Abstract
Rotaing bending and axial fatigue tests have been performed using solution treated and aged materials in two beta titanium alloys, Ti-15Mo-5Zr-3Al and Ti-22V-4Al alloy. Subsurface crack initiation was found to be a microstrructure and a stress ratio related phenomenon. In the former alloy, cracks were initiated internally only in a material that was solution treated above the beta transus and at a certain stress ratio in relatively short life regime. The effects of grain size and mean stress were not characterized entirely by the Hall-Petch relationship and the Goodman relationship, respectively, which was attributed to different crack initiation mechanisms. In the latter alloy, subsurface crack initiation was also seen in the long life regime around or above 10^7 cycles, resulting in a step-wise S-N curve. A smooth facet invariably existed at the crack initiation site. The occurrence of this mode of failure is discussed based on microstructural and mechanical considerations.
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