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Wind turbine rotor blades are subject to highly dynamic loads and designed for life cycles of at least 20 years, which means that materials are subjected to high-cycle fatigue. Fatigue is a design-driving loading for current and future blades. Bond lines of blades are exposed to a multi-axial stress-state due to the anisotropic thin-walled blade structure and curved, tapered, twisted, and airfoil-shaped blade geometry. To eliminate undesirable failure modes and thus increase the reliability of wind turbine rotor blades, standards and guidelines recommend that the multi-axial stress-states be taken into consideration for the limit state analysis. In addition, thermal residual stresses that develop during manufacture can have a significant impact on the fatigue life of the bond line. By means of a cyclic full-scale blade test of a commercial 81.6m long offshore blade, we validate a crack initiation model, which takes into account multi-axial thermal and mechanical stress-states, as well as the probabilistic stress-life, to predict the edge of crack initiation in the adhesive as well as the span-wise position. Both observations agreed well with the simulations. All residual normal stress components and cross-sectional plane shear stress made up the major part of the mean equivalent stress, while the mechanical stress amplitude components - longitudinal, peel, and cross-sectional plane shear stress - made up the major part of the equivalent stress amplitude.

Thermal residual stresses have a major impact on the bond line fatigue of wind turbine blades, which can initiate tunneling cracks in the adhesive layer of the bond lines early in the operational life of the blade. This work investigates the simulation accuracy for predicting thermal residual stresses within a thick bond line. The trailing-edge bond line strip of a 34 m blade was modeled with classical laminated plate theory (CLT) on the one hand and with finite element (FE) plate models of different fidelities on the other. For the model benchmark, the thermal residual stresses were on the basis of a thermal simulation. These develop during the cooling after a typical curing cycle of a wind turbine blade manufacturing process. It was found that the analytical model on the basis of CLT was in good agreement with the plate models of higher fidelity. Additionally, a full 3D FE blade model was used to calculate the shape distortion and the thermal residual stresses. It was found that the analytical model, which did not take into account effects stemming from the whole blade structure, underestimated the full 3D FE model.

to initiate your request. See also AIAA Rights and Permissions www.aiaa.org/ randp. *Technical Head, Department of Rotor Blades, Am Seedeich 45.

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