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Numerical Modelling of porosity generation, movement, and compaction during the RTM process.

Thesis

Numerical Modelling of porosity generation, movement, and compaction during the RTM process.

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Thesis
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Thick Laminates (above 6mm) are increasingly present in large composites structures such as wind turbine blades. Designs are based on static and fatigue coupon tests performed on 1-4mm thin laminates. However, a thickness effect has been observed in limited available experimental data. For this reason standard experimental data cannot automatically be transferred to thicker laminates.Different factors are suspected to be involved in the decrease of static and dynamic performance of thick laminates. These include the effect of self-heating, a mechanical scaling effect and the manufacturing process influence.Self-heating during fatigue is related to the material energy loss factor. During dynamic loading a certain percentage of mechanical energy is dissipated into heat, leading to a rise in material temperature. When the temperature approaches the maximum service temperature of the material, a reduction in fatigue life can be observed. The work proposes an FE method to forecast self-heating, which is validated by using empirical data.Scaling effects and coupon geometry influence the results of thickness scaled coupon tests. The thickness effect was studied with the help of compression and tension tests on thickness scaled coupons. In order to reduce the test effects of the scaled coupon tests the coupon geometry and clamping system are designed for optimal load introduction.The manufacturing process and curing cycles are reported as one of the leading causes to explain possible scaling effects. Through-thickness lamina properties were studied using the sub-laminates technique. In this way, it was possible to relate the in-plane lamina properties with the manufacturing properties conditions. A relation between the mechanical properties and the process conditions is proposed.In the case of static and fatigue properties, the sub-laminates tests report a large variation in resin related properties which is dependent on the manufacturing process. Scaled tests are studied from this point of view; the scaling effect is related to the manufacturing process, and the assumption of uniform strength fields is considered not valid for thick laminates in comparison with thin laminates.
Article
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A bifluid–solid contact model involving surface tension and wetting effects is developed within a finite element framework, in order to provide an accurate characterization of the fluids and fibrous behaviours during Liquid Composite Molding processes. This model is based on a Eulerian approach of two immiscible fluid (resin/air) domains with boundary conditions which prescribe wetting phenomena at fluid/fiber interfaces. The fluid interface is described by the Level Set method, on which capillary force is considered. Numerical simulations of a drop evolution with wetting effects are used to illustrate this challenging physical problem.
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Mold filling in anisotropic porous media is the governing phenomena in a number of composite manufacturing processes, such as resin transfer molding (RTM) and structural reaction injection molding (SRIM). In this paper we present a numerical simulation to predict the flow of a viscous fluid through a fiber network. The simulation is based on the finite element/control volume method. It can predict the movement of a free surface flow front in a thin shell mold geometry of arbitrary shape and with varying thickness. The flow through the fiber network is modeled using Darcy's law. Different permeabilities may be specified in the principal directions of the preform. The simulation permits the permeabilities to vary in magnitude and direction throughout the medium. A modified version of Darcy's law, based on the capillary model, is used to describe the flow of power-law fluids through the porous medium. Experiments were carried out to measure the characteristic permeabilities of fiber preforms.
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The study of the capillary flow developed during the processing of composite materials is critical because it acts as an important driving force for the impregnation of the fiber tows. It is also a key mechanism on the void formation during infiltration of the fibers. In this study, capillary pressure of jute/vinylester composites was measured and the impact of capillary forces on fabric permeability was experimentally analyzed. It was found that the capillary pressure was significantly higher than in synthetic fiber fabrics. In addition the permeability resulted higher when the water/glycerin solution was used, because its higher compatibility with the fibers leaded to negative capillary pressures and enhanced flow. Therefore, the capillary pressure obtained in the constant pressure experiments was used to correct the applied pressure gradient in the permeability tests and thus a corrected value of permeability independent of the tests fluid was obtained.
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The focus of this paper is the development of numerical schemes for tracking the moving fluid surface during the filling of a porous medium (e.g., polymer injection into a porous mold cavity). Performing a mass balance calculation on an arbitrarily deforming control volume, leads to a general governing filling equation. From this equation, a general, fully time implicit, numerical scheme based on a finite volume space discretization is derived. Two numerical schemes are developed: (1) a fully deforming grid scheme, which explicitly tracks the location of the filling front, and (2) a fixed grid scheme, that employs an auxiliary variable to locate the front. The validity of the two schemes is demonstrated by solving a variety of one- and two-dimensional problems; both approaches provide predictions with similar accuracy and agree well with available analytical solutions.
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In this investigation, a comprehensive Vacuum Assisted Resin Transfer Molding (VARTM) process simulation model was developed and verified. The model incorporates resin flow through the preform, compaction and relaxation of the preform, and viscosity and cure kinetics of the resin. The computer model can be used to analyze the resin flow details, track the thickness change of the preform, predict the total infiltration time and final fiber volume fraction of the parts, and determine whether the resin could completely infiltrate and uniformly wet out the preform. Flow of resin through the preform is modeled as flow through porous media. Darcy's law combined with the continuity equation for an incompressible Newtonian fluid forms the basis of the flow model. During the infiltration process, it is well accepted that the total pressure is shared by the resin pressure and the pressure supported by the fiber network. With the progression of the resin, the net pressure applied to the preform decreases as a result of increasing local resin pressure. This leads to the springback of the preform, and is called the springback mechanism. On the other side, the lubrication effect of the resin causes the rearrangement of the fiber network and an increase in the preform compaction. This is called the wetting compaction mechanism. The thickness change of the preform is determined by the relative magnitude of the springback and wetting deformation mechanisms. In the compaction model, the transverse equilibrium equation is used to calculate the net compaction pressure applied to the preform, and the compaction test results are fitted to give the compressive constitutive law of the preform. The Finite Element/Control Volume (FE/CV) method is adopted to find the flow front location and the fluid pressure. The code features the ability of simultaneous integration of 1-D, 2-D and 3-D element types in a single simulation, and thus enables efficient modeling of the flow in complex mold geometries. VARTM of two flat composite panels was conducted to verify the simulation model. The composite panels were fabricated using the SAERTEX multi-axial warp knit carbon fiber fabric and SI-ZG-5A epoxy resin. (Abstract shortened by UMI.)
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Mold-filling simulation of unsaturated flows in LCM is important for optimizing mold design quickly and cost-effectively in the virtual space. For the first time, a true multiscale approach is developed for simulating the unsaturated flow under isothermal conditions in the dual-scale fiber-mats of RTM. To solve the coupled macro–micro equation-set, a coarse global mesh is used to solve the global flow equations over the entire domain while fine local meshes in form of the periodic unit-cells of fabrics are employed to solve the local tow-impregnation process. A multiscale algorithm based on hierarchical computational grids has been proposed to simulate the unsaturated flow in the dual-scale fiber mats under isothermal conditions. The predictions are compared with measurements for a 1-D flow experiment which indicates that the proposed approach can be used to simulate the unsaturated flow accurately through dual-scale fiber mats in LCM without the use of any fitting parameters.
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A new methodology is presented to simulate mold filling in resin transfer molding (RTM) using a combination of the level set and boundary element methods (BEMs). RTM is a composite manufacturing process where a liquid resin is injected in a closed rigid mold containing a dry fibrous reinforcement. Process simulation is motivated by the importance of tracking accurately the motion of the flow front during the mold filling stage. The BEM solves the equation governing the resin flow and the level set method is implemented to track the resin front in the mold. This formulation opens up new opportunities to improve RTM flow simulations and optimize injection molds. The present paper focuses on isothermal resin flow in undeformable porous medium. The implementation of the numerical algorithm is described and several examples of two-dimensional filling with single or multiple injection gates are presented. The robustness of the coupling and the ability to predict accurately the position of the front by this new model are discussed. It is also shown how dry spot formation can be tracked precisely during the simulation and how a generalization of this approach allows predicting resin flow across obstacles.