Effects of Main Spar Geometry and Layup Parameters on Bend–Twist Response and Structural Performance Balance in Wind Turbine Blades

Geometric and laminate modifications can increase bending-induced twists in wind turbine blade spars, but the associated changes in deformation, stress, mass, and vibration are not always assessed on a consistent basis. This study therefore compares the structural trade-offs of four established design parameters under a common finite-element framework. An equivalent thin-walled composite box-spar model based on the NREL 5 MW reference blade was developed using a midsurface representation with spanwise-varying cross-sections, zoned layups, and thickness tapering. Spar-cap thickness, shear-web inclination angle, spar-cap off-axis ply angle, and off-axis ply fraction were varied individually under consistent loading and boundary conditions. The results show that spar-cap thickness primarily influences flapwise deflection and structural mass. Shear-web inclination produces the largest twist-to-deflection response index, reaching approximately 1.17–1.21 °/m at ±20°, but also increases tip deflection and peak equivalent stress. Increasing the off-axis ply angle or fraction does not improve this response for the investigated layups. For the C2 configuration, the response index decreases to approximately 22% of the baseline value, accompanied by increased deflection and stress and reduced natural frequencies. These findings reveal parameter-dependent structural trade-offs under the prescribed model and loading conditions, providing a comparative basis for further blade-spar design and validation.