Influence of stress ratios and residual stress on the fracture pattern of Ti-6Al-4V alloy in high cycle regime

Experiments are conducted on the high cycle fatigue behavior of Ti-6Al-4V in air at room temperature. Fatigue strengths are determined at various stress ratios for specimens prepared by hard turning plus polishing and residual stress relieved in vacuum after polishing. For various stress ratios and stress amplitudes investigated, the relaxation of residual stresses is measured during the interrupted fatigue test. The relationship of residual stresses relaxation with the cycles and the effects of residual stress as long as stress ratios on fatigue strength are analyzed. Results indicate that the improvement of residual compressive stress on fatigue strength decreases and vanishes finally with the increase of stress ratio. Compared to fatigue crack originating from surface for annealed specimens, the fatigue crack initiation sites are located in the interior of the specimen due to the effect of residual stress at lower stress ratios. At R=-1.0, the residual stresses are relaxed after several cycles and then maintained stable. The improvement of fatigue strength is attributed to surface crack closure induced by compressive residual stress. For R=-0.6-0.1, in contrast, the relaxation of residual stress is insignificant and fatigue failure is dominated mainly by the internal defect and local stress concentration. ©, 2015, Editorial Office of Chinese Journal of Mechanical Engineering. All right reserved.