Reliability analysis of bonded joints in wind turbine blades. An experimental and numerical pyramid approach for static and cyclic strength.

Abstract:

The necessity of reducing fossil fuel dependency has become an important issue in the last years. To achieve this goal, the renewable energy production, in particular wind energy generation, has seen a dramatic expansion. However, wind energy systems face a difficult competition with traditional carbon-based energy sources regarding cost competitiveness. Wind turbine systems are being scaled up with increasing size and power. However, new challenges with major complexity are expected to show up. Structural integrity of all components becomes critical, especially for the case of the blades. This growth in size has a direct impact on the loads that have to be withstood; therefore new designs with improved materials are one option. In addition, the wind turbines are installing off-shore. To keep the cost of maintenance and repair within reasonable limits the intrinsic reliability of structural design needs to be guaranteed.

This research focuses explicitly on a particular aspect of structural design which is reported to be very critical in many designs of wind turbine blade wing box structures, namely the connection between the spar web and the spar cap. A pyramid structured approach is developed which links local phenomena of stress transfer and failure in the connection to overall loads on the entire machine.

With the objective of better understanding large wind turbine blade behaviour in different loading conditions, a new methodology is proposed. The aim of this fully automated procedure is to model the relevant aspects of both structural design and load application in a comprehensive way and to propagate the effect of external loads to internal stresses in the critical components. The major advantage of the proposed procedure is that as the wind loads are introduced as a smooth pressure distribution and that undesirable stress concentrations are avoided which is critical when software like Bladed or FAST is used. While a CFD calculation can take hours or days, the iterative procedure described in this work needs a much shorter computation time. Comparison with the results which are obtained with the established procedures using Bladed and FAST shows a good correlation and validates the approach.

Bonded joints in wind turbine blades have particular characteristics including unusually high thicknesses in the adhesive layers and a multi-axial stress state. At the lowest level of the pyramid, an extensive experimental campaign is performed in materials such as pure adhesive and bonded joint specimens which are developed specifically to take into account the particular requirements for application in wind turbine blades. Coupons are subjected both to quasi static and fatigue loads at different conditions. 

In order to take into account variability present in the experimental campaign, a probabilistic approach is used to identify the most appropriate failure criterion. The strength prediction method considers a statistical size effect in the strength of the material by considering not only the magnitude of the stress distributions, but also the volume over which they act. The material strength is modelled using a Weibull statistical function. Both quasi-static and fatigue results are analysed using probabilistic tools, leading to quite fine results.

Following the study of the bonded joint between the spar cap and the shear web, a subcomponent based on this joint is developed. A similar geometrical structure is designed as a C-beam, where the flanges represent the spar cap and the web represents the shear web. Results from a finite element model are compared to experimental data and satisfactory results are obtained.