Verification of Structural Properties for Bend-Twist Coupled Wind Turbine Blades

Bend-twist coupling (BTC) for wind turbine blades has been extensively researched with a focus on passive load alleviation, and many advantages are presented in literature. An important step towards integration of this new technology into structural blade design is to experimentally verify it. The structural properties of blades are calculated using numerical models and verified through tests. The main goal of this thesis is to evaluate the methods used for testing. Hence, the work presented in this thesis is positioned in the interface between design and testing of wind turbine blades.

To gain understanding about the verification of structural properties for bend-twist coupled wind turbine blades, a common terminology used throughout the thesis is defined first. Then, the tools used in the thesis are reviewed briefly, followed by an extensive literature study on structural properties, bend-twist couplings, laboratory testing and prediction methods. After which a review is included, to explain how bend-twist coupling can be achieved with material bend-twist coupling or geometric bend-twist coupling, or a combination of both. Material bend-twist coupling is accomplished by rotation of the main fibre direction, which would be otherwise aligned with the blade span.

Having established the theoretical basis for the thesis, the impact of material bend-twist coupling on shear centre, flexural centre, dynamic properties and stiffness for simple BTC structures is presented. The methods for measuring the shear centre and the flexural centre for material bend-twist coupled specimens are demonstrated experimentally on 1 m beams. The 8 m long beam is used to demonstrate the method for identifying material bend-twist coupling using mode shapes. The 8 m long, prismatic beam is material bend-twist coupled and has an airfoil cross-section. The method for measuring bend-twist coupling stiffness with pure torque and bending moment is demonstrated on that 8 m coupled beam. Conducting the pure bending moment test on the same 8 m beam is a novelty in research. Chapter 6, sections 6.5.2.3 and 6.6.3 present and discuss the novel method conducted in this thesis for testing bending stiffness and bend-twist coupling stiffness. A follow up test of an uncoupled version of the 8 m beam was set up and carried out to gather knowledge about the coupled specimen. A large dataset was generated. The data can be used for further development to verify structural property testing methods in uncoupled specimens. The project revealed uncertainties related to torsional stiffness of the 8 m beams, and torsional stiffness of large, compliant blades in general, which inspired the subsequent study on the torsional stiffness of a blade. The test method which uses combined loading to determine torsional stiffness is evaluated on a 14.3 m wind turbine blade.

The experimental campaigns performed in this PhD project, together with the in-depth analysis of the structural input properties, lead to a holistic understanding of bend-twist coupling, and its effect on structural properties. To round up the thesis, general conclusions are drawn, the research questions are answered and suggestions for future work are given.