Fatigue Strength Analysis of Wind Turbine Main Shaft Considering Surface Topography

• • The proposed harmonic superposition method yields fatigue damage that varies with surface topography wavelength at constant Rz = 25 µm, whereas DNVGL's empirical formula produces a fixed damage value, exposing a critical limitation in current certification practice. • • Maximum cumulative fatigue damage of 0.378 is located at the main shaft inner hole edge, satisfying the Miner's rule threshold of 1.000, but this margin may be illusory if wavelength effects are ignored. • • The DNVGL surface roughness correction factor Fo = 1 - 0.22 (log Rz / 0.64) · log(σb + 0.45 log Rz / 0.53) fails to capture wavelength-dependent stress concentrations, potentially underestimating damage by up to 30% for long-wavelength topographies. • • The derived fatigue notch factor βk and stress concentration factor Kt enable direct modification of S-N curve slopes m1 = 5.5 / Fok² and m2 = 2m1 - 1, providing a computationally efficient alternative to high-cost fine finite element models.