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 (0.315TM) and were controlled between constant torque limits of equal but reversed magnitudes.Below a critical strain rate, pCrl,, the fatigue life obeys the law for a constant torque range. When p exceeds ycrlt 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 Ycrit.A relaxation behaviour was observed after the first quarter cycle when torque ranges were greater than a critical level of 2T,,,,. A similar phenomenon was observed at ranges less than 2Tc,,, with straining rates less than P~,~~ 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 2T,,,,.Cumulative strain (cyclically induced creep) occurred at all torque ranges and straining rates. Minimum cumulative strains occurred at a torque range of 2TCr1, with the value of TcrIt 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.

Strain Rate and Loading Waveform Effects on an Energy-Based Fatigue Life Prediction for AL6061-T6

“…Previous studies [8,14,15] indicate that hysteresis energy values decrease after an initial "break in" period that lasts for about the first 20% of the specimen lifetime at any stress level. The break in period is followed by a relatively stable strain energy period until an exponential increase in energy during the last 20% of the specimen's life.…”

“…Miller and Rizk [14,15] studied commercially pure aluminum in torsion under several constant strain rates (triangular). The specimens in this study were subjected to fully reversed torque controlled fatigue cycles at high stress levels.…”

“…According to their tests, this phenomenon increased as stress level increased and became less noticeable at lower stress levels. Miller and Rizk [14] also conducted several full life tests monitoring plastic strain. They noted that at moderate stress levels, plastic strain decreased for approximately the first 25% of the specimen life then reached a stable region.…”

Cyclic behaviour of materials

“…For details of test rigs and experimental techniques readers are referred to earlier papers (24) (30) (32). Let it be sufficient to say that torsion is considered to be a more fundamental mode of testing than either push-pull or bending (36) and in c .-L 400 5 this context it is interesting to note that two recent publications concerning the design of high-pressure, high-temperature vessels recommend the use of cyclic shear-stress-shear-strain data (38) (39).…”

An experimental and numerical study of the strain rate effects on mild steel for cyclic loading

“…44 Enhanced view o f the shifted final half cycle at each strain range for part 2 o f 3 o f the strain range sequence for ATE2E and ±1% strain range for ATE2I45 Enhanced view o f the shifted final half cycle at each strain range for part 3 o f 3 o f the strain range sequence for ATE2E and ±1% strain range for ATE2I----------------" ...... 1" " ----------------...........................!............................ * ................ ............... •........... ....... A s ? '…”