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 regardless of scatter of data. This paper elucidates the essential structure of S-N curve and the causes of scatter of fatigue life and fatigue limit based on the prediction model for materials containing various defects. The validity of the model was verified by testing specimens containing small defects with various sizes and by analyzing the difference of failure lives due to defect size. The model was successfully applied to the data reported by other researchers. It was revealed that the statistical nature of the scatter of fatigue data should be expressed by the statistics of extremes, because the source of the scatter is originated from the largest defect contained in specimens.
