Abstract
The effect of mean stress or the stress ratio, R, (ratio of minimum stress to maximum stress) on the high-cycle fatigue properties of Ti-6A1-4V extra-low interstitial (ELI) forging was investigated at 293 K and 77 K. Fatigue tests were carried out for up to 107 cycles at stress ratios of R=-1, 0.01 and 0.5, and also under the condition where the maximum stress was fixed at 0.2% proof stress ( Crmax = cr02 test). The high-cycle fatigue strength at 107 cycles decreased at each temperature as the stress ratio increased, when the fatigue strength was evaluated in terms of stress amplitude (aa). In the R=0.01 and 0.5 tests, however, the high-cycle fatigue strength at 107 cycles exhibited deviations of aa below the modified Goodman line at both temperatures, indicating that this alloy exhibited an anomalous mean stress dependency. In particular, the amounts of negative deviations from the modified Goodman line at 77 K were larger than those at 293 K. Thus, it is concluded that the anomalous mean stress dependency of this alloy becomes remarkable at low temperatures.
