复合材料带筋壁板预成型体压缩性与渗透性对树脂流动的影响

以航空碳纤维增强树脂基复合材料典型结构件带筋壁板为研究对象,通过对U3160单向织物的组织结构进行分析,根据纤维束的受压变形状态对其压缩响应进行理论建模,然后以纤维束压缩模型为基础,预测了U3160单向织物按0°/45°/90°/-45°铺层时预成型体在压缩应力作用下厚度变化的响应行为。建立了压缩应力作用下纤维预成型体的渗透率解析模型:在织物压缩模型的基础上,建立了纤维束等效渗透率模型;根据张量理论,分别建立了0°、±45°和90°铺层织物等效渗透率模型;运用渗透介质串并联关系,建立了带筋壁板各特征区域渗透率综合表征模型。基于PAM-RTM流动模拟软件,进行分区渗透率定义,在充模过程中对树脂在带筋壁板预成型体中的流动行为进行模拟,优化工艺参数,确定出最终充模方案,并制作带筋壁板实验缩比件进行成型实验,验证了充模方案的合理性。研究结果为制件的成功制作提供理论依据,从而指导生产实践。      

液体模塑成型工艺用纤维织物厚度方向饱和渗透率的预测模型液体模塑成型工艺用纤维织物厚度方向饱和渗透率的预测模型

树脂在复合材料预成型体厚度方向的渗透能力对复合材料液体模塑成型工艺（LCM）的成功实施至关重要。本文采用连续加载的方式,研究了玻璃纤维增强树脂基复合材料液体成型过程中多轴向无屈曲织物（NCF）和斜纹织物（WF）的压缩响应行为,并建立描述该行为的数学模型。采用自制测试装置对预成型体在重力等不同注射压力驱动下的厚度方向渗透率进行测试,考察了预成型体纤维体积分数、测试流体注射压力等对预成型体厚度方向渗透率K_z的影响。基于预成型体压缩响应数学模型和厚度方向渗透率与注射压力的关系,对Kozeny-Carman公式进行修正,提出了变注射压力条件下的厚度方向渗透率预测模型。结果表明:预成型体厚度方向渗透率随着纤维体积分数的增大而减小,与Kozeny-Carman方程结果相符合。当纤维体积分数为0.42≤V_f≤0.58时,注射压力对厚度方向渗透率影响较大,实验结果验证了本文提出的预测模型;当纤维体积分数V_f≥0.58时,注射压力对厚度方向渗透率影响较小,厚度方向渗透率趋于恒定。      

导流介质对真空导入模塑工艺树脂流动行为的影响

采用可视化流动实验方法研究了高渗透率导流介质对真空导入模塑工艺中树脂流动行为的影响。结果表明: 导流介质能较大幅度地减少树脂的充模流动时间, 且充模时间随着导流介质使用比例的增加而呈线性减少的关系; 导流介质的提速作用随着预成型体厚度的增加而逐渐减弱; 预成型体上下表面树脂流动前沿位置差距与预成型体厚度呈良好的线性增加关系, 说明导流介质的影响作用具有明显的厚度效应。厚度效应原理为真空导入模塑工艺过程的参数优化和保证制品质量提供了理论依据。      

复合材料液体模塑成型工艺中预成型体渗透率张量的数值预测

结合均匀化理论和计算流体动力学技术, 实现了对复合材料液体模塑工艺中预成型体渗透率张量的预测。首先, 采用均匀化理论分析了流体在多孔介质内的流动问题, 推导出广义达西定律, 证明以施加周期性边界条件的单胞为研究对象, 可以预测预成型体的渗透率张量, 并以单向纤维织物为例, 对该方法进行了验证。对于复杂结构的预成型体, 渗透率的预测分为两步, 首先分别确定预成型体中流道和纤维束的渗透率, 然后计算其整体宏观渗透率。对于二维平面织物, 该方法与其他预测方法及实验的结果吻合较好。本文还考察了单胞的微观结构对渗透率的影响, 微观结构相似的预成型体如果孔隙率相同, 但束间流道的结构不同, 其整体宏观渗透率也存在很大差别。      

结构化增韧层增韧RTM复合材料预成型体的渗透特性

选用尼龙无纺布(Polyamide Nonwoven Fabric,PNF)作为结构化增韧层,研究了增韧层的引入对纤维预成型体在树脂传递模塑成型(RTM)工艺过程中渗透特性的影响。结果表明:在径向非饱和流动模式下,层间增韧预成型体沿纤维方向的渗透率为5.2×10-12 m2,略低于非增韧预成型体的7.1×10-12 m2,而沿垂直于纤维方向的渗透率为2.3×10-12 m2,略高于非增韧预成型体的1.6×10-12 m2。此外,层间增韧预成型体的单向饱和流动渗透率为2.6×10-12 m2,较非增韧预成型体的1.9×10-11 m2下降了约1个数量级,z向饱和流动渗透率较非增韧预成型体的1.3×10-13 m2下降至2.5×10-14 m2,同样下降了约1个数量级。对复合材料层间微观形貌的分析结果表明:造成预成型体渗透率下降的主要原因首先是PNF引入至层间之后将阻碍层间树脂的快速流动,同时增韧层将使层内纤维含量明显升高,由55.3vol%上升到63.7vol%。     

预成型体纵向渗透率统计模型

预成型体渗透率是LCM工艺重要的材料工艺参数,建立渗透率模型对认识纤维/树脂流动浸润机理,更准确地预报渗透率及优化工艺和材料参数具有重要意义。本文作者以LCM工艺复合材料的微观结构分析为依据,建立纵向渗透率的统计模型,系统研究单向预成型体材料微观结构及参数对其渗透特性的影响规律,并进行了实验验证。      

基于FEPG有限元分析系统的树脂充模过程模拟

为优化复杂预成型体结构的液体成型工艺,基于有限元法/生死节点法模拟了复合材料液体模塑成型过程树脂流动,并针对典型矩形平板、圆板结构、I型加筋壁板充模过程进行了仿真与验证。结果表明:典型矩形平板和圆板结构的充模过程模拟结果与理论解一致性较好,验证了生死节点法跟踪树脂流动前锋的有效性。含有方腔的变厚度圆柱体和正方体三维实体结构的充模过程模拟验证了有限元方法对三维结构的适用性。基于有限元法/生死节点法的液体充模过程模拟方法对于复杂求解区域具有更好适应性,可用于复杂实体结构的液体模塑成型工艺过程树脂流动规律预测、指导模具设计及工艺优化。      

基于混合网格方法的VARTM工艺充模仿真与实验验证

针对VARTM工艺的特点,建立了充模过程树脂流动和预成型体变形行为数学模型。提出了基于混合网格方法的VARTM充模仿真算法,在该算法中,模具型腔几何模型进行二维或三维网格划分,在每个真空袋表面单元上增加一个一维附属单元,用于在仿真过程中实时地吸收或挤出因真空袋变形而产生的局部树脂体积变化,形成混合网格仿真模型;求解过程中,对树脂流动和预成型体变形分别进行求解后,基于上述混合网格模型进行两者耦合操作,实现了仿真精度和速度的统一。搭建了VARTM充模实验平台,进行了一维充模实验,通过仿真结果与实验测量结果对比,验证了本文算法的正确性。最后,通过三维仿真算例,验证了算法对三维复杂结构和顺序浇口策略仿真的可行性。     

z向流动RTM工艺树脂的流动浸润行为

研究了层间“离位”附载多孔薄膜结构形式增韧层的大厚度纤维预成型体中等代流体（树脂）沿预成型体厚度方向（z向）的流动行为,通过压力传感器监测z向流动RTM（z-RTM）工艺注射过程中进、出胶口压力的变化规律,进一步反推树脂在层间“离位”增韧与非增韧预成型体中的宏观流动及微观浸润模式。结果表明,在 z-RTM工艺注射过程中,树脂在沿纤维束间z向快速流动的同时完成对纤维丝束内部的浸润。层间“离位”附载的增韧层虽延缓了树脂的宏观流动,但使流动前锋曲面更加平滑。层间“离位”增韧预成型体z向渗透率为3.5×10^-15m²,与非增韧预成型体的z向渗透率2.9×10^-14m²相比,降低约一个数量级。     

VIP工艺中L型构件渗透规律的实验研究

VIP(Vacuum Infusion Process)作为一种新型的低成本液体模塑成型技术由于具备一系列优点已广泛应用于大型复合材料零件的生产中。不同于平板构件,VIP工艺中L型构件的渗透特性在各个部分并不均匀。本文通过对L型构件预成型体渗透率进行实验研究,得到了三种几何结构的L型预成型体的渗透规律,由于拐角处纤维被压实使得L型构件的渗透率比相同铺层和相同工艺下平板构件的渗透率小;并采用几何平均方法分析了L型预成型体拐角处的渗透率,对异型结构件VIP工艺的实施具有一定的指导作用。     

The effect of fabric and fiber tow shear on dual scale flow and fiber bundle saturation during liquid molding of textile composites

In Liquid Composite Molding (LCM) processes, a fibrous reinforcement preform is placed or draped over a mold surface, the mold is closed and a resin is either injected under pressure or infused under vacuum to cover all the spaces in between the fibers of the preform to create a composite part. LCM is used in a variety of manufacturing applications, from the aerospace to the medical industries. In this manufacturing process, the properties of the fibrous reinforcement inside the closed mold is of great concern. Preform structure, volume fraction, and permeability all influence the processing characteristics and final part integrity. When preform fabrics are draped over a mold surface, the geometry and characteristics of both the bulk fabric and fiber tow bundles change as the fabric shears to conform to the mold curvature. Numerical simulations can predict resin flow in dual scale fabrics in which one can separately track the filling of the fiber tows in addition to flow of resin within the bulk fabric. The effect of the deformation of the bulk fabric due to draping over the tool surface has been previously addressed by accounting for the change in fiber volume fraction and permeability during the filling of a mold. In this work, we investigate the effect of shearing of the fiber tows in addition to bulk deformation during the dual scale filling. We model the influence of change in fiber tow characteristics due to draping and deformation on mold filling and compare it with the results when the fiber tow deformation effect is ignored. Model experiments are designed and conducted with a dual scale fabric to characterize the change in permeability of fiber tow with deformation angle. Simulations which account for dual scale shear demonstrate that the tow saturation rate is affected, requiring longer fill times, or higher pressures to completely saturate fiber tows in areas of a mold with high local shear. This should prove useful in design of components for applications in which it is imperative to ensure that there are no unfilled fiber tows in the final fabricated component.     

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.     

RTM工艺注模过程边缘效应模拟分析

RTM工艺需将纤维预制体预置到模具中,由于纤维预制体结构不均匀性和模具形状、尺寸等影响,极易产生边缘效应。边缘效应会严重影响树脂流场发展和压力场分布。本文作者采用等效渗透系数方法模拟边缘效应,得到了其影响下的树脂流动前峰曲线和压力场。研究表明:一方面边缘效应可能导致不期望的树脂流场发展而形成工艺缺陷——干斑;另一方面可以利用边缘效应提高工艺效率:常流率注射时减小合模压力和注射压力,常压力注射时可以减少注模时间。      

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.     

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.     

Fabric structure and mold curvature effects on preform permeability and mold filling in the RTM process. Part II. Predictions and comparisons with experiments

In Part I, an experimental study was completed in a series of five molds, each having corners of different radii (from 0.06 to 8.0 in.). The primary goal of this work has been to determine whether corners in LCM molds significantly affect the filling process, by altering the structure of the preform locally in such regions. Consistent trends were found for each series of experiments completed in the mold, for which the same preform type, and number of layers were used. For constant flow rate injection, the required injection pressures to fill the two molds with tighter radii was significantly increased as compared to the other molds. Composite parts were manufactured in these molds, measurements made on these parts revealing design flaws, the cavity thicknesses not being equal in all sections of the molds. Several numerical simulations are presented in this paper, the goal being to separate any effects due to the varying thicknesses from effects due to the corners present. Careful simulations have been completed, taking into account the actual thicknesses in each mold, and the resulting preform volume fraction. Experimentally measured permeability data was employed, and the predicted injection pressures match very well for all four molds studied, seeming to indicate that the corners present have not affected the filling. A simple model for preform compression in corners has been developed, which predicts local permeability modifications due to in-plane compression of the fabric layers. These predictions have been employed in conjunction with an existing tool to analytically predict the permeability components of a preform in a flat cavity. This code requires from the user a geometrical model of a preform unit cell, this data being measured from samples cut from the parts manufactured. The resulting predictions for injection pressure are good for an entirely predictive approach, underpredicting experimental values by only 30–60%. Sensitivity analyses have demonstrated the strong relationship between permeability and the details of the preform cell. Two numerical studies were completed to determine how sensitive the injection pressure curves are to reduced permeabilities in the corner regions. For the two injection schemes having two different gate locations, pressures were not significantly affected, while the permeabilities in this region were reduced up to 100 times. Though the molds used were not ideal for isolating effects on mold filling due to corner radii, the evidence presented does not show the existence of any strong behavior related to mold radii.     

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.     

Analysis and minimization of void formation during resin transfer molding process

In the resin transfer molding process, residual air in the pores of fiber preform results in dry spots and microvoids in the finished product. The dry spots are usually formed due to irregular permeability of fiber mat and improper injection locations. The microvoids result from non-uniform microarchitecture of the fiber preform, and they are transported through the gap between fiber tows during infiltration of the resin. In this study, a real-time simulation/control method was proposed to actively control the formation and the transport of air voids during the mold filling. The flow equations were solved in real time to predict the change of the flow front shape. The flow front was detected by optical sensors and the control actions were taken based on the sensor signals. Through this automated simulation/control scheme, a real-time control of resin flow could effectively avoid the dry spots and minimize the formation of microvoids by modulating the injection pressure.     

In situ measurement and monitoring of whole-field permeability profile of fiber preform for liquid composite molding processes

For liquid composite molding (LCM) processes, such as resin transfer molding (RTM), the quality of final parts is heavily dependent on the uniformity of the fiber preform. However, the conventional permeability measurement method, which uses liquid (oil or resin) as its working fluid, only measures the average preform permeability in an off-line mode. This method cannot be used to create an in situ permeability profile because of fiber pollution. Further, the conventional method cannot be used to reveal preform's local permeability variations. This paper introduces a new permeability characterization method that uses gas flow to detect and measure preform permeability variations in a closed mold assembly before resin injection. This method is based upon two research findings: (1) resin permeability is highly correlated with air permeability for the same fiber preform with well-controlled gas flow, and (2) the whole-field air permeability profile of a preform can be obtained through measuring the pressure field of gas flow.In this study, first the validity of the gas-assisted, in situ permeability measurement technique was established. Then the technique was demonstrated as effective by qualitatively detecting non-uniformities and permeability variations in fiber performs. Finally, a two-dimensional flow model, based on the finite difference scheme, was developed to quantitatively estimate the whole-field preform permeability profile using predetermined pressure distribution. The efficacy of the new method was illustrated through experimental results.