Fatigue Life Prediction of Al319-T7 Subjected to Thermo-Mechanical Loading Conditions

Abstract

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 (5x10-5, 5x10-4 and 5x10-3) 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, (Δεp)n. The relationship between the plastic strain range (Δεp) and the number of cycles to failure (Nf) is presented as λ(T)•(Δεp)n•Nf = 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(To) for low cycle fatigue test results at various isothermal conditions. To 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.