Effect of Crack Initiation Size on Fatigue Strength in 0.85% Carbon Steel

The fatigue limit of a plain specimen of a tool steel JIS SK85 was investigated. The initial crack length in the fatigue process is related to the size of a single crystal in the case of low carbon steel. However, the crack initiation size is not related to the crystal size in the case of pure aluminum, a lamellar pearlitic steel, etc. In this study, the relationship between the initial crack length and the fatigue limit was investigated. The original microstructure of this material includes a spherical microstructure. However, after heat treatment under some conditions, that microstructure changed to a lamellar microstructure. The fatigue limit of heat-treated SK85 could be evaluated by an equation in which the parameters are the hardness and initial crack length. Finally, this result was discussed based on the observations of crack growth behavior and fracture surface.