# Experimental and Numerical Study of Vacuum Resin Infusion for Thin-Walled Composite Parts
## 1 Introduction
The construction of the resin flow theory in these works always begins with [PERSON]'s law, which determines the dependence of the superficial resin velocity on the pressure gradient in the liquid phase, on the porous media permeability tensor and the resin viscosity. Some works, for example [42], use [PERSON]'s correction to [PERSON]'s law to take into account the pressure drop due to shear stresses at the interface liquid--porous region. Heat transfer and thermal-kinetic processes affecting the resin rheology have been studied in a number of works [6; 13; 19; 20; 21; 43; 44; 45] to reduce the filling time of the preform and eliminate the dry spots. The influence of process parameters, in particular heating rates and holding times, on the resin viscosity and laminate infiltration was evaluated during experimental tests and confirmed by monitoring the process. There are also a number of works (e.g., [3; 36]) focused on the construction of analytical models of infusion processes. Despite the relative simplicity and impossibility of using analytical approaches for modeling the real structures, they can be used to test more complex models on simple systems, as well as in experimental evaluations of the properties of preforms and resins. The work [46] presents the lumped model of VARTM with parameters, developed using the mechanical-electrical analogy. This model was intended to describe the flow of fluid through preform's macropores and the absorption of resin by micropores in fiber bundles. To characterize the double fiber reinforcement, two physical phenomena are used, described in the work by lumped elements: resistance to the flow of liquid resin and the effect of its absorption during infusion. In some works, the problem of process optimization is considered by which, in the broad sense, we mean the determination of the values of controlled parameters (design variables) that ensure the achievement of the best results (objectives). Optimization approaches and technologies can be divided into the following four kinds. The first kind of optimization problem includes the classical investigations of the process output at varying controlled parameters (inputs) using some experimental methods. The paper [28] reports about a microscopic study of the void content at the locations near the inlet as well as near the outlet of the infused preform at the varying independently controlled resin gates and vacuum vents pressures. A minimum filling time and absence of the dry spots were accepted as objectives in the experimental work [6], where design variables were: parameters of the temperature cure cycle (heating rate at the first and second ramps, temperatures and durations during dwell sections). The second kind is based on the numerous computer simulations and their results' analysis to choose better process conditions. [PERSON] [9; 10] optimizes the filling time of preform and tries to eliminate the dry spots by varying the resin gate and vacuum vent diameters, their numbers and locations using FE/CV-based software. Influence of the heating rates and isothermal holds on the uniformity of the temperature and degree of cure fields during resin impregnation were studied in [21] using the FE software ABAQUS. In order to provide a resin flowing profile uniformity and make the infusion time less than resin gel time, the authors of [2; 24] used RTM-Worx software to optimize the manufacturing process of the boat hulls by manual changing injection gate position, length and spacing of minor pipes for parallel and fish-bone arrangements of the pipes. The third kind combines the methodology of the two previous ones: computer simulation and experiment, the results of which are used to set parameters and validate the model of the infusion process. In the paper [38], [PERSON] with co-workers informs about the minimization of the filling time in grooved foam sandwich composites changing the resin sources pattern at the experiments and numerical simulations.