Abstract. The European Academy of Wind Energy (eawe), representing universities and institutes with a significant wind energy programme in 14 countries, has discussed the long-term research challenges in wind energy. In contrast to research agendas addressing short-to medium-term research activities, this eawe document takes a longer-term perspective, addressing the scientific knowledge base that is required to develop wind energy beyond the applications of today and tomorrow. In other words, this long-term research agenda is driven by problems and curiosity, addressing basic research and fundamental knowledge in 11 research areas, ranging from physics and design to environmental and societal aspects. Because of the very nature of this initiative, this document does not intend to be permanent or complete. It shows the vision of the experts of the eawe, but other views may be possible. We sincerely hope that it will spur an even more intensive discussion worldwide within the wind energy community.

In this work, a glass/epoxy material system applied in wind turbine blades was used to evaluate degradation processes induced by water ingression. Composite and neat epoxy specimens were conditioned in demineralised water at 50 • C for 4800h and tested quasi-statically and in fatigue. Comparing results from mechanical tests in composite specimens, significant degradation was found, with up to 36% lower static shear strength and three orders of magnitude shorter fatigue life. For neat epoxy specimens, a lower degree of degradation was observed, with up to 17% lower tensile and bending moduli and strength. Specimens dried after having been immersed were also tested. For composite samples, recovery of shear stiffness and strength was incomplete. For neat resin, stiffness and bending strength were completely recovered but a decrease in the strain at failure was observed. It is hypothesised from differences in magnitude and reversibility of degradation between composite and neat resin that matrix degradation is accompanied by high differential swelling stresses and damage to the fibre/matrix interface in composites. The damage due to moisture ingression and the subsequent changes in failure behaviour are further investigated through thermal analysis (DSC, DMA) and optical microscopy.

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The reliability of rotor blades is the pre‐condition for the development and wide use of large wind turbines. In order to accurately predict and improve the wind turbine blade behavior, three main aspects of the reliability and strength of rotor blades were considered: (i) development of methods for the experimental determination of reliable material properties used in the design of wind turbine blades and experimental validation of design models, (ii) development of predictive models for the life prediction, prediction of residual strength and failure probability of the blades and (iii) analysis of the effect of the microstructure of wind turbine blade composites on their strength and ways of microstructural optimization of the materials. By testing reference coupons, the effect of testing parameters (temperature and frequency) on the lifetime of blade composites was investigated, and the input data for advanced design of wind turbine blades were collected. For assessing the residual strength and stiffness of wind turbine blades subjected to irregular cyclic loads, a shell‐based finite element numerical methodology was developed, taking into account the non‐linear response of plies, and experimentally validated. Two methods of structural reliability estimation of the blade, which take into account the stochastic nature of the anisotropic material properties and loads, were developed on the basis of the response surface method and the Edgeworth expansion technique, respectively. The effects of fiber clustering, misalignments, interface properties and other factors on the strength and lifetime of the wind turbine blade materials were investigated in the micromechanical finite element simulations. The results described in this paper stem from the Rotor Structure and Materials task of the UPWIND project. Copyright © 2011 John Wiley & Sons, Ltd.

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