2003 Volume 52 Issue 11 Pages 1298-1304
To investigate the fatigue characteristics and the fracture feature of Ti-6Al-4V in very high cycle region, uniaxial tension fatigue tests were carried out. The S-N diagram showed a complex folding shape, which corresponded to the locations of the fracture origins; i. e., surface-originating fractures in high stress level and interior-originating fractures in low stress level. In this study, observed also were interference type fractures in which the main crack was formed by the coalescence of a surface-originating crack and interior-originating cracks in low stress level. Interference type fractures occurred in shorter life region than interior-originating fractures. Therefore, it is inappropriate and risky to evaluate a fatigue life in low stress level without consideration of interference type fractures. When characteristics of this type of material are investigated, it is important to make a model in consideration of distribution of crack initiation sites. A clear difference was observed on the fracture surfaces of structure sensitive stage 2a regions between the surface-originating fracture and the interior-originating fracture. The initial interior crack propagation is characterized by unclear slip marks and a generally round ductile feature with aggregation of a few micro-meter size granular substances. This tendency was not observed on the surface cracks. This seemed to result from the different surrounding environments of surface and interior cracks. Synergistic effects of a vacuum-like environment inside of material and compressive load may cause forming of granular substances.