Lightweight wind turbine blades play a crucial role in improving wind turbine technol- ogy. They enforce lower loads on the overall system, making material and cost sav- ings possible. Also, increasingly longer blades can be used, enabling more efficient, economically viable turbines. This thesis investigates how the mass of a rotor blade can be reduced without compromising its structural integrity. The work is based on the OLW934 blade from Olsen Wings A/S. Its mass is reduced using a comprehensive optimization framework .

A structural and aeroelastic optimization process was applied using the gradient-based AESOpt algorithm, finite element analysis (FEM), and aeroelastic checks with HAWC2S. The framework reduced the blade’s mass by 35% while keeping its structural integrity. The redesigned blade was found with improved internal structure and layup. Since the optimization mainly focused on the structure, the aeroelastic performance was checked manually throughout the process. Due to stiffness assumptions in the tip re- gion, no definite validation on the aeroelastic stability of the turbine with the redesigned blade has been found yet, but the results indicate sufficiency.

This behaviour should be verified. Also, the internal structure setup, especially the spar caps and shear webs, should be studied in more detail, as only initial design con- siderations were explored. Additionally, varying initial layup regions and evaluating alternative material options could yield further improvements. Still, the results show that the structural optimization framework used can significantly reduce blade mass and can be applied to other designs. This supports the development of lighter blades and better turbine performance.

This thesis explores the structural implications of utilising natural fibre-reinforced polymer composites (NFRCs) in wind turbine blades through a numerical investigation. To perform the research, the DTU 12.6 m research wind turbine blade is taken as case study. A sensitivity study is performed, investigating the impact of different material engineering constants on the blade's performance. A structural redesign of the DTU 12.6m blade with NFRCs followed to further investigate the implications of utilising bio-based materials. The findings reveal that the reduced mechanical properties of NFRCs restrict the direct replacement of traditional glass fibre composites. The structural redesign process showed that the torsional rigidity and material strengths are the primary drivers for the bio-based blade design. This differs from the conventional blade design where the tip deflection and therefore specific stiffnesses are the main drivers. In order to meet the design requirements the structural blade design will need to be rethought.

The trailing edge of wind turbine blades are commonly manufactured as an adhesive joint of the pressure-side and suction-side composite panels of the blade. Under some conditions, a lead-to-trailing (LTT) edgewise bending moment can induce buckling at the trailing edge adhesive joint, which may lead to early failure of the blade due to delamination. As a structural instability, buckling in wind turbines has been the focus of much research especially in full-scale tests and more recently at the sub-component level. These higher-level tests, however, are done on pre-manufactured wind turbine blades and require extensive preparation in order to adapt the testing rig to each blade section, as well as incurring into elevated costs.
An additional test level has been suggested for elements and details of wind turbine blades. It has been suggested that this level can fulfill many purposes: New concepts, modifications, material combinations and orientations can be tested, partial safety factors of larger scale tests can be reduced or even certifying minor details of the blade can be done at the element and detail level. As such, the focus of this project is to develop a testing method for a simplified trailing edge bonded joint with a custom designed hinged clamping system upon which a compressive moment can be imposed to induce buckling.
The design of this test will initially be based on a semi-analytical buckling plate model, where in-plane and out-of-plane displacements are coupled through the Von Karman strain-displacement relations. This semi-analytical tool is employed to quickly estimate the buckling loads for plates of varying dimensions. Strain-free imperfections can be included in the model for twisted/pre-bent plates in order to estimate their effect on reducing the load-bearing capacity of the structure. The semi-analytical tool is complemented with FE models for all the design parameters.
The semi-analytical and numerical results are compared to demonstrate the agreement of both approaches aimed to provide a sturdy base for the research. Next, the experimental buckling loads and force-displacement curves are shown against the predictions from the previous approaches with good agreement. Nevertheless, the observable discrepancies between the experimental and numerical results showed that the desired joint-fixity at the boundaries was not fully realized, therefore leading to a slightly different post-buckling behavior. In the end, suggestions are given to improve on the experimental clamping system in order to improve and expand the scope of this research.