Stress ratio, elevated temperature and microstructural effects on the near-threshold fatigue behavior of titanium alloys

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.