۱- Introduction

۲- Calculation methods

۳- Comparison of the two approaches

۴- Conclusion

References

Abstract

Fatigue design optimization of composite material structures is limited by the classical approaches used, derived from knowledge based on metal fatigue. Other approaches exist to describe damage mechanisms of composites but they cannot always be applied at the structure scale because of their complexity. However, assumptions can be made in the case of beam structures to reduce the structural investigation at the section scale. With these assumptions this paper proposes to compare a progressive fatigue damage model written at the ply scale to the normative approach for the assessment of wind turbine blade section design. It is shown that the normative approach is very conservative and the progressive fatigue damage model provides very useful information to understand damage propagation at the structure scale.

Introduction

At the present time, the wind power industry is facing the double challenge of further increasing the size of turbines for off-shore production and making them lighter for land-based production, so that they can be adapted for use in less windy areas. The main stumbling block has been identified in the hope that the design of the blades can be optimised to meet these challenges, and it concerns the precision of their design under fatigue. A wind turbine blade is generally designed to be in rotational operation for 20 years in wind fields that by their very nature are variable. This creates structures that are highly subject to fatigue. We are talking about 108 cycles approximately, which makes fatigue over a long lifetime the main design criterion [1]. In addition to high resistance to fatigue, the material used for the blades should have a low mass, which has a direct impact on stress, sufficient stiffness so that it does not impair the aerodynamic properties of the turbine, and a cost that makes wind energy competitive. With all these elements combined, it soon became clear that glass fibre non-crimp fabrics (NCF), with possibly carbon fibres locally with polyester or epoxy matrices, were required for medium and large sized blades. Although long fibre composites have high resistance to fatigue, there is as yet no commonly accepted design method devoted specifically to these materials and the normative approach currently used in the context of blade certification is based on a transposition of existing knowledge of the fatigue behaviour of metals [2]. In this study, the normative approach is compared with a progressive fatigue damage model written at the ply scale.