注射条件对LCM工艺非饱和流动特性影响

双尺度多孔纤维预制体填充过程中延迟浸润的非饱和流动现象,对基于树脂流过区域为完全饱和区域的充模理论及模拟方法提出了挑战。通过控制体/有限单元（CV/FE）法结合沉浸函数实现了液体模塑成型工艺（LCM）中非饱和填充浸润的数值模拟,并对比了恒压下的实验结果,验证了其可靠性。分析讨论了注射口压力、流量和液体黏度对双尺度多孔纤维织物非饱和填充浸润特性的影响。结果表明:在允许误差内,该数值模拟结果可靠,可用于分析讨论各因素对双尺度多孔织物非饱和流动特性的影响;填充浸润过程中,纤维织物内部非饱和区域长度并非保持不变,而是随着填充浸润的进行经历了4个变化过程;不同注射条件下,压力、流量及黏度对非饱和流动特性影响不同。研究结果对合理控制注射条件及流体特性实现双尺度多孔纤维预制件的完全浸润具有指导意义。      

Analysis of non-isothermal mold filling process in resin transfer molding (RTM) and structural reaction injection molding (SRIM)

In this paper, we present a modeling and numerical simulation of a mold filling process in resin transfer molding/structural reaction injection molding utilizing the homogenization method. Conventionally, most of the mold filling analyses have been based on a macroscopic flow model utilizing Darcy's law. While Darcy's law is successful in describing the averaged flow field within the mold cavity packed with a porous fiber preform, it requires experiments to obtain the permeability tensor and is limited to the case of porous fiber preform-it can not be used to model the resin flow through a double porous fiber preform. In the current approach, the actual flow field is considered, to which the homogenization method is applied to obtain the averaged flow model. The advantages of the current approach are: parameters such as the permeability and effective heat conductivity of the impregnanted fiber preform can be calculated; the actual flow field as well as averaged flow field can be obtained; and the resin flow through a double porous fiber preform can be modelled. In the presentation, we first derive the averaged flow model for the resin flow through a porous fiber preform and compare it with that of other methods. Next, we extend the result to the case of double porous fiber preform. An averaged flow model for the resin flow through a double porous fiber preform is derived, and a simulation program is developed which is capable of predicting the flow pattern and temperature distribution in the mold filling process. Finally, an example of a three dimensional part is provided.     

Numerical simulation of metal matrix composites and polymer matrix composites processing by infiltration: a review

The injection of a liquid metal through a fibrous preform is one of the techniques used to manufacture metal matrix composites (MMCs). The flow of metal through fibrous preform is a problem of fluid mechanics in porous medium. Numerical simulations of this process were developed in particular for non-isothermal infiltrations which take into account the phenomena of phase change. In addition, numerical models were developed to predict the appearance of defects in the end product and to study the evolution of the deformation of the fibrous preform during metal infiltration. After pointing out the analogous numerical studies devoted to the Resin Transfer Moulding (RTM) process, we give a progress report on the models developed to date for MMCs.     

基于多相流的LCM工艺气泡生成仿真

分析了液体模塑成型工艺（LCM）下织物预成型体中双尺度流动以及由此造成的空气裹入,进而产生细观及微观气泡的现象。基于多相流（VOF）方法建立了树脂空气两相流体在单胞内部流动的数学模型,并确定了该模型中多孔介质阻力源项和毛细力源项的具体形式。基于Fluent软件的UDF功能实现了上述两相流模型的数值求解,研究了平纹织物单胞中的两相流动以及2种气泡的生成过程。对Rovcloth 2454织物的气泡生成仿真结果显示,毛细数 Ca 对气泡的产生有决定性作用：当毛细数接近临界毛细数 Ca^c时,气泡产生量最低,而当Ca小于Ca^c时,容易产生细观气泡,反之容易产生微观气泡。通过与文献中的理论预测和实验数据对比,验证了本文算法的正确性。     

基于VOF/PLIC界面追踪方法的RTM工艺充模过程数值模拟

针对基于Darcy定律的树脂传递模塑(RTM)工艺的充模过程数值模拟的局限性,将纤维预制体内的充填流动作为两相流(树脂相和空气相)处理,在动量方程中考虑了惯性项和粘性项,采用有限体积方法(FVM)离散控制方程,并与VOF/PLIC界面追踪方法相结合,发展了求解树脂在纤维预制体内非稳态流动问题的数值模拟方法.在此基础上开发了RTM工艺的充模过程数值模拟程序,其算例的数值模拟结果与解析解或实验结果吻合良好,验证了此数值模拟方法的有效性和可靠性.     

A numerical study of filling process through multilayer preforms in resin injection/compression molding

In resin injection/compression molding (RI/CM), a preform often comprises layers of different fiber reinforcements. Each fiber reinforcement has unique through thickness and in-plane permeabilities as well as compressibility, creating a heterogeneous porous medium in the mold cavity. In the present article, numerical simulation is utilized to investigate the filling process of RI/CM in such a heterogeneous porous medium. The filling stage is simulated in a full three-dimensional space by using control volume/finite element method and based upon an appropriate filling algorithm. The flow in the open gap which may be present in the mold cavity is modeled by Darcy’s law using an equivalent permeability. Numerical simulations of filling process for preforms containing two and three layers of different reinforcements in various stacking sequences are conducted with the aid of computer code developed in this study. Results show that the injection time as well as flow front progression depends on fiber types in the whole preform, fiber stacking sequence and open gap provided in the mold cavity. Simulated results also suggest that the presence of open gap at top of reinforcement can lead to both low injection time and uniform flow pattern.     

Non-isothermal injection molding with resin cure and preform deformability

In this paper, a non-isothermal model to simulate some injection molding processes used to fabricate composite materials is deduced. The model allows the solid constituent in both the dry and the wet region to deform during infiltration. The dry porous material is assumed to behave elastically, while the mixture of resin and preform is assumed to behave as a standard linear solid. The model also takes into account the fact that the liquid undergoes an exothermic cross-linking reaction during infiltration and eventually gels stopping the infiltration process. Focusing then on one-dimensional problems it is shown that the integration of the mechanical problem in the uninfiltrated region can be reduced to the integration of an ordinary differential equation defining either the space-independent volume ratio or the location of the infiltration front, depending on whether the flow is driven by a given infiltration velocity or by a given inlet pressure. The remaining system of partial differential equations in the two interfaced and time-dependent domains is then posed with the proper interface and boundary conditions. After writing the problem in a Lagrangian formulation fixed on the solid constituent, domain decomposition techniques are used for the simulation.     

Experimental assessment of dual-scale resin flow-deformation in composites processing

In this paper we are concerned with the assessment of sub-models within a two-phase continuum mechanical FE framework for process modeling of composites manufacturing. In particular, the framework considers the inclusion of two deformation dependent models describing resin flow related to: (1) meso-scale wetting and compaction of individual plies and (2) overall preform deformation and macroscopic Darcian flow. Using micro-mechanical modeling, we model the physics of these sub-processes in relation to the recently developed Out-Of-Autoclave (OOA) prepergs. The models are placed in context with a compression–relaxation experiment, employed to study the preform deformations considered separated from other sub-processes. Finally, calibrations and model validations are carried out against the relaxation experiment to relate the FE framework to the mechanical response of the preform. Therefore, using the above experiment, parameter values out of the literature and those estimated from micrographs gave a fair agreement between the simulation and experiments.     

Numerical prediction of saturation in dual scale fibrous reinforcements during Liquid Composite Molding

This paper presents a fractional flow model based on two-phase flow, resin and air, through a porous medium to simulate numerically Liquid Composites Molding (LCM) processes. It allows predicting the formation, transport and compression of voids in the modeling of LCM. The equations are derived by combining Darcy’s law and mass conservation for each phase (resin/air). In the model, the relative permeability and capillary pressure depend on saturation. The resin is incompressible and the air slightly compressible. Introducing some simplifications, the fractional flow model consists of a saturation equation coupled with a pressure/velocity equation including the effects of air solubility and compressibility. The introduction of air compressibility in the pressure equation allows for the numerical prediction of the experimental behavior at low constant resin injection flow rate. A good agreement was obtained between the numerical prediction of saturation in a glass fiber reinforcement and the experimental observations during the filling of a test mold by Resin Transfer Molding (RTM).     

Characterization of preform permeability in the presence of race tracking

For realistic simulation of resin flow in a stationary fibrous porous preform during Liquid Composite Molding (LCM) processes, it is necessary to input accurate material data. Of great importance in simulating the filling stage of the LCM process is the preform permeability; a measure of the resistance the preform poses to the flowing fluid. One method to measure permeability values is by conducting one-dimensional flow experiments, and matching the flow behavior to known analytical models. The difficulty is the edge effects such as race tracking disrupt the flow and violate the one-dimensional flow assumption. The new approach outlined in this paper offers a methodology to obtain accurate bulk permeability values despite any race tracking that may be present along the edges of the mold containing isotropic fabrics. Further, a method of approximate equivalent isotropic scaling is explained to extend the use of this method to determine permeability of anisotropic materials with race tracking present. Both approaches are validated with computer simulations, and then utilized in laboratory experimentation. The values calculated from this approach compare well with permeability values obtained from one-dimensional permeability experiments without the presence of race tracking.     

变形多孔介质流固耦合模型及数值模拟研究

假定骨架、固体颗粒和水均是可压缩的,在此基础上采用两相不混溶流体的理论推导了水的连续方程,通过引入气压恒定这一假定进一步简化为水的非饱和渗流连续方程。基于广义Biot理论给出了固体骨架积分形式的平衡方程,结合非饱和渗流连续方程采用加权残值法推导了流固耦合方程组的有限元列式。通过干燥介质吸水的数值模拟来考察非饱和流固耦合模型的预测能力,数值模拟的结果表明耦合模型可以准确地反映吸水过程的规律。将耦合模型应用于水下大断面隧洞开挖的瞬态分析,可以模拟出开挖引起的EDZ区域孔隙水压力急剧升高、有效应力减小、渗透系数动态变化以及排水对洞室稳定性的影响,计算的结果与国外大型原位实验的一般性观测结论相吻合。     

Monitoring Density and Temperature in C/C Composites Processing by CVI with Induction Heating

Carbon/carbon composites are processed by chemical vapor infiltration (CVI) with radio-frequency inductive heating, which leads to inside-out temperature gradients, suitable for the production of homogeneously densified pieces if properly controlled throughout the whole processing. We present here a 2D axisymmetrical case where a comprehensive numerical model is tested against experimental runs. The numerical thermal model takes into account induction heating, radiative, conductive, and convective effects, intermediate regime diffusion and densification reactions in the pores, and the evolution of the porous medium. The results are the time evolution of the temperature, concentration, and composite material density field, as well as the input power necessary to ensure a given maximal temperature in the preform. Experimental data are measurements of the temperature and density fields at various infiltration stages. Comparison between experience and simulation, yielding an useful agreement, shows that porosity becomes trapped inside the preform as densification proceeds, because of the progressive lowering of the temperature gradient steepness. The discrepancies between computations and experimental data rely on the only approximate knowledge of some quantities, principally the reaction kinetics, which are currently under investigation.     

挤压铸造硼酸铝晶须增强Al基复合材料浸渗过程理论分析

本文作者采用挤压铸造法,制取了硼酸铝晶须增强Al基复合材料,并根据Laplace方程及多孔体的渗流物理基本原理,从多孔渗流动力学角度分析了挤压浸渗过程中金属液在多孔体中的流动,认为紊流才是其主要的表现形式。并较详细地分析了晶须长度对预制件孔结构及浸渗的影响。     

基于数值模拟的热梯度CVI制备C/C 复合材料致密化行为

根据热梯度化学气相渗透(CVI)工艺制备C/C复合材料的特点, 建立了均相与非均相反应的多场耦合数学模型。以2D炭毡为预制体, 天然气为前驱体, 炉压为100 kPa的工艺条件下, 通过计算获得了预制体致密化过程中密度的演变规律; 分析了沉积温度及气体流量对致密化的影响, 获得了合理的沉积温度和气体流量范围。致密化100 h后, 预制体整体密度的计算值与实验值基本一致, 径向密度分布的模拟值与实验值呈相同的变化规律, 验证了模型的可靠性和模拟的预测能力。      

Efficient Permeability Measurement and Numerical Simulation of the Resin Flow in Low Permeability Preform Fabricated by Automated Dry Fiber Placement

We propose a new experimental method using a Hassler cell and air injection to measure the permeability of fiber preform while avoiding a race tracking effect. This method was proven to be particularly efficient to measure very low through-thickness permeability of preform fabricated by automated dry fiber placement. To validate the reliability of the permeability measurement, the experiments of viscous liquid infusion into the preform with or without a distribution medium were performed. The experimental data of flow front advancement was compared with the numerical simulation result using the permeability values obtained by the Hassler cell permeability measurement set-up as well as by the liquid infusion experiments. To address the computational cost issue, the model for the equivalent permeability of distribution medium was employed in the numerical simulation of liquid flow. The new concept using air injection and Hassler cell for the fiber preform permeability measurement was shown to be reliable and efficient.     

Modeling of Liquid Metal Infiltration of Porous Fiber Preform During Squeeze Casting

The present work adopts a new approach to the analytical modeling of infiltration of porous fiber preforms by liquid metal in the squeeze casting of metal matrix composites, with the assumption that the process is adiabatic and that the flow is unidirectional. Fluid dynamics is described on the basis of Darcy's law, while separate equations are derived to explain the thermal behavior of the liquid metal and the fiber, assuming that the thermal interactions between the two are interfacial. Unlike earlier models, this approach does not consider the thermal behavior of a “composite,” but instead studies the behavior of the liquid metal and the fiber preform separately. In addition to the conventional application of heat balance techniques and development of partial differential equations involving temperatures, this work introduces supplementary conditions for temperature calculations, specifically at the entry and front points during infiltration. Differential equations are solved by a method of finite differences, and the problem of additional unknowns (preform temperature) at the infiltration front position is overcome using the “virtual point” concept. Simple expressions are derived for the calculation of process parameters like total time for complete infiltration and time for solidification, on the basis of which the occurrence of complete infiltration is predicted. A novel attempt in generating the profiles of the preform and liquid temperatures at specific instants during infiltration has also been made. The relative influence of the liquid superheat temperature, the preform preheat temperature, and the squeeze pressure on the infiltration mechanism is analyzed by studying the infiltration characteristics for various squeeze conditions.     

Film stacking impregnation model for a novel net shape thermoplastic composite preforming process

An impregnation model has been developed to evaluate the infiltration phenomena that occur during a novel near net shape preforming method. This process comprises automated deposition of thermoplastic resin and unidirectional (UD) carbon fibres to a pre-programmed stacking sequence, thereby forming tailored preforms for subsequent stamp-forming. Infiltration kinetics have been simulated to study the effect of different stacking scenarios, materials, and pre-consolidation routes on the novel preforming process. Isothermal infiltration of a liquid thermoplastic polymer into a compressed UD fibre bed has been examined and the experimental results have been used to validate an infiltration model based on local fluid flow in compressible porous media. This enables simulation of infiltration in alternating matrix film and fibre layers, relating pressure, time, and temperature with the local fibre volume fraction, pressure, and liquid and solid velocities in the stacked material. For a given set of processing conditions, the model fibre volume fraction distribution prediction enables the optimum matrix stacking layer thicknesses to be determined. It was shown that infiltration is inhibited above a limiting pressure which leads to increased fibre bed compaction and hence decreased permeability.     

Analysis of the Edge Effect on Flow Patterns in Liquid Composites Molding

In liquid composites molding, the cutting of the fiber preform is often not sufficiently precise leaving a small clearance between the reinforcement and the mold edges. This clearance creates a preferential flow path for the resin which may disrupt the filling of the mold cavity. Experience has shown that even a small clearance of 1 or 2 mm could have a significant effect, especially if the preform has a high fiber content. A model is thus needed to predict this channeling effect in order to take it into account in computer simulations of the mold filling process.This paper presents a model to describe this phenomenon. The idea is to characterize simultaneously the flow in the channel and through the reinforcement. The model is derived using Navier–Stokes equations in the open channel and Darcy's law in the porous preform. From this model, an equivalent porous medium was defined for which an equivalent permeability tensor can be computed as a function of the channel geometry. Numerical simulations performed with the computer software RTMFLOT developed in our laboratory have shown that in some limit cases, i.e., a large channel (5–6 mm) or for very low porosity reinforcements, the transverse flow from the channel to the preform can be neglected in the model while still obtaining quite a good prediction of the flow edge effect. In other cases, however, namely for a clearance of intermediate size (2–3 mm) which is the most common case in RTM, or for a higher porosity of the reinforcement, the transverse flow from the channel to the rest of the preform must be taken into account. Experimental data to validate the proposed model are also presented.     

铝熔体在多孔介质中的渗流过程

在流体流动相似原理的基础上，提出了铝熔体在多孔介质间隙（简称多孔介质）中渗流模拟的原理和方法．设计建造了用于铝液和模拟液在外加压力下进行单向渗流的两套试验装置模拟试验揭示渗流时铝液在多孔介质中渗流的若干规律实践表明理论模型与试验吻合良好．     

液体模塑成型工艺二维径向非饱和流动数值模拟

通过引入沉浸函数建立了双尺度多孔介质非饱和流动模型,并采用有限元/控制体积法实现了恒压及恒流注射条件下液体模塑成型（LCM）工艺二维径向非饱和流动的数值模拟,得到了不同注射条件下纤维织物内的压力场分布及半饱和区域长度随时间的变化规律,并将双尺度非饱和理论结果与单尺度饱和理论结果进行对比。结果表明：非饱和流动过程中,半饱和区域内的压力和压力梯度明显下降;半饱和区域长度随时间逐渐增加随后保持稳定,当流动前沿到达出口后半饱和区域长度开始逐渐减小;当两个主方向渗透率不同时,沿主方向半饱和区域长度也不同,渗透率越大该方向的半饱和区域长度也越大,纤维织物完全浸润时间取决于较小的渗透率。研究结果对合理预测树脂填充过程中压力分布及纤维预制件的浸润具有指导意义。