It's well known High Strength Steel shows internal origin type fatigue failure called Fish eye failure in low stress and high cycle region. In this failure, ODA(Optically Dark Area) is formed and this ODA occupies the great portion of fatigue life of Fish eye failure. Although the explication of the mechanism of ODA formation is needed for the explication of the Fish eye failure mechanism, the explication is not yet accomplished. It's reported that internal fatigue crack in high strength steel is propagating extremely slowly by recent investigations. However, it is not clear the relationship between the extremely slow fatigue crack growth behavior and the mechanism of ODA formation, and whether it is a peculiar phenomenon of high strength steel. In study, fatigue test was done in vacuum environment. As a result, fractography suggest that more obvious ODA is formed by smaller stress ratio when internal crack is extremely slowly propagating.
