双尺度纤维织物二维非饱和流动的数值模拟与实验

液体模塑成型工艺(LCM)中非饱和流动的填充模拟对于在虚拟空间中快速、高效地优化工艺参数具有重要意义。采用了一种模拟双尺度纤维织物在等温条件下非饱和流动的双尺度计算模型,通过引入沉浸函数求解宏观-微观流动控制方程组,同时考虑了在微观浸渍中毛细压力的影响,在有限元/控制体积算法中实现了对非饱和流动的数值模拟。随后对三向缝合纤维织物进行了二维径向填充实验,将实验结果与数值模拟的预测值对比。结果表明,该计算模型可以较精确地模拟双尺度纤维织物中的非饱和流动。在此计算模型的基础上,讨论了流体黏度、注射流量及纤维束孔隙率对非饱和填充浸润的影响。结果表明,不同流体黏度、注射流量及纤维束孔隙率对纤维织物填充过程中非饱和区域长度、入口压力曲线及填充时间影响不同。研究结果可以对合理预测纤维织物的浸润及树脂填充过程中入口压力提供指导。     

复合材料热压工艺多物理场耦合数学模型

针对热压工艺特点 , 将预浸料视为可变形的多孔介质 , 通过体积平均 , 建立了固化过程中温度、 纤维应力和树脂流动多物理场耦合的数学模型。该模型考虑了纤维变形和体积分数变化的影响 , 反映了渗透率和纤维体积分数的关系。对于复合材料层合平板热压工艺 , 通过进一步简化 , 给出了一维固化方程 , 并进行了有限元数值分析。数值模拟中采用 AL E移动网格方法来处理动边界问题。计算结果表明 , 与非耦合的经典模型相比 , 该模型给出的结果能更好地与实验吻合 , 层合板逐层压缩现象也和实验结果一致。而且该模型能预测树脂的排出量和纤维层间纤维体积分数的变化。     

基于表面张力和流体压力耦合分析的树脂传递模塑过程的细观模拟

树脂传递模塑工艺(RTM)是纤维增强聚合物基复合材料的重要制备工艺之一。当树脂的宏观、细观流动前沿不一致时易于形成孔隙缺陷,从而降低复合材料性能。文中根据RTM工艺过程特点,建立树脂在纤维束间流动的轴对称模型,进而根据流体力学和表面科学,建立树脂流体的表面张力和流体压力耦合分析的方法,并采用流体体积函数(VOF)方法追踪流动前沿,分析温度、纤维束间距、流动速度等工艺参数对流体压力和前沿的影响规律。     

恒压下树脂在双尺度多孔介质中的流动特性

基于复合材料液态模塑(LCM)工艺过程中存在半饱和区域的实验现象以及对预制体双尺度效应的逐步认识, 一些学者提出用沉浸模型来研究双尺度多孔介质的不饱和流动。通过体积均匀化方法描述了双尺度多孔介质复合材料液态模塑工艺模型的特征, 得到含有沉浸项的双尺度多孔介质的质量守恒方程, 并采用有限元法对方程进行数值求解, 通过具体算例计算了考虑双尺度效应时恒压树脂注射下不同时段的压力分布状态, 得到树脂在填充过程中流动前沿半饱和区域从出现到消失的过程, 采用不同注射压力进行模拟并比较。结果表明, 与单尺度多孔介质模型不同, 双尺度多孔介质模型更能反映实际树脂填充过程中出现的半饱和区域现象。     

基于材料性能时变特性的复合材料固化过程多场耦合数值模拟

针对热固性树脂基复合材料固化过程中各种复杂的物理化学变化之间的相互影响,建立了基于材料性能时变特性的复合材料固化过程的二维多场耦合计算模型。该模型由已知的3个经典复合材料固化过程子模型构成,包括热-化学模型、树脂黏度模型和树脂流动模型。在此基础上,将固化过程中材料性能的时变特性引入多场耦合计算模型中。通过与文献中实验结果的比较,证明了所建立的模型具有较高的可靠性。对AS4/3501-6复合材料层合平板的固化过程进行了数值模拟,重点研究了固化过程中纤维体积分数变化及材料参数的时变特性对固化过程中温度、固化度和树脂压力等参量的影响。分析结果表明:考虑纤维体积分数变化和材料性能的时变特性后,固化过程中复合材料层合板中心温度峰值明显减小,树脂压力随时间的变化将有所滞后。     

螺杆式粉料计量包装机粉料流动分析与数值仿真

分析了粉料在定量充填系统内的流动过程。在PRO/E中建立几何模型,利用流体仿真软件FLUENT对粉料物料在螺杆旋转条件下的流动过程进行数值模拟,分析不同的螺杆参数对充填计量的影响规律,从而为确定合理的螺杆几何参数及外部条件提供依据。     

基于化学流变学的复合材料液态模塑成型过程的灵敏度分析

为了探讨复合材料液态模塑成型（LCM）过程中充填时间和树脂流动前锋形状对材料参数及工艺参数的敏感程度,考虑树脂非稳态浸润过程中的边缘效应以及固化反应现象,引入灵敏度分析方法,推导了模腔内流体压力灵敏度和流体速度灵敏度等关键物理量参数之间所满足的数学关系,构建了充填时间灵敏度方程以及表征材料浸润缺陷形成可能性的树脂流动前锋形状函数及其灵敏度方程,并设计了各物理量的耦合求解方法及灵敏度分析的技术路线。在此基础上,自主开发了数值模拟软件,数值分析了关键材料和工艺参数对树脂流场发展的影响规律和程度。模拟结果表明,在恒压注射边界条件下,提高流体注射温度是提高生产效率最有效的方法,减少边缘区域渗透率则是最能改善树脂流动前锋形状以及充填浸润效果的途径。     

缝合泡沫夹芯结构复合材料VARTM工艺树脂充填模拟及验证

采用"长条"模型代替泡沫中沿缝线方向树脂流动模型并对其等效渗透率及孔隙率进行计算;然后用PAM-RTM软件对缝合泡沫夹芯结构真空辅助树脂传递模塑成型(VARTM)工艺树脂充填过程进行数值模拟,最后建立了流动可视化实验装置对模拟结果进行实验验证。对比模拟与实验结果表明,模拟与实验完成整个充填过程的时间非常接近,并且树脂在预成型体中流动方式及在预成型体上、下面板流动状态的数值模拟与实验结果也非常地一致。故建立的"长条"模型及其相关参数的计算是合理的,可以用来准确地预测缝合泡沫夹芯结构VARTM工艺树脂充填过程。另外,树脂在上面板流动前沿在沿宽度方向较为统一,而树脂在下面板流动前沿呈现锯齿状,容易产生空隙及干斑等缺陷,影响制品的质量。     

考虑孔隙缺陷的CFRP微观切削仿真与实验研究

碳纤维增强树脂基复合材料(CFRP)在航空航天等领域应用广泛。在CFRP制造过程中难以避免会产生孔隙等缺陷,对后续的切削加工造成一定影响。在考虑了CFRP成型过程形成的孔隙缺陷基础上,运用有限元仿真模拟方法,从纤维-树脂-界面尺度建立了含孔隙缺陷的CFRP微观切削仿真模型,研究了不同孔隙率条件下不同纤维排布方向的CFRP微观切削行为,并通过实验验证了仿真模型的正确性。研究结果表明:孔隙的存在会增加刀具的“空切”现象,从而对CFRP切削过程的切削力、材料破坏及亚表面损伤、材料能量等产生影响。随孔隙率的增加,切削力呈下降趋势,孔隙边缘的纤维产生整体断裂的倾向增加;孔隙对0°、45°和135°纤维排布方向的CFRP切削加工的面下损伤影响不大,在纤维排布方向为90°条件下,孔隙率高于3vol%时对加工表面的面下损伤具有较大影响;在材料内部能量耗散方面,“顺切”(纤维方向角小于90°)时的总耗散能低于“逆切”,随孔隙率增加,总耗散能降低。      

基于树脂流动的变截面复合材料结构固化过程热-流-固多场强耦合数值仿真

在考虑树脂流动对固化温度场影响的基础上,将树脂流动引入经典热-化学模型,并在考虑了固化过程材料性能时变特性条件下,建立了复合材料热-流-固多场强耦合有限元模型。通过对比文献中未考虑树脂流动对温度场的影响,本文所建模型温度场较实际结果的最大温差更低,厚度密实精度更高,模型可靠性更好。基于所建热-流-固强耦合有限元模型,对变截面复合材料结构固化过程进行数值仿真。研究发现,变截面复合材料结构较厚区域存在明显温度场、固化度场及树脂流场分布梯度,纤维体积分数分布不均性较大,这与结构不同区域的厚度、固化过程温度传递滞后及局部树脂流动受固化效应不同步产生的影响有关。变截面复合材料结构厚度由3.52 mm增加至42.24 mm,截面最大温差由0.3℃增加到34.3℃,纤维体积分数分布不均匀性由0.1%增加到1.3%。     

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.     

Governing equations for unsaturated flow through woven fiber mats. Part 1. Isothermal flows

Correct modeling of resin flow in liquid composite molding (LCM) processes is important for accurate simulation of the mold-filling process. Recent experiments indicate that the physics of resin flow in woven (also stitched or braided) fiber mats is very different from the flow in random fiber mats. The dual length-scale porous media created by the former leads to the formation of a sink term in the equation of continuity; such an equation in combination with the Darcy's law successfully replicate the drooping inlet pressure history, and the region of partial saturation behind the flow-front, for the woven mats. In this paper, the mathematically rigorous volume averaging method is adapted to derive the averaged form of mass and momentum balance equations for unsaturated flow in LCM. The two phases used in the volume averaging method are the dense bundle of fibers called tows, and the surrounding gap present in the woven fiber mats. Averaging the mass balance equation yields a macroscopic equation of continuity which is similar to the conventional continuity equation for a single-phase flow except for a negative sink term on the right-hand side of the equation. This sink term is due to the delayed impregnation of fiber tows and is equal to the rate of liquid absorbed per unit volume. Similar averaging of the momentum balance equation is accomplished for the dual-scale porous medium. During the averaging process, the dynamic interaction of the gap flow with the tow walls is lumped together as the drag force. A representation theorem and dimensional analysis are used to replace this drag force with a linear function of an average of the relative velocity of the gap fluid with respect to the tow matrix for both the isotropic and anisotropic media. Averaging of the shear stress term of the Navier–Stokes equation gives rise to a new quantity named the interfacial kinetic effects tensor which includes the effects of liquid absorption by the tows, and the presence of slip velocity on their surface. Though the gradient of the tensor contributes a finite force in the final momentum balance equation, a scaling analysis leads to its rejection in the fibrous dual-scale porous medium if the permeability of flow through the gaps is small. For such a porous medium, the momentum equation reduces to the Darcy's law for single-phase flow.     

Prediction of optimal flow front velocity to minimize void formation in dual scale fibrous reinforcements

Liquid Composite Molding (LCM) is an increasingly used class of processes to manufacture high performance composites. Engineering fabrics commonly used in LCM generally have a dual scale architecture in terms of porosity: microscopic pores exist between the filaments in the fiber tows, while macroscopic pores appear between the tows. Capillary flows in fiber tows play a major role on the quality of composites made by resin injection through fibrous reinforcements. This paper reports on an investigation on fabric imbibition characterization and subsequent evaluation of the optimal flow front velocity during resin injection through fibrous reinforcements. The goal is to devise more robust LCM processes and improve part quality. In order to evaluate a priori the injection conditions that minimize void formation, an impregnation model is developed based on imbibition characterization. This approach allows predicting the optimal front velocity without having to model complex dual scale flows through fibrous reinforcements and without performing expensive and time-consuming fabrication tests. After a summary of previous imbibition results obtained with a probe fluid, the optimal modified capillary numbers are computed by the new predictive model and the values are compared with results reported in the literature on void formation in LCM processes. Afterwards, capillary rise measurements are carried out with four infiltration fluids in order to evaluate the range of optimal flow front velocity that minimizes void formation. This characterization is implemented with vinyl ester resin, epoxy anhydride resin, styrene and anhydride. Finally, the optimal flow front velocity is evaluated for several fabric configurations.     

Variations in unsaturated flow with flow direction in resin transfer molding: An experimental investigation

The dual-scale nature of fiber preforms due to the presence of large continuous gaps between fiber tows gives rise to the unsaturated flow in resin transfer molding (RTM) process which is characterized by a droop in the injection pressure history due to the delayed absorption of fiber tows (the ‘sink’ effect). In this study, we experimentally investigate the effect of change in flow direction on the unsaturated flow in three anisotropic dual-scale fiber mats. A series of 1-D mold-filling experiments involving a constant flow rate were conducted for a unidirectional woven fiber-mat, a biaxial stitched mat, and a triaxial stitched fiber-mat along with a reference single-scale random mat. In the case of the unidirectional mats, the droop in the inlet-pressure history, signifying the strength of the sink effect, is found to be strongest for flow along the micro-channels aligned with fiber tows. The droop, and hence the sink effect, is observed to weaken progressively for flow-directions at 45° and 90° to this principal direction. In the case of the biaxial and triaxial mats, the situation is more complex due to the multi-layer construction of such mats: maximum droop is found when mats are oriented at a 45° angle with respect to the fiber-mat coordinate, and it weakens in the 0° and 90° directions. The unsaturated flow effect is also quantified by measuring percentage deviation in the area under the experimental curve from that of the predicted curve. A clear correlation between the droop (through the percentage deviation) and the permeability along a flow direction in the unidirectional mats is observable, though such a relationship eludes the triaxial mat. The effect of unsaturated flow on liquid-front progress during the 1-D experiment was also studied. In contrast to the reference single-scale random mat where the observed front progress closely follow the prediction based on the single-scale physics, a small difference was observed between the observed and predicted front progress for the three dual-scale mats considered. However the difference was too small to yield any significant correlation with the flow direction.     

Three-dimensional (3D) modeling of the thermoelastic behavior of woven glass fiber-reinforced resin matrix composites

The thermoelastic behavior of glass fiber-reinforced resin matrix composites is very important in several applications such as electronic packaging. Simulation of the composite behavior is complicated because of the complex nature of woven fiber architecture. In this study, we have conducted a numerical simulation of elastic and thermal expansion behavior of woven glass fiber-reinforced resin matrix composite. The simulations were compared to experimental data, showing excellent agreement with elastic properties and fairly good results for the thermal expansion coefficient of the composite.     

Modeling forces generated within rigid liquid composite molding tools. Part A: Experimental study

The term liquid composite molding (LCM) encompasses a growing list of processes, including resin transfer molding (RTM), injection/compression molding (I/CM), and resin infusion (a.k.a. VARTM). All LCM techniques involve compressive deformation of the fiber reinforcement prior to, and in many cases during mold filling. Forces acting on molds are primarily due to the requirement to compact the reinforcement, and pressure generated due to resin flow through these fibrous structures. An experimental study of the forces exerted on a mold during the RTM and I/CM processes is presented here. Two reinforcing materials have been considered, exhibiting significantly different resistance to compaction. The evolution of mold clamping force has been shown to be strongly influenced by the complex, non-elastic compaction behaviour of fiber reinforcements. The important effects include stress relaxation, an apparent lubrication by the injected fluid, and permanent deformation. Efforts to simulate the experiments will be presented in Part B of this study.     

A model for predicting uniaxial compression behavior of fused fibrous networks

Fused fibrous networks are increasingly being used for emerging industrial applications ranging from thermal/sound insulation, fluid filtration/separation, and energy conversion to tissue scaffolds. Majority of these applications need a deeper understanding of fused fibrous networks under compression loading. In this research work, a compression model of fused fibrous networks has been proposed by defining two distinct regions displaying the bending of free fiber segments between the fiber-to-fiber contacts followed by the transverse compression of fiber contacts through classical Hertzian contact mechanics approach. The mechanistic models developed in this study, have clearly elucidated the main fiber and structural parameters that control the compression behavior of fused fibrous networks. A comparison has also been made between the theoretical and experimental pressure–strain curves of randomly and preferentially aligned fused fibrous networks.     

Three-Dimensional Flow Simulations for the Resin Transfer Molding Process

In resin transfer molding (RTM) a stack of fiber mats or woven rovings is laid inside the mold cavity. Then the mold is sealed and resin is injected. The computer simulation of the injection phase in resin transfer molding (RTM) can help the mold designer to position properly the injection ports and the air vents, to select an adequate injection presssure and to optimize the cycle time. The purpose of this article is to present a finite element simulation model of the filling process that can be applied to three-dimensional “thin shell” molds. An application to a subway seat is described to illustrate the various stages of the simulation     

三维机织碳纤维/环氧树脂复合材料在两种测量方法下的热响应机制对比

采用瞬态热线法和闪光法分别测量了多种结构参数的三维机织碳纤维/环氧树脂复合材料的导热系数。通过对3D正交机织碳纤维/环氧树脂复合材料的有限元模拟可以看出,3D正交机织碳纤维/环氧树脂复合材料内经纱、纬纱和Z向纱的导热作用在不同的受热形式下会发生变化。采用瞬态热线法测量时,2.5D机织碳纤维/环氧树脂复合材料的导热系数低于2.5D经向增强结构,同时高于3D正交结构,而采用闪光法测量时,2.5D经向增强和3D正交碳纤维/环氧树脂复合材料的导热系数均小于2.5D机织结构。这是由于在使用不同的测量方法时,三维机织碳纤维/环氧树脂复合材料内部相同的纱线系统在导热过程中所起的作用并不相同。随着纤维体积含量的提高,瞬态热线法和闪光法测得的2.5D机织碳纤维/环氧树脂复合材料的导热系数都在不断提高。由于经纱的屈曲,采用闪光法测量时,导热性能提升更加明显。研究结果表明,三维机织碳纤维/环氧树脂复合材料在不同受热形式下具有不同的热响应机制。     

Coupling filtration and flow during liquid composite molding: Experimental investigation and simulation

Molding composites constituted of fiber reinforcements, resin and fillers is of prime interest for many transportation applications. Dealing with the flow of particle-filled resin in a fibrous network raises the issue of particle retention and viscosity increase. The present study aims at simulating such molding through an efficient coupling between a filtration model, that has been previously described, and a flow model (Darcy’s law). First, filling experiments are realized so as to separate cases: cake filtration, retention and no retention for two types of single-scale porous materials (polyester felt and glass fiber mat) injected with a resin filled with micro-beads. Then results of filler content, viscosity, permeability, pressure, retention profiles are simulated from the coupling between filtration and flow. Experimental data of filler profiles in the final parts, resin flow front evolution and injection times are compared with predictions obtained from the simulation.