Effect of Stress Ratio on Subsurface Fatigue Crack Initiation Behavior of Beta-Type Titanium Alloy

The effect of stress ratio (R=σmin/σmsx) on subsurface fatigue crack initiation behavior in high-cycle fatigue of beta-type titanium alloy, Ti-15V-3Cr-3Sn-3Al, was investigated using smooth specimen subjected to axial reversed loading in air at room temperature. Two types of microstructure prepared using different aging time conditions included the partially precipitated alphaphase in beta-phase and the fully precipitated one. The first part of S-N curves at high stress amplitude region tested under R=-1 and 0.05 looks similar to curves reported in the literature, however after the curve flattens out there was a sharp drop in the fatigue strength at low stress amplitude region, accompanying a transition of crack initiation site from surface to interior. The number of cycles to the transition depended on R but did not depend on the microstructure. Fatigue fracture of all specimens tested under R=0.5 and 0.7 showed the subsurface crack initiation. No defect was observed at the subsurface crack nucleation sites, but the region of the nucleation site had a grain morphology of facet pattern. Limit of fatigue fracture occurred by surface crack initiation was controlled by the threshold of formation of slip in beta-phase due to cyclic hardening, and described by the Gerber-relationship characterizing mean stress effect.