Unified understanding of the formation of unique fracture surface region in vacuum environment

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To clarify the formation mechanism and the process of forming the granular region, which is a unique phenomenon in the very high cycle fatigue, fatigue tests using specimens with an artificial defect were conducted in vacuum. The fatigue tests were based on the idea that the environment around sub-surface fatigue crack is vacuum-like environment. During the tests, fracture surfaces were intentionallycontacted. Fracture surface observations revealed that repeating contacts of the fracture surfaces and vacuum environment are necessary for the formation of the granular region.A new mechanism for the formation of the granular region was proposed.