2016 年 2 巻 p. 15-00730
Fatigue cracks often initiate inside materials in the very high cycle region (VHCF), although they always initiate at the surface of materials in usual high cycle fatigue. However, the mechanism behind internal crack propagation is not yet clear due to difficulties in observation. The environment inside internal cracks is likely similar to a vacuum environment since it is shut off from air, leading to negligible effect of oxidation or gas absorption. In the present work, the effect of vacuum on fatigue fracture surfaces of high strength steel SNCM439 was investigated quantitatively to estimate the effect of the vacuum-like environment inside the internal crack on the crack propagation process. Uniaxial fatigue tests were carried out in vacuum and air environments and then 3D fractography was performed to measure surface roughness of the fracture surface. These analyses targeted three typical fracture surfaces: (a) surface fracture in air, (b) surface fracture in vacuum, and (c) subsurface fracture. Results of fatigue tests showed that fatigue lives were longer in vacuum than in air. Fracture surface roughness of surface fracture in air was greater than that in vacuum, while fracture surface roughness of subsurface fracture agreed quite well with that of surface fracture in vacuum. These results indicate that the effects of vacuum environment and the environment inside internal crack on fatigue crack propagation are almost the same. This leads us to conclude that the behavior of internal crack propagation can probably be estimated from surface crack propagation in vacuum environment.
