# Experimental and Numerical Study of Vacuum Resin Infusion for Thin-Walled Composite Parts
## 1 Introduction  
Vacuum-assisted composite fabrication processes are widely used for producing large components with good quality, and widely applied in many industries due to being low cost, time-consuming, simplicity of the required equipment and better ecology conditions at the manufacturing. The technical details and components of the vacuum infusion in composite manufacturing are described in many papers, technical reports, dissertations [1; 2; 3; 4; 5] and others. The history of the vacuum infusion process in the production of composites should be dated back to 1945, when two engineers at Marco Chemical, Inc. [PERSON] and [PERSON] proposed a new method of molding, patented by them in 1950 [3]. Despite the extremely great attention of specialists and scientists, which was manifested in the large number of patents and publications issued, in the development of its various kinds [6], the wide industrial use of the process is constrained by its lack of reliability in terms of quality assurance and the absence of irreparable defects, especially in the manufacture of the critical load-carrying composite structures. Meanwhile, some modifications of the VARTM (Vacuum-Assisted Resin Transfer Molding) processes have been developed to improve its quality, versatility and performance that allowed to significantly expand the scope of the process in the practice of composites manufacturing. Among the developed contrivances, which were able to overcome the difficulties associated with low permeability of the preforms, are a high permeability (HPM) and flow distribution media [7; 8], air-permeable membrane [7], use of multiple injection lines and vents with independent control of resin and vacuum pressure [2; 7; 9; 10; 11; 12; 13; 14]. A distributed thermal control sometimes is used. Due to the above-mentioned improvements while ensuring a low degree of the resin cure at the end of the preform filling [15; 16; 17; 18; 19; 20; 21] they were successfully implemented in the manufacture of aircraft structures (usually shell type) [1; 22; 23], blades of the wind turbines [1; 20; 24; 25; 26; 27], ship's hulls [1; 2; 20; 24; 25], details of spacecraft [1; 28], large-sized parts of cars bodies [29; 30] and in the defense industry [7; 27]. Despite the abundance of completed and increasing number of ongoing studies of the processes, in each case, the developer must solve a number of difficult problems due to the features of vacuum infusion. The most important feature of VARTM is due to the presence of a flexible vacuum bag that covers the upper surface of the infused preform. This causes the compressive effect of atmospheric pressure on the preform, and the result of this action is its compression deformation, which changes according to the location of advancing front of the resin. In the dry region, where the low air pressure \(p_{vac}\) is supplied due to connection to the vacuum vent, the compressive pressure exerting a compression effect on the preform is equal to \(p_{comp}^{void}=p_{atm}-p_{vac}\), whereas in the region filled with liquid resin, the pressure approaching the atmospheric pressure counteracts the external, atmospheric pressure \(p_{comp}^{fill}=p_{atm}-p_{res}\)[6]. This phenomenon, which significantly changes the permeability of the preform for the moving liquid resin, can contribute to the creation of incompletely saturated zones and dry spots [7; 12; 24; 28; 30; 31]. Another reason for the preform compaction is a wetting compaction, which can arise due to the lubricant effect of the resin when it wets the fibers [32]. This compaction accompanies the moving front of the resin, further worsening the conditions of its propagation, and also causes the hysteresis behavior of the preform when it is unloaded from external pressure. The presence of differently oriented layers in laminated preform leads to the fact that the impregnation of the layers depends on their permeability in the direction of the resin front propagation, as well as leads to the layers deformation that disrupts their packing [5; 16].