Computation modelling of repetitive raindrop impact and predicting the ensuing damage in the coating and substrate due to fatigue is an essential step in determining the life of the wind turbine blade. The blade is multi-layered with a viscoelastic coating layer, a viscoelastic elastic putty layer and an elastic composite substrate with an interface between the layers. The damage usually takes the form of erosion of the coating and debonding at the various interfaces between the layers of the wind turbine blade.

The modelling of impact and damage involves a number of steps beginning with generating a stochastic rain scenario which provides information on raindrop size distribution and velocity hitting a volume of the blade. A coupled eulerian-lagrangian (CEL) analysis of a single raindrop and a representative region of the blade provides impact forces on and resulting stresses in the blade. The CEL analysis is repeated for drops of different diameters to generate a library of stresses. This library, combined with a stochastic rain scenario, generates stresses at any location of the representative blade due to the repetitive impact of different-sized raindrops hitting arbitrary locations. Continuum damage mechanics is used to predict the damage evolution in the coating, and when damage reaches a critical value, the element is deleted to represent the erosion of the coating. A cohesive zone model for fatigue debonding is employed for interfaces between coating, putty, and composite substrate. Both the damage evolution and a Cohesive zone model for fatigue are implemented through a user-defined subroutine in ABAQUS/Explicit.

Using the model, the interaction between erosion and debonding is studied. Using the properties of materials from the literature, the analysis shows that damage always initiates from the bottom portion of the coating layer close to the adhesive interface. This eventually creates displacement jumps at the interface of coating and putty, causing damage initiation at the interfaces. Repetitive raindrop impact of different sizes at arbitrary locations causes damage accumulation and propagation faster at the interface, finally damaging the coating layer.