A Study on Relationship among Crack Initiation Size, Fatigue Limit and Hall Petch Relation

Abstract

Relationship between microstructure and fatigue limit of a smooth specimen of tool steel SK85 (0.83% carbon steel) was investigated. Also, fatigue limit of a pure copper was evaluated. Push-pull tests were performed under stress ratio R = -1. The initial length of crack which initiate during fatigue process is related to the size of a single crystal in the case of carbon steel, etc. In this study, SK85 was chosen for a testing material to investigate the relationship between the microstructure and fatigue limit due to the following reason. The original microstructure of the material used includes spherical microstructure. However, after annealing or normalizing under some conditions, that microstructure changed to a lamellar microstructure. The fatigue limit of heat treating SK85 could be evaluated by a relation in whom the parameters are the hardness and initial crack length. The crack initiation sizes were related to the microstructure. The evaluation method of fatigue limit was investigated in consideration of initiation size of crack or sizes of microstructure. Also, it was discussed that the crack growth rate of SK85 was related to the condition of ferrite grain distributions. For the purpose of investigation of evaluation method of fatigue limit, experiments of smooth and holed specimens of copper were performed. Furthermore, we investigated the relationship between the Hall Petch relation and evaluation method of fatigue limit in this study.