A qualitative analytical investigation of geometrically nonlinear effects in wind turbine blade cross sections

• The Brazier effect induces stress intensity factors in curved, thin-walled beams under biaxial bending which can exceed the fatigue threshold.
• A second order bend-twist coupling was found to be a vital part of the solution.
• An increase of the camber of airfoils decreases the fatigue lifetime of the trailing edge joint.
• The proposed slice approach facilitates computationally efficient fracture analysis of wind turbine blade cross-sections.

This paper analytically investigates the Brazier effect on asymmetric thin-walled sections subject to biaxial bending. In the latter case a torsional moment – in this paper referred to as Brazier torsion – is induced, which proved to be a vital part of the solution. By means of a generic cross section, that was inspired by a wind turbine blade, it is demonstrated that geometric nonlinear effects can induce an in-plane opening deformation in re-entrant corners that may decrease the fatigue life. The opening effect induces Mode-I stress intensity factors which exceed the threshold for fatigue crack growth at loads well below the load-carrying capacity of the beam.The findings in this paper are twofold: Firstly, the investigated analysis procedure can be integrated into the design process of wind turbine blade cross sections. Secondly, the proposed approach serves as a basis for computationally efficient numerical analysis approaches of structures that comprise complex geometry and anisotropic material behaviour – such as wind turbine rotor blades.