A study of the back stress and the friction stress behaviors of Ti-6Al-4V alloy during low cycle fatigue at room temperature

Low cycle fatigue (LCF) tests were performed on Ti-6Al-4V alloy at room temperature. The fatigue hardening/softening of the Ti-alloy were determined. Both the back stress (X) and the friction stress (σF) were evaluated from analysis of fatigue hysteresis loops and their cycle-dependent behaviors during fatigue tests were obtained experimentally. It is shown that the cyclic stress response or the fatigue hardening/softening of the Ti-alloy actually results from the competition effect of the back and the friction stress behaviors during fatigue cycling. A two-slope behavior in both the εpa-2Nf and the X-2Nf plots was also obtained. The micro-mechanisms associated with the back and friction stress behaviors as well as the two-slope behavior in the εpa-2Nf and X-2Nf plots were further discussed with the help of OM and TEM observations of fatigue failure specimens at representative strain amplitudes. In this study, the DIC technique was also introduced to characterize quantitatively the cyclic deformation heterogeneity of the Ti-alloy at the grain scale.