Permeability Prediction Of Fiber Preform And Infiltration Process Simulation By Finite Element Method

Advanced resin matrix composites are indispensable strategic materials in the field of aerospace due to their advantages of high specific strength, high specific modulus, corrosion resistance, fatigue resistance, easy design and large area molding. But its further development is restricted by the high cost of the composite manufacturing. Resin transfer molding (RTM), as a typical representative in the advanced composites molding process, has become one of mainstream technologies to realize low-cost. However, the products in RTM often have the defects such as dry spots or voids, which will affect the product quality and performance. The permeability inhomogeneity is one of important reasons of dry spots or voids. The permeability of the fiber preform, which represents the ease of flowing through a porous medium, is an important parameter influencing the resin impregnation process. The permeability is one of preform’s key properties, which is closely related to its structure. Many challeges are faced for permeability prediction due to its complex correlation with multi-level structure of preform. Besides, the sufficient infiltration flow of the resin in the fiber is a key factor directly affecting the performance of RTM products. Therefore, it is an urgent issue to discuss the permeability’s correlation with structural parameters of the preform and further accurate prediction of RTM process by numerical simulation.In this paper, two types of cells with or without stitch of Non-Crimp Fabric (NCF) are built. Aimed at the dual porous charateristic of the fiber preform and based on intra-tow and inter-tow coupling flow of dual scale porous medium, the infiltration coupling flow behaviour of the resin in the meso-level and micro-level of the preform is numerically simulated. Brinkman equation is used to describe intra-tow flow, while Stokes equation is used to discribe inter-tow flow. The mathematical models of the resin flow in the intra-tow and inter-tow of the preform are built. The velocity field and the pressure field are solved by the volume averaging theory and the prediction for NCF’s in-plane permeability is developed combined with Darcy’s law.The key structural parameters affecting permeability are discussed. The effect of the different NCF’s ply orientation on the in-plane permeability is investigated. The influencing law of multi-level structural parameters of the preform on the in-plane permeability is revealed, which result in the breakthrough in the permeability prediction. The permeability of two cell of the preform with or without stitch is compared. The effect of the structural factors including the width (B), the height (h), the semi-major axis length of the ellipse section of fiber bundle (c) and the distance between fiber bundles (b) and fiber ply orientation on the in-plane permeability of the preform is investaged. The influencing degree of different structural parameters on the permeability is quantized by Morris sensitivity analysis method, which proves an important theoretical basis for further optimization and design of the fiber preform structure. The effect of crossing equivalent radius between the fiber bundle and flow channel of the cell with stitch, size and distribution of the stitch, off-center distance of stitch on preform permeability is focused. The results show that in-plane permeability of the preform cell without stitch is larger than that of the preform cell with stitch. The preform permeability decreases with the increase of the fiber bundle width, increases with the increase of the fiber bundle height and the semi-major axis length of the ellipse section of fiber bundle and the distance between fiber bundles. The permeability of the cell without stitch decreases about52percent and the permeability of the cell with stitch decreases about54percent when the fiber bundle width inreases by1.2times. The permeability of the cell without stitch increases about two times and the permeability of the cell with stitch increases about1.7times when the fiber bundle height doubles. The permeability of the cell without stitch increases by30%and the permeability of the cell with stitch increases by29%when the semi-major axis length of the ellipse section of fiber bundle increases by3.2times. The permeability of the cell without stitch increases by11.3times and the permeability of the cell with stitch increases by14times when the distance between fiber bundles increases by4.7times. The order of permeability sensitivity degree corresponding to different structural parameters is as follows:b>h>B>c. The in-plane permeability is most sensitive to the distance between fiber bundles, while the semi-major axis length of the ellipse section of fiber bundle is relatively unsensitive. The fiber bundle crossing greatly hampers the flow velocity. Compared with the preform permeability without crossing, the preform permeability with crossing lowers by80%when the ratio of equivalent diameter of crossing bundle of flow channel to distance between the bundles is0.7. The preform permeability increases slightly with the increase of off-center distance, while decreases slightly with the increase of inclination angle of the stitch.The analytical model of the in-plane permeability of the fabric correlated with its structure is built on basis of the relation between the pressure drop and geometric parameters of the flow channel between the flow channels. The analytical model correlated with stitched fiber preform structure is built by the introduction of the modifying factor and equivalent channel method for the stitched fabric cell. The predicted permeability by the proposed model is compared with the results by the finite element method. The results show that the proposed mathematical model can accurately predict in-plane permeability of the fiber preform, which makes it possible to fast and accurately predict the permeability of the preform with multi-level structure. It play an important role in promoting RTM molding mold filling theory, improving composite material technology, optimizing the process parameters and reducing costs.Aiming at ex-situ toughening technology proposed by National Key Laboratory of Advanced Composites of Beijing Institute of Aeronautical Materials, the influencing law and the mechanism of the interlaminar toughening layer on preform permeability are investigated. The effect of the toughening layer thickness and permeability and fiber ply orientation on the Z-direction permeability of the preform is analyzed emphatically. The results show that the Z-direction permeability of the preform decreases with the increase of toughening layer thickness, while increases with the increase of the toughening layer’s permeability. The Z-direction permeability of the preform with [0]2ply orientation is larger than the permeability corresponding to other ply orientations. Among [0/30],[0/45],[0/60] and [0/90] ply orientations, the Z-direction permeability of the preform with [0/45] ply orientation is the largest, while [0/30] ply orientation is the lowest. Fiber volume fraction, specific surface areas of the flow channel and flow channel structure determine the preform permeability. The proposed numerical simulation model can accurately predict Z-direction permeability. It is a key parameter of RTM process flow simulation along the Z-direction of the preform, which will provides an important guiding role for further RTM process design and optimization.Aimed at the characteristics of the resin flow through multi-scale fiber preform, Darcy’s law is modified by the unsaturated factor. The partial differential equation (PDE) describing the resin unsteady flow in the fiber preform is established. The control equation of resin flow in the preform for RTM is solved by COMSOL software as a solver. The resin flow front evolution is investigated, which is compared with analytical solution and experimental result. The results show that the model can accurately predict resin unsteady flow in fiber preform. Aimed at ex-situ toughening technology and Z-direction RTM process, the inlet pressure and time relation curve of untoughening and toughening unsteady infiltration process is investigated and compared with experimental results. The resin flow intra-tow and inter-tow of the preform with toughening layer and untoughening layer is simulated. The infiltration visualization of resin flow through meso-scale and micro-scale fiber preform is realized, which provides an important supplement for prediction of macro-flow in fiber preform and guidance for actual process. The toughening layer makes flow front smoother, which reduces defect of the product. The influencing rule of the process parameters (including the resin viscosity, injection pressure and preform permeability) on the flow front is studied, which makes the model become a powerful tool to guide RTM mould design and process design, and will prove technical support for the actual production.