Structure-related Modeling Of The Permeability Of Fiber Composite Preforms

Polymer matrix composites, which were firstly produced in the 40s last century, are widely used in the areas of aerospace, automobile manufacturing, and other domains due to its high specific strength and stiffness, excellent fatigue resistance, outstanding design ability, easy molding and many other performance advantages. In recent years, as an advanced manufacturing technology for composites, resin transfer molding (RTM) attracts much attention due to its integration, low cost and net forming in the manufacturing process, under the background of national key project "Large aircraft" and the development trend of materials technology "structural and functional properties combination". However, infiltrating defects, such as dry patches and voids, often occur in RTM products and threaten their quality and performance.The formation of the infiltrating defects in RTM products are related to the penetration of the fiber preform. As a physical quantity characterizing the penetration of fiber preform, permeability is an important property parameter which depends on the fiber preform's geometry structure, and an essential key parameter to analyze the resin flow behavior in fiber preform so as to predict the defects'formation and distribution. Therefore, it has important theoretical significance and engineering practical value to investigate the structure-relation of preform permeability and establish mathematical models describing the relation. The models will not only reveal the quantitative relation between structure and permeability, but also can be used to analyze the resin flow behavior in the preform, based on which the production cycle and formation of defects can be predicted to raise productivity and improve quality and performance of composite products. It can be used to direct the design, preparation and using of the preform as well, so the design ability of composites material can be fully exploited.The permeability of non-crimp fabrics (NCF) which has excellent combination properties and wide application in practice, was investigated in the present work. Based on the analysis of NCF's structure, a unit cell model characterizing the NCF's structure was established, and the meso-level resin flow behavior within it was described by coupling the inter-tow and intra-tow flows. A numerical model of NCF's permeability was founded by simulating the resin flow behavior in the unit cell and using the Darcy's law to calculate the equivalent permeability. And then the model was verified and applied to investigate the relation between permeability and the structure of NCF, and to explore the degree of the influence of each structure parameter on permeability by local sensitivity analysis method. The main conclusions are as follows:a) The permeability of NCF increases with the increase of the permeability of fiber bundles, but the influence is very small, so that it can be neglected if the variation of fiber bundles'permeability is small.b) The permeability of NCF increases with the increase of the length of the elliptic edge's long axis and the height of fiber bundle, but with the decrease of the width. These influences on the permeability of NCF are obvious. c) The permeability of NCF increases with the increase of the distance between fiber bundles, and the influence is great.d) The permeability of NCF increases with the decrease of the radius of stitched yarn, and the influence can be obvious when the distance between fiber bundles is relatively large, but the influence of the dimension variation of stitched yarn is very little.e) The permeability of NCF decreases with the increase of compression in normal direction, and the influence is significant.On the ground of the above conclusions, the NCF's structure was simplified, and two different models of NCF permeability were derived by theoretical analysis method. One is based on the solution of Stokes equation, which describes the flow behavior in the void between fiber bundles, and the other is based on an approximate computing formula of Poiseuille number for flow in micro-channels. The previous one is denoted as modelⅠwhen the latter one is modelⅡ. Comparatively analyzing modelⅠ, modelⅡand the numerical model comes to the conclusion that modelⅡis better than modelⅠfor modelⅠis unable to reveal the influence of fiber bundle shape on the NCF permeability. The modelⅡhas described the dependence of NCF's permeability on the length of the long axis of elliptic edge, the width, and the height of fiber bundle, and the distance between fiber bundles. As the calculated results of modelⅡare consistent with the numerical results, it can be used to calculate the permeability of NCF, analyze the resin flow behavior within it and predict the formation of infiltrating defects.On the basis of above work, the NCF permeability model was applied to calculate the permeability of plain, twill, and satin weave preform in an approximated way by analyzing their structural characteristics.