Materials--Fatigue--Testing

The effect of stress ratio, elevated temperature and differences in microstructure on the near-threshold fatigue crack growth behavior of Titanium alloys was investigated. Near-threshold fatigue testing was conducted on Ti-6Al-2Sn-4Zr-2Mo and Ti-6Al-2Sn-4Zr-6Mo at stress ratios of 0.1 and 0.5 and at temperatures of 400 ℉ and 800 ℉. It was found that an increase in stress ratio resulted in an increase in growth rates and a decrease in the fatigue threshold. Increasing the temperature also resulted in faster growth rates and a lower threshold. There was slight differences in growth rates between the two alloys due to microstructural differences. Crack closure could effectively remove some of the effects of stress ratio and microstructural differences but could not remove the effect of the higher temperature. Fractographs revealed the difference in microstructure but did not show significant changes in fracture modes between temperatures.

This research investigated the effects of stress ratio, temperature and microstructure on the near-threshold fatigue crack growth characteristics of three differently processed titanium alloys. The materials investigated included one hot-worked Ti-6Al-2Sn-4Zr-2Mo alloy, one cast Ti-6Al-4V alloy and one cast Ti-6Al-2Sn-4Zr-2Mo alloy. It was found that overall crack closure could account for apparent differences in fatigue threshold for stress ratios up to about 0.7. Tests run at stress ratios of 0.9, however, showed a noticeably lower fatigue threshold. Crack closure was not able to account for differences due to elevated temperature. Fractographic evidence is also presented which shows the extremely beneficial effects of larger grain size on the fatigue threshold through crack branching, deflection and bifurcation.