2017 年 66 巻 5 号 p. 365-371
The relationship between the microstructure and fatigue limit of a smooth specimen of tool steel SK85 (0.83% carbon steel) was investigated. Also, the fatigue limit of pure copper was evaluated. Push-pull tests were performed under stress ratio R = -1. The initial length of a crack which initiates during the fatigue process is related to the size of a single crystal in the case of carbon steel. In this study, SK85 was chosen as a testing material to investigate the relationship between the microstructure and fatigue limit for the following reason. The original microstructure of the material used includes spherical microstructure. However, after annealing or normalizing under certain conditions, that microstructure changed to a lamellar microstructure. The fatigue limit of the heat treated SK85 could be evaluated by a relation in which the parameters are the hardness and initial crack length. The crack initiation sizes were related to the microstructure. The method of evaluating fatigue limit was investigated in consideration of the crack initiation size or the sizes of microstructures. Also, it was discussed the fact that the crack growth rate of SK85 was related to the condition of ferrite grain distributions. For the purpose of investigating the method of evaluating the fatigue limit, experiments with smooth and holed specimens of copper were performed. Furthermore, we investigated the relationship between the Hall Petch Relation and the method of evaluating fatigue limit in this study.
