2007 年 56 巻 12 号 p. 1111-1117
In order to simulate interior-originating cracks of Ti-6Al-4V in very high cycle fatigue, crack growth tests using CT specimens were conducted in a high vacuum. Morphogenetic factors on granular fracture surface of interior-originating fatigue were investigated based on the effect of vacuum on crack growth process. As a result, the following were obtained. (1) Vacuum decreased da/dN and increased ΔKth compared with air. This results from a stronger closure effect induced by plasticity in a vacuum. (2) Fine concavo-convexo patterns consisting of a few micrometer size granular features were observed on the fracture surface only in a vacuum. This granular region is same as the region observed in the interior-originating fatigue. (3) The magnitude of da/dN or ΔK had no relation with the formation of granular region. (4) To clarify the effect of crack closure on granular region, an additional test, which makes one of fracture surfaces contact with other, was conducted. This clarified that the area of granular region increased and the fine concavo-convexo patterns became more distinct. Therefore, granular fracture surface of interior-originating fracture in Ti-6Al-4V results from repeating contacts of crack surfaces in a high vacuum or similar environment.