Effects of temperature and stress ratio on stage II fatigue crack propagation in bimodal Ti-6Al-4V

The temperature dependence of the fatigue crack propagation rate in stage IIb in bimodal Ti-6Al-4V was investigated at different stress ratios R. Fatigue tests were conducted between room temperature and 550 K at R of 0.1, 0.7, 0.8, and 0.9, and two phenomena were elucidated consequently. First, the fatigue crack growth rates were nearly temperature independent for R = 0.1, 0.7, and 0.8 while it is temperature dependent at R = 0.9. This difference in the temperature dependence can be explained by the assumptions that the fatigue crack growth is controlled by the dislocation activities associated with work-hardening for R ¯ 0.8 while it is controlled by dislocation glide at R = 0.9. Second, the fatigue crack growth rates at R = 0.9 was higher than those at R = 0.1, 0.7, and 0.8. This increase in the fatigue crack growth rate at R = 0.9 can be explained by the change in the stress intensity factor of crack opening. Both the controlling mechanisms emanated from the change in the dislocation structure in front of the crack tip.