Method for calculating fatigue test loads of large-scale wind turbine blades considering pre-bending and geometric nonlinearity

The increasing size of wind turbine blades has intensified the influence of pre-bending and geometric nonlinearity in fatigue testing, where traditional methods neglect their coupling effects and lead to significant load calculation deviations. This study develops a calculation method that considers both the static deformation caused by pre-bending and the softening effect induced by geometric nonlinearity. The method establishes a multi-degree-of-freedom system model and determines test loads through combined static equilibrium and dynamic response analyses. Validation with a 90-meter blade shows that the method reduces the first flapwise natural frequency calculation error from 6.1% to 0.3%, and achieves high accuracy in predicting test bending moments with errors within ±2% in the highly loaded root region (0-40% span). The method also reveals the variation of stress ratios from −6 at the root to −1 near the tip, capturing the asymmetric loading characteristics in actual testing conditions. These findings establish a theoretical basis for fatigue testing of large-scale wind turbine blades and improve testing efficiency by reducing empirical adjustments.