Abstract
Uniaxial tension fatigue tests and fracture surface observations were carried out on Ti-6Al-4V. The effects of stress ratios R(=σ_<min>/σ_<max>) on fatigue properties and initial crack propagation processes on interior-originating fractures were investigated. As a result, the following were obtained : (1) S-N curves under R<0 showed a steep incline in a long life region where interior-originating fractures occurred although those under R<0 revealed a gentle slope in a whole life. (2) Multiple cracks initiated from plane facets were observed on the fracture surface under R<0,however those multiple facets could not be found under R<0. The result of (1) was probably explained by the idea that positive stress rations facilitated the initiations of many small cracks and the linkage of those cracks.
