Fatigue Strength Analysis of Wind Turbine Main Shaft Considering Surface Topography
Surface topography critically influences fatigue crack initiation in wind turbine main shafts, yet DNVGL certification relies on empirical roughness corrections that neglect full geometric features. This study reconstructs measured surface topography via harmonic superposition, derives analytical expressions for stress concentration factor and fatigue notch factor, and modifies the empirical terms in DNVGL S-N curves. A 25 µm ten-point height roughness (Rz) is applied to a finite element model of an external rotor generator main shaft. Cumulative fatigue damage is computed using FKM mean stress correction, multiaxial critical plane method, and rainflow counting. Under identical Rz, the proposed method reveals that fatigue damage decreases with increasing surface topography wavelength, whereas DNVGL yields invariant damage. The maximum damage of 0.378 occurs at the inner hole edge, below the critical value of 1.000, confirming design compliance. The results demonstrate that single roughness parameters are insufficient for quantitative fatigue assessment, and full surface morphology must be incorporated to avoid over- or under-conservative designs.