## 2 Numerical Modelling Techniques  
Finite Element (FE) models are developed in Abaqus using explicit analysis to simulate the single rain droplet impact event. In this modelling, LEP, filler and glass fibre reinforced plastic (GFRP) substrate, and an aluminium shim, at the base, are considered. The rain droplet velocity is applied at multiple droplet nodes. The droplet and plate are modelled with tetrahedral element C3D4 and hexahedral elements C3D8R, respectively. The 3D mesh element is considered to capture the normal and shear stress near the impact zone. The water droplet impacts the plate perpendicularly, with a 90\({}^{\circ}\) angle, in the model where the pressure exerted is at its highest [29]. This is because when a droplet hits at a normal angle, it generates the highest peak pressure due to momentum transfer on the impacted surface. This leads to a sudden pressure rise due to water hammer pressure. The model is constrained on each side to avoid unrealistic lateral deformation. The greater the thickness of these layers, the more the substrate will be protected from each impact. The FE model represents a 27 mm diameter test coupon, which is standard for RET (rain erosion testing) at the University of Limerick [30]. While each layer of aluminium and filler measures 1 mm thick, each layer of GRFP+45, GRFP\(-\)45, and LEP has a thickness of 0.5 mm, as depicted in Figure 3. The LEP has been described as a surface layer made with rigid epoxy, allowing a pure elastic stress behaviour. The model is initialised with an impact velocity of 100 m/s, no gravity, a friction coefficient of 0.2, a tie type of contact between layers, and a droplet diameter of 2 mm. The mesh distortion is ruled by the ALE, and the surface is defined as flat with no previous damage. The material properties used in this model are presented in the Tables 1 and 2 and were inspired by the work of [PERSON] et al. and [PERSON] et al. [13; 28]. Using similar properties allows a decent comparison with other researchers in the same domain. The water droplet properties have been defined as follows: density \(\rho_{w}=1000\) kg/m\({}^{3}\), speed of sound \(C_{0}=1450\) m/s. Regarding the different layers of the blade, all the properties are listed in the following tables. A mesh convergence study (MCS) is a vital process in FEA models to ensure that simulation results are independent of mesh size and are precise, accurate, and reliable. \begin{table}
\begin{tabular}{c c c c} \hline \multirow{2}{*}{**Layers**} & \multicolumn{3}{c}{**Properties**} \\ \cline{2-4}  & **Density \(\rho\) (kg/m\({}^{3}\))** & **Young’s Modulus \(E\) (GPa)** & **Poisson’s Ratio \(\
u\)** \\ \hline LEP (Gelcoat Epon E862 Epoxy) & 1150 & 2.5 & 0.4 \\ \hline Filler & 1452.8 & 8 & 0.3 \\ \hline Aluminium (Al) & 2820 & 70 & 0.3 \\ \hline \end{tabular}
\end{table} 
Table 1: Isotropic materials layers’ properties [13; 28]. Figure 3: Schematic of the material composition of the FEA Abaqus model. \begin{table}
\begin{tabular}{c c} \hline \multicolumn{2}{c}{**Properties GFRP**} \\ \hline Density \(\rho\) (kg/m\({}^{3}\)) & 1864 \\ \hline Young’s modulus \(E_{11}\) (GPa) & 44.87 \\ \hline Young’s modulus \(E_{22}\) (GPa) & 12.13 \\ \hline Young’s modulus \(E_{33}\) (GPa) & 12.13 \\ \hline Poisson’s ratio \(\
u_{12}\) & 0.3 \\ \hline Poisson’s ratio \(\
u_{13}\) & 0.3 \\ \hline Poisson’s ratio \(\
u_{23}\) & 0.5 \\ \hline Shear modulus \(G_{12}\) (GPa) & 3.38 \\ \hline Shear modulus \(G_{13}\) (GPa) & 3.38 \\ \hline Shear modulus \(G_{23}\) (GPa) & 3.38 \\ \hline \end{tabular}
\end{table} 
Table 2: Anisotropic materials layers’ properties GFRP [28]. For the plate, the mesh refinement process ranges from 0.02 mm to 0.00008 mm, with convergence observed at a mesh size of 0.0001 mm, which optimises the computational time while ensuring result accuracy. Similarly, mesh refinement is applied to the droplet, while the plate mesh size is fixed to the previously obtained value of 0.0001 mm. The CPRESS is generated since neither von Mises nor S33 stresses are applicable to liquid materials.