Salivar, Gary C.

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.

The deformation and fatigue crack growth behavior of amorphous metals are presented. A discussion of magnetic domains, and their influence on magnetostriction is also included. Testing of Metglas 2605SC, utilizing magnetostriction to generate DeltaK, indicated near-threshold (Region I) fatigue crack growth behavior for as-cast and annealed specimens. A Delta Kth on the order of 0.6-1.0 kg/mm^3/2 was also indicated. The best crack growth rate behavior was obtained for transverse field-annealed specimens. Significant variability in the data, however, prevented a determination of Delta Kth for this condition. A life prediction model based on a Paris equation was developed. Crack growth data were consistently lower than predicted by the Region II model, which would be expected for near-threshold data. Fractographic analyses supported the influence of domains on crack growth behavior. The scatter in the Region I data appears to be contributed by the effects of domains and domain walls.

This investigation concentrated on the fatigue crack growth properties of ARALL laminates. The influence of stress ratio and elevated temperature on fatigue crack growth rates were addressed. It was found that elevated temperature had an effect on the delamination of the fiber/resin lamina from the aluminum sheets resulting in a change in crack growth rate behavior. A stress ratio effect was also present at elevated temperature that could not be explained completely by crack closure concepts.

Near-threshold fatigue crack growth behavior was examined in Titanium 8Al-1Mo-1V in laboratory air at room temperature and at an elevated temperature for various stress ratios. It was found that at the elevated temperature, the threshold stress intensity, ▲K_th, varied linearly with the stress ratio. At low stress ratios increasing the temperature increased the intermediate and near-threshold crack growth rates but did not affect ▲K_th. Crack closure was able to account for the effects of stress ratio, but not completely for the effect of temperature. It appears that a unique value of ▲K_th,eff is available for this material under the test conditions investigated.