# Experimental and Numerical Study of Vacuum Resin Infusion for Thin-Walled Composite Parts
## 4 FE Simulation of the Vacuum Infusion Process: Results and their Analysis  
The study on the capabilities of the developed approach was carried out using CAD models of a group of rotorcraft composite structures with a geometry like thin-walled shells of complex shapes (see Figure 12). The original CAD models were adjusted to correct topological errors (poor joining of the whole surface's patches), which prevent high-quality finite element meshing, as well as some local simplifications to exclude mesh thickening in areas with increased curvature. This part of the work is very important because violation of C\({}^{0}\) or C\({}^{1}\) continuity of the surface and sharply changed FE meshing can cause front distortion and a change in the direction of its spreading. The remainder of this article presents and discusses the modeling of the infusion process in the last cowling shown in Figure 13. The system modeling was implemented in Comsol Multiphysics environment, where the phase field in Equations (6) and (7), which are built-in simulation modes, were accepted as a ruling equation coupled with the modified Richards equation (Equation (14)), which was built in the coefficient form PDE mode. FE mesh and resin gate with vacuum vent locations are present in Figure 13. All boundary conditions and the numerical value of the model parameters correspond to the above. The HPM tape 5 cm wide was laid around the part's perimeter, which was taken into account at the HPM locations by increasing the preform permeability. Figure 12: A group of the composite rotorcraft cowlings intended for manufacture by vacuum infusion technology. Figure 13: The finite element meshing of the infused part geometry (**a**); resin gate and vacuum vent location (**b**). Some snapshots of the resin volume fraction and driving pressure distribution in the preform during model simulation are shown in Figure 14. The left column of this Figure shows the plots of the resin pressure distribution in the preform combined with the lines of two \(V_{r}\) levels: 0.1 and 0.9, whereas the right column contains the resin filling \(V_{r}\) distribution plots at the different time instants. Several important features of the simulation can be identified when considering these images. Figure 14: Cont. The use of HPM tapes located around the perimeter of the infusing preform can lead to a significant local expansion of the front of the resin (see time 600 s) and even to the formation of low pressure and saturation areas inside the resin flow front (see time 1400 s). With the closure of some internal areas by an inner front, the filling of these areas with a resin gradually increases, although the nature of the pressure distribution inside such partially filled areas is not violated (see times 2800 s, 3600 s, 6000 s). It can be explained by the real diffusion of the resin as well as by the mobility \(\gamma\), which determines the time scale of diffusion in the Cahn-Hilliard equation (Equation (4)). Estimation of the actual  
Figure 14: Some snapshots of the driving pressure combined with the level lines of the resin volume fraction at \(V_{r}\) = 0.1 (green lines) and \(V_{r}\) = 0.9 (red lines) (left column) and resin volume fraction (right column). magnitude of this mobility presents significant difficulties, but the occurrence of such closed contracting areas indicates the possibility of the formation of dry spots. A similar phenomenon is observed when the outer front approaches the protruding corners of the preform contour (see times 600 s and 2800 s). This slow mixing of two phases on the surface of their contact is inherent for all interacting systems. In real infusion processes, such mixing is constrained by an increase in air pressure inside the contracting empty area and a gradual increase in the viscosity of the resin over time under the influence of temperature due to polymerization.