Revealing the mechanism of critical root radius in notch fatigue limit based on crack closure concept

The critical root radius, named as branch point, exists in notch fatigue limit, below which the fatigue limit appears to be independent of the root radius. Two theoretical models were proposed in this paper to discuss the characteristics of critical root radius and notch fatigue limit. The first one was the qualitative model by modifying the Dugdale model for fatigue crack growth from notch tip and it was used to analyze the forming mechanism and notch size dependence of critical root radius. Upon on the above systematical analysis for critical root radius, the second model was presented, based on crack closure concept, to quantitatively predict the notch fatigue limit for various notched specimens. Its effectiveness and capability were demonstrated by fatigue experimental data from ten kinds of materials under different stress ratios.