## 1 Introduction
would protect, at least temporarily, the blade from the damaging effects of environmental particles, such as water, dust, sand, or hail pellets. According to [PERSON] et al. [13], erosion in surface texture begins earlier in hyperelastic coatings than in viscoelastic coatings. [PERSON] et al. and many other researchers agree that flexible coating represents better protection against wear due to its better damping properties of impact. However, flexible coatings may present a higher risk of internal defects, favouring debonding [13; 14]. Some innovative coatings are being developed and investigated according to [PERSON] et al. [2], such as metallic and metallic-polymer coatings, viscoelastic polyurethane coatings and multilayer hybrid polyurethane protections. Some repairing technologies have also been developed, such as the low viscosity resin injections, or non-destructive inspection techniques if the damage is not visible [15]. [PERSON] et al. [16] quantify the economic potential of the safe-erosion method, an operational technique for mitigating erosion by significantly reducing the rotational speed of wind turbines under extreme environmental conditions. Additionally, [PERSON] et al. divide the coating into soft and hard layers, helping to reduce erosion. It has been discovered that the inclusion of a primer between the LEP and the filler has improved adhesion between the layers of the wind turbine blade [17]. Finally, innovative manufacturing techniques, such as vacuum-assisted resin transfer moulding or 3D woven composite manufacturing, are being thoroughly investigated to ensure better quality standards and fewer internal defects in the production process [18]. The consideration of stress history and the impact of stress waves are crucial criteria to understand how stress waves affect fatigue damage. In order to analyse and evaluate the positive and negative aspects of a given protection system, we will consider in this work the propagation of the stresses caused by the impacts through the materials which compose the protection system. The stress history and the criteria to consider how the stress waves affect fatigue damage are based on the coating capability to reduce or enhance the surface and interface stress developments under impact. When a rain droplet impacts the blade's surface, its kinetic energy changes suddenly as the droplet decelerates. The rapid deceleration and compression of the liquid droplet create a high-pressure shock zone at the impact location. The shock wave propagates within the liquid and into the leading edge protection coating, thus influencing the compressive stress development in the coating. The droplet impact generates high pressure, which is known as water hammer pressure. This leads to erosion in the leading edge blade surface. When a droplet hits the surface, compressed normal wave and transverse shear wave are developed, as shown in Figure 2, along with a third Rayleigh wave along the surface. [PERSON] et al. [19] explain that a compressive shock wave travels through the blade material each time a droplet hits its surface. Several researchers, including [PERSON] et al. [20], have explored numerical models to predict peak stress evolution with time, even though studies from [PERSON] et al. [21] indicate that creating high-fidelity models remains rare and very complex. New modelling methods describing the computational liquid drop impact are tackled in many articles; they mainly compare the Lagrangian method, the Coupled Eulerian-Lagrangian method and the smoothed particle hydrodynamics (SPH). The Lagrangian Method is a standard finite element technique which uses standard Lagrangian meshing to describe the deformation of the bodies. However, this technique is limited to models where large deformation is unlikely [24]. SPH is a meshless method of impact modelling where numerous small particles are attached to a mass. According to [PERSON] et al.