Co-authors​ :

 Alexander SEIDEL (GERMANY), Bent F. SØRENSEN (DENMARK), Dr.Klaus DRECHSLER (GERMANY) 

Composite structures are used for a multitude of applications where fatigue resistance is a primary design criterion. Wind turbine rotor blades, especially, have to withstand tens to hundreds of millions of cycles during their lifetime. The design of such structures is usually carried out using empirical design approaches where macroscopic properties have to be determined experimentally on the coupon level. Besides being time- and cost-intensive, these methods can underestimate the actual fatigue performance of the composite material as side effects (like failure initiating in the tab region) lead to a premature failure. This work introduces a combined analytical-numerical modelling approach on the microscale to better understand the development of fatigue damage zones in unidirectional fibre-reinforced plastics (UD-FRPs) and investigates the influence of different material characteristics (e.g. inter-fibre distance or interface properties) on their fatigue behaviour. Conceptually, the model is based on the observations made by Jespersen and Mikkelsen [1], showing that fatigue in unidirectional composites manifests as a progression of adjacent fibre breaks. The underlying mechanism has been modelled previously by Sørensen et al. [2], utilising a micromechanical analytical approach. It shall only be introduced briefly: An initial fibre break leads to a debond crack between a fibre and its surrounding matrix under tensile loading. The debond crack tips at either end of the debond crack create a stress concentration field (as observed in e.g. van den Heuvel, Peijs and Young [3]). As the debond crack grows under cyclic loading, the stress concentration field moves along with the crack tip until eventually reaching a defect in a neighbouring fibre. This causes the adjacent fibre to break and the fatigue damage zone to grow. The methodology of this work combines the analytical model by Sørensen et al. to describe the debond crack initiation and growth with a numerical finite element model to calculate resulting stresses in the discrete fibres, the matrix and the intact interface regions. As the material behaviour in the debonded interface region varies spatially and cyclically due to wearing effects, an adapted cohesive material model is additionally introduced. The proposed methodology allows for overcoming several limitations of the purely analytical model (elastic to plastic matrix behaviour, hexagonal fibre packing to statistical fibre distribution, spatially constant to varying material properties). A central aspect of this work is the utilisation of the methodology to study the influence of the aforementioned material characteristics on the composites’ fatigue behaviour.