Effect of Defect Shape on Fatigue Limit Evaluation of High-Strength Steel Using Vacuum Quenching and Tempering

The √area parameter model is widely used for predicting the fatigue limit of materials containing small defects based on the assumption that a small defect can be regarded as a crack. Although the model was successfully applied to various materials, its applicability to high-strength steel requires further validation. In this study, the fatigue limit was evaluated using specimens of vacuum-quenched and tempered martensitic steel in which a drill hole, an electric discharge machined (EDM) notch, and a pre-crack were introduced. The fatigue limit of the specimen with the pre-crack was consistent with the prediction of the √area parameter model, whereas the specimens with the drilled hole and EDM notch exhibited higher fatigue limits, indicating that these defects could not be regarded as cracks in the fatigue limit evaluation. Fracture surface observations confirmed that the fatigue limits were determined by the crack non-propagation limit rather than the crack initiation limit. Furthermore, finite element analysis indicated that differences in defect-induced stress fields influenced the fatigue crack propagation, leading to deviations in the fatigue limits. These findings contribute to an accurate estimation of the fatigue limit of high-strength steels.