Effects of Existence of Inclusions on Fatigue Strength

There is a good correlation between the static strength and fatigue limit in many carbon steels. However, in the case of some commercially sold carbon steel, the fatigue limit cannot be evaluated from the static strength. Care should be taken when those materials are used for ma-chine equipment. In the case of some materials, there is decrease in fa-tigue limit from the expected values of Vickers hardness. In this study, one such case was investigated by push-pull. The examinations of the S-N curves and crack growth behavior were performed. Then, it was discussed that decrease in fatigue limit was related to the existence of inclusions or defects of unexpectedly large size. When such material as used in the present study is used for a machine equipment, the decrease of fatigue limit should be taken into account to determine an applied load.