Essential structure of S-N curve: Prediction of fatigue life and fatigue limit of defective materials and nature of scatter

Historically, S-N curve has been expressed in the form of an exponential relationship between applied stress σ and number of cycles to failure Nf. This paper elucidates the essential structure of S-N curve based on the prediction method for fatigue life and limit of materials containing defects from the viewpoint of mechanics of small crack. The extended application of the proposed method to step loading and variable amplitude loading is discussed. The question about the applicability of Miner’s rule is also discussed from the viewpoint of continuous decrease in fatigue limit with number of stress cycles and crack growth.