Effects of Defect, Mean Stress and Lower Loading on High Cycle and Very High Cycle Fatigue Behavior of Ti-6Al-4V Alloy

Abstract

In this study, the effects of defect, mean stress and lower loading are investigated for high cycle (HCF) and very high cycle fatigue (VHCF) behavior of Ti-6Al-4V alloy. It indicates that the S–N curve of Ti-6Al-4V alloy exhibits a linear decreasing trend or a plateau characteristic in HCF and VHCF regimes, which depends on the defect size and stress ratio. VHCF strength decreases with increasing the defect size, and it is irrespective of stress ratios. The fatigue crack initiates from specimen surface at R = −1 in both HCF and VHCF regimes. While the fatigue crack initiates from the subsurface or the interior of the specimen at R = 0.1 in VHCF regime. A sequence of lower stress amplitude below the fatigue strength at 10⁹ cyc has no or negligible influence on the fatigue life of 10⁵–10⁹ cyc. The lower stress amplitude in variable amplitude loadings does not affect the failure mechanism. The residual compressive stress relaxation is not observed after a large number of lower loadings under ultrasonic frequency fatigue test. Gerber formula and Goodman formula give dangerous predictions of VHCF strength for both smooth specimens and specimens with defects.