Fatigue Life Prediction of A1319-T7 Subjected to Thermo-Mechanical Loading Conditions

摘要

This study investigated a fatigue life prediction method based on extensive experiment results of cast aluminum alloy Al319-T7 subjected to repeated thermal and mechanical loading. Cyclic tests and fully reversed fatigue test results of the material were obtained from the specimens subjected to three different strain rates (5×10~(-5), 5×10~(-4) and 5×10~(-3)) and various temperature conditions. At each strain rate the specimens were subjected to room temperature (25°C), 150°C, 200°C, 250°C and 300°C. Thermo-mechanical fatigue (TMF) tests were also conducted for in-phase and out-of-phase conditions of the temperature and mechanical loading. During the thermo-mechanical fatigue tests, the effect of loading phases and dwell time on fatigue life of the specimens was also observed. This study modified Taira's fatigue damage model for thermo-mechanical loading condition to include the strain rate effect on the fatigue damage. Taira assumed that fatigue damage per reversal is proportional to the damage factor, λ(T), and plastic strain range powered by n, (Δε_p)~n. The relationship between the plastic strain range (Δε_p) and the number of cycles to failure (N_f) is presented as λ(T)·(Δε_p)~n·N_f = C. Where C is a temperature independent material constant. The temperature effect is included in the damage factor, λ(T) that can be determined from the ratio of λ(T)/λ(T_o) for low cycle fatigue test results at various isothermal conditions. T_o is the reference temperature and can be determined by experiment. This study used stress range applied instead of plastic strain range in the original equation. Furthermore, the modified equation includes the effect of strain rate, phase, and dwell time. The new fatigue damage equation was well correlated with the experiment results.