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.     

Transient gas flow technique for inspection of fiber preforms in resin transfer molding

A transient gas flow method was developed to determine the quality of fibrous preforms in resin transfer molding (RTM) prior to resin injection. The method aims at detecting defects resulting from preform misplacement in the mold, accidental inclusions, preform density variations, race tracking, shearing, etc. Unlike the previously developed method based on steady-state gas flow, the new method allows for the acquisition of continuous time-varying pressure data from multiple ports during a single test. The validity of the method was confirmed by one-dimensional flow experiments.     

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.     

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.     

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。      

LCM 充模过程中的边缘效应

边缘效应是复合材料液体模塑成型技术(Liquid composites molding , LCM) 中常见的纤维预成型体铺敷缺陷之一。采用单向流动法研究了边缘效应对纤维预成型体渗透率及充模过程的影响, 结合其等效渗透率的理论预测模型对不同纤维体积含量、不同缝隙宽度条件下的边缘效应进行了模拟与分析, 提出了一边缘效应强弱的表征因子, 并以一较复杂的模腔的充模过程为实例提出了对边缘效应的在线监控策略及处理方案。      

Mathematical Analysis of Resin Flow through Fibrous Porous Media

Resin flow through fiber preforms was analyzed mathematically. Closed form solutions for fiber volume fraction distribution and pressure field during resin infusion into fiber preforms were suggested, and a new effective permeability was defined. The effect of preform compressibility on the fiber volume fraction and pressure distributions in resin-saturated region was investigated analytically. The findings show that the compaction behavior of preforms has significant impact on the resin infusion process. The solutions derived analytically in this study can provide insight into a liquid composites molding (LCM) process.     

Flow analysis during compression of partially impregnated fiber preform under controlled force

Compression resin transfer molding (CRTM) is an alternative solution to conventional resin transfer molding processes. It offers the capability to produce net shape composites with fast cycle times making it conducive for high volume production. The resin flow during this process can be separated into three phases: (i) metered amount of resin injection into a partially closed mold containing dry fiber preform, (ii) closure of the mold until it is in contact with the fiber preform displacing all the resin into the preform and (iii) further mold closure to the desired thickness of the part compacting the preform and redistributing the resin. Understanding the flow behavior in every phase is imperative for predictive process modeling that guarantees full preform saturation within a given time and under specified force constraints.     

Analysis of vacuum bag resin transfer molding process

An analytical model is developed to analyze the resin flow through a deformable fiber preform during vacuum bag resin transfer molding (VBRTM) process. The force balance between the resin and the fiber preform is used to account for the swelling of fiber preform inside a flexible vacuum bag. Mold filling through multiple resin inlets is analyzed under different vacuum conditions. The formation of dry spots is demonstrated in the presence of residual air. Molding of a three-dimensional ship hull with lateral and longitudinal stiffeners is simulated to demonstrate the applicability of the model.     

Simulation of void formation in liquid composite molding processes

Air entrapment within and between fiber tows during preform permeation in liquid composite molding (LCM) processes leads to undesirable quality in the resulting composite material with defects such as discontinuous material properties, failure zones, and visual flaws. Essential to designing processing conditions for void-free filling is the development of an accurate prediction of local air entrapment locations as the resin permeates the preform. To this end, the study presents a numerical simulation of the infiltrating dual-scale resin flow through the actual architecture of plain weave fibrous preforms accounting for the capillary effects within the fiber bundles. The numerical simulations consider two-dimensional cross sections and full three-dimensional representations of the preform to investigate the relative size and location of entrapped voids for a wide range of flow, preform geometry, and resin material properties. Based on the studies, a generalized paradigm is presented for predicting the void content as a function of the Capillary and Reynolds numbers governing the materials and processing. Optimum conditions for minimizing air entrapment during processing are also presented and discussed.     

Numerical simulation of injection/compression liquid composite molding. Part 1. Mesh generation

This paper presents a numerical simulation of injection/compression liquid composite molding, where the fiber preform is compressed to a desired degree after an initial charge of resin has been injected into the mold. Due to the possibility of an initial gap at the top of the preform and out-of-plane heterogeneity in the multi-layered fiber preform, a full three-dimensional (3D) flow simulation is essential. We propose an algorithm to generate a suitable 3D finite element mesh, starting from a two-dimensional shell mesh representing the geometry of the mold cavity. Since different layers of the preform have different compressibilities, and since properties such as permeability are a strong function of the degree of compression, a simultaneous prediction of preform compression along with the resin flow is necessary for accurate mold-filling simulation. The algorithm creates a coarser mechanical mesh to simulate compression of the preform, and a finer flow mesh to simulate the motion of the resin in the preform and gap. Lines connected to the top and bottom plates of the mold, called spines, are used as conduits for the nodes. A method to generate a surface parallel to a given surface, thereby maintaining the thickness of the intermediate space, is used to construct the layers of the preform in the mechanical mesh. The mechanical mesh is further subdivided along the spines to create the flow mesh. Examples of the three-dimensional meshes generated by the algorithm are presented.     

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%。     

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。     

Preform permeability variation with porosity of fiberglass and carbon mats

An experimental investigation on fiber bed permeability variation with porosity for three types of reinforcement mats is performed. The reinforcements consist of plain-weave carbon, plain-weave fiberglass, and chopped fiberglass mats. Resin flow experiments are performed in a rectangular cavity with different fiber volume fractions. RL 440 epoxy resin is used as the working fluid in the experiments. Several layers of mats are laid inside the mold in each experiment and resin is injected at a constant pressure. The effects of reinforcement type and porosity on fiber bed permeability are investigated. Fiber mat permeability of woven mats show large degrees of anisotropy. Resin flow in chopped fiberglass mats is circular, suggesting an isotropic permeability tensor. In all the three cases, preform permeability increases with fiber bed porosity in a non-linear fashion. The results of this investigation could be employed in optimization of liquid composite molding manufacturing processes.     

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.     

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。      

Measurement of transverse permeability using gaseous and liquid flow

Accurate measurement of transverse permeability is important for processes such as resin film infusion and vacuum-assisted resin transfer molding. In these liquid composite molding processes the out-of-plane flow is dominant and thus the transverse permeability is needed for flow prediction. This paper introduces an apparatus to measure saturated permeability for fibrous preforms using both gaseous and liquid flow. The setup creates a uniform one-dimensional flow through-the-thickness of the reinforcement by integrating a high permeability layer on the mold surfaces. A wide range of permeability as a function of fiber volume fraction can be measured in one experiment while applying a known load under a hydraulic testing machine. The system has been designed using process simulation. The measurements using the gaseous medium are comparable to the saturated fluid flow results. The measurement system can also be used to measure changes in dry fabric permeability prior to infusion due to debulking or application of binders on the fabric surface.     

Permeability measurement of textile reinforcements with several test fluids

In liquid composite moulding (LCM) techniques, the liquid resin has to flow a long distance to impregnate the dry fibres. The measure for the resistance of the fibre preform to the resin flow is the permeability of the fibre preform. Because of the dual-scale porous structure of the textile preforms, test fluid can influence the unsaturated permeability values through the interaction of the fluid and fibres. In this study, the influence of test fluid on the permeability measurement of several types of textile reinforcements is investigated. First the contact angle of various fluids and fibres was measured. Then the permeability measurement of the textile reinforcements was carried out. The results showed that the influence of test fluid is small under the test conditions.     

In-plane permeability characterization of the vacuum infusion processes with fiber relaxation

In vacuum infusion processes fiber preforms are placed onto the single molding surface and enveloped with a non-rigid polymer bag which is sealed to the molding surface. The flexible bagging film does deform during the resin infusion process thus changing the compaction of the fabric. However, one can also relax the preform by drawing a partial vacuum in a rigid chamber placed on top of the flexible bag which will increase the permeability of the fabric under the chamber. A numerical model is presented to characterize the change in permeability and describe the mold filling for such processes in which the fabrics undergo controlled relaxation by external stimuli. The predictions from the simplified model agreed reasonably well with the experiments. This characterization and resin flow front prediction with time method should prove useful in processes such as Vacuum Induced Preform Relaxation (VIPR) process which can be used to actively manipulate flow in a vacuum infusion process.     

Numerical simulation of injection/compression liquid composite molding. Part 2: preform compression

In the injection/compression liquid composite molding process (I/C-LCM), a liquid polymer resin is injected into a partially open mold, which contains a preform of reinforcing fibers. After some or all of the resin has been injected, the mold is closed, compressing the preform and causing additional resin flow. This paper addresses compression of the preform, with particular emphasis on modeling three-dimensional mold geometries and multi-layer preforms in which the layers have different mechanical responses. First, a new constitutive relation is developed to model the mechanical response of fiber mats during compression. We introduce a new form of nonlinear elasticity for transversely isotropic materials. A special case of this form is chosen that includes the compressive stress generated by changes in mat thickness, but suppresses all other responses. This avoids the need to model slip of the preform along the mold surface. Second, a finite element method, based on the principle of virtual displacement, is developed to solve for the deformation of the preform at any stage of mold closing. The formulation includes both geometric and material nonlinearities, and uses a full Newton–Raphson iteration in the solution. An open gap above the preform can be incorporated by treating the gap as a distinct material layer with a very small stiffness. Examples show that this approach successfully predicts compression in dry preforms for three-dimensional I/C-LCM molds.