Investigation of Structural Behavior due to Bend-Twist Couplings in Wind Turbine Blades

One of the problematic issues concerning the design of future large composite wind turbine blades is the prediction of bend-twist couplings and torsion behaviour. The current work is a continuation of a previous work [1,2], and it examines different finite element modelling approaches for predicting the torsional response of the wind turbine blades with built-in bend-twist couplings.

Additionally, a number of improved full-scale tests using an advanced bi-axial servo-hydraulic load control have been performed on a wind turbine blade section provided by Vestas Wind Systems A/S. In the present work attention was aimed specifically at shell element based FEA models for predicting torsional behaviour of the blade.

Three models were developed in different codes: An ANSYS and ABAQUS model with standard section input and an ANSYS model with matrix input. All models employed the outer surface of the blade cross section as the defining surface, off-setting the location of the shell elements according to the specified thickness.

The experimental full-scale tests were carried out on an 8 m section of a 23 m wind turbine blade with specially implemented bend-twist coupling. The blade was tested under considerably larger load levels compared to earlier tests and showed linear-elastic response during flap-wise bending and combined bending-torsion tests which made it possible to employ the principle of superposition to extract the torsional characteristics of the blade from these tests.

Additionally, pure torsion tests were carried out on the blade employing a more advanced bi-axial servo-hydraulic load application control. The use of shell-solid models for the prediction of torsional response was recommended based on earlier investigations. However as these models in practise are cumbersome to apply in design, the numerical models mentioned above were compared with previous experiments and the new experiments presented in this paper.

Additionally, the models were verified against two older MSC.Nastran models developed in. All shell models performed well for flap-wise bending, but performed poorly in torsion with deviations in the range of 15 to 35%, when employing the section input for the off-set definition. However, the ANSYS model generated using matrix input for the off-set definition was found to perform adequately.