TORSIONAL PERFORMANCE OF WIND TURBINE BLADES-EXPERIMENTAL INVESTIGATION
The complete 3D static responses of two different eight meter long wind turbine blade sections were tested. To experimentally investigate the 3D response, an advanced 3D digital optical deformation measuring system (ARAMIS 2M and 4M) was applied in this work. This system measures the full-field displacements (ux, uy and uz ) of the blade surface. A least squares algorithm was developed, which fits a plane through each deformed cross section, and defines a single set of displacements and rotations (three displacements and rotations) per cross section. This least squares algorithm was also used to accommodate problems with a flexible boundary condition by determining the displacements and rotations for a cross section near the boundary. These displacements and rotations are subtracted from all other cross sections along the blade and thereby making the blade section fully fixed at the chosen cross section near the boundary.
Modern wind turbine blades are constructed using a combination of different materials. Typically glass fiber reinforced plastic is used for most of the structure, with most of the fibers in the longitudinal direction to limit tip deflections. As wind turbines increase in size the torsional eigenfrequency becomes lower and the torsional mode may couple with some of the lower bending modes. This can lead to catastrophic collapse due to the flutter instability. For larger wind turbines it therefore becomes gradually more important to be able to make reliable prediction of the torsional behavior of the blade and to calculate any structural couplings that may exist, such as the bend-twist coupling. However, correct modeling of torsional stiffness and bend-twist couplings based on FEM is subjected to some uncertainties, when numerical results are compared with experimental modal analyses of blades. In  the response of a shell finite element (FE) model was compared with a number of measured modal modes and the correlation related to torsional response was limited, including the 1 st torsional mode and especially for the higher modes. To investigate these uncertainties regarding the torsional stiffness and the bend-twist couplings, a number of static tests applying different load cases have been performed on a section of a fullscale wind turbine blade provided by Vestas Wind Systems A/S. Fig. 1. A flapwise bending test performed on a wind turbine blade section. This blade section was furthermore modified by adding some additional angled UD layers on the suction and pressure side of the blade. These UD layers introduce measurable bend-twist couplings, which the original blade did not have. The primary aim of this experimental testing was to validate a FE-model design of the two blade sections (the original and modified). The