Fatigue strength evaluation of small defect at stress concentration

The effect of individual large notches on the fatigue strength of components is one of the oldest and most studied topics in the history of metal fatigue. When a small defect is present at the notch root, both the stress concentration of the main notch and the effect of the small defect interact and simultaneously influence the fatigue strength. The effect of the main notch can be evaluated from the viewpoint of stress concentration and stress gradient. Both have a strong influence on the fatigue notch factor. The parameter model has been successfully applied to fatigue limit evaluation of materials containing small defects under uniform stress condition. If a small defect is present at the notch root, the effect of stress gradient must be also considered in the application of the model. In the present study, the fatigue tests and fatigue crack growth analyses are carried out for specimens containing a small defect with the size = 46.3µm at the root of notch with 1mm depth and root radius of 1.0mm or 0.3mm. Fatigue limit predictions are made based on the parameter model and the stress intensity factor analyses for a small crack subject to a steep stress gradient. Existing fatigue notch effect methods are reviewed and used in fatigue limit predictions for comparison. Moreover, new fatigue notch effect method based on the parameter model is proposed. The greatest advantage of the proposed method is that it can predict fatigue limit using easily obtainable parameters and without requiring fatigue tests or troublesome analyses. Suggestions for the extension of the proposed method to practical engineering problems are also made.