Extremely low rate fatigue-rack propagation of high strength steel in ODA crack propagation regime

Fish eye failure that is unique fatigue fracture occurs in very long life region of high strength steel. The fatigue life of fish eye failure dominated ODA (Optically Dark Area) around the fracture origin. A granular looking area of ODA is visible close to the crack initiation, fatigue life of fish eye failure is almost exclusively spent in the propagating in ODA. To predict fatigue life of fish eye failure, therefore, it is important to know the formative mechanism ODA. In this study, fatigue test was performed under vacuum environment that simulates environment of subsurface failure. The observation of process when ODA is formed on the precrack tip was examined in order to make clear a mechanism of ODA formation. Fatigue crack growth rate has depended on maximum stress by range of ΔK ≤ 4MPa√m. Under the fixed maximum stress of value, initiated fatigue cracks became non-propagating cracks. Moreover, fractography results suggest that ODA is formed when internal crack is propagating extremely slowly.