically induced creep) occurred at all torque ranges and straining rates. Minimum cumulative strains occurred at a torque range of 2 T_crit with the value of T_crit having a minimal dependence on the straining rate. The ratio of time to cyclic-dependent processes that contribute to failure is itself dependent on both the torque range and the straining rate. However, a single curve suffices to represent all results if a total strain path to failure criterion is adopted.