Fatigue life prediction of notched specimens using the critical distance based on S–N curve evolution

Purpose This study aims to develop a robust notch fatigue life prediction model for hydro turbine components, specifically addressing fatigue failures in welded joints induced by severe stress concentrations at transition fillets. The research focuses on overcoming the inherent limitations of the conventional theory of critical distance (TCD) in the finite life regime and its heavy dependency on empirical fitting parameters. Design/methodology/approach An enhanced TCD framework is proposed by incorporating the evolutionary characteristics of the baseline S–N curve to formulate a novel expression for the characteristic length L. A stress gradient threshold is introduced to physically define the effective damage zone, and the effective stress is refined using a root-mean-square formulation. Additionally, a path resampling strategy is implemented to optimize finite element data extraction and eliminate numerical errors associated with mesh sensitivity. The model is validated using experimental results from 04Cr13Ni5Mo/ER316 L welded specimens. Findings Results demonstrate that predicted fatigue lives exhibit strong agreement with experimental observations, significantly outperforming the conventional line method in accuracy and robustness. The proposed model effectively captures the accelerated damage at notch roots without requiring additional empirical parameter calibration. It achieves high predictive precision comparable to extensively calibrated mainstream frameworks. Originality/value The study establishes a novel mathematical formulation for critical distance derived from S–N curve evolution, enabling the autonomous determination of L and eliminating empirical fitting requirements. This provides a promising and efficient theoretical framework for the structural health assessment of hydroelectric infrastructures.