铺层方式对织物渗透率的影响

增强材料的渗透率是研究和模拟RTM工艺的关键参数。采用单向玻璃纤维织物,在恒定的注射压力下,通过记录树脂在织物内流动前缘椭圆长、短轴随时间的变化,利用Darcy定律测定了不同铺层方式下织物的渗透率,研究了铺层方式对织物渗透率的影响。实验证明,铺层方式改变时,织物的渗透率大小和主轴方向发生改变,其关系符合椭圆的叠加原理。      

国产CCF300碳纤维单向织物液体成型工艺性及其复合材料力学性能

选取国产碳纤维CCF300所制备的2种单向织物,单向无纬织物U3160及单向无屈曲织物KUC160,分别对其预成型体进行压缩特性和渗透特性测试,以研究2种单向织物的液体成型工艺性,并采用树脂传递模塑(RTM)工艺制备2种单向织物/双马来酰亚胺树脂基复合材料,测试并对比其面内力学性能。结果表明:预成型体压缩试验中,嵌套效应受压力及织物层数影响较大,压力越高、层数越多,嵌套效应越显著。U3160织物的嵌套效应较KUC160织物更为明显,在较高压力下,KUC160织物预成型体的纤维体积分数较U3160织物的下降了约20%。渗透率测试结果表明:相比U3160织物,KUC160织物0°方向的渗透率较高,而90°方向的渗透率有所降低;这是由于经编线的绑缚作用能促进0°方向的宏观流动,而阻碍90°方向的微观渗透。此外,KUC160织物的经编线与U3160织物的纬向纱线的导流作用也对渗透率有影响。力学性能试验结果表明:相比U3160织物增强复合材料,KUC160织物增强复合材料0°方向的拉伸、弯曲和压缩性能均有所下降,拉伸强度和弯曲模量降幅最大,分别约为11%和21%;而层间剪切强度有小幅提高,增幅约为8%。      

传感监测LCM成型厚铺层结构增强体内部渗透性能研究

采用传感监测系统辅助对厚铺层结构纤维增强体内部流动情况进行了实验研究.结果表明:渗透率随铺层厚度的增加逐渐减小,并趋于平缓;测试液在铺层厚度增强体顶层和底层渗透存在一定量的差异,并随铺层厚度的增加而增大.在不影响结构要求的前提下,导流布与长丝毡的配合使用是改善厚铺层结构渗透性能的有效方法.     

Complex three‐dimensional microstructural permeability prediction of porous fibrous media with and without compaction

Much of the work in permeability prediction has so far been done with respect to the in‐plane properties. Guided by Darcy's law, the in‐plane permeabilities (i.e. Kxx and Kyy) have been well characterized by researchers both experimentally and, to some extent, analytically and numerically. Work on transverse or through thickness permeability, however, has been sparse. Owing to the fact that the limited length scale in the through thickness direction of most fibre preforms makes transverse permeability a difficult value to measure experimentally, the objective of the present development is to study the feasibility of applying the methodology proposed by the authors in a previous work for in‐plane permeability prediction to the estimation of transverse, as well as the in‐plane, permeability of a typical 3D woven fibre preform. The additional objective of this work is to present a preliminary study on the effects of fibre mat compression on the fibre preform permeability. Copyright © 2004 John Wiley & Sons, Ltd.     

织物预成型体厚度方向渗透特性研究

以达西定律为基础,研究了不同类型织物的织物层数、定型剂质量分数以及铺层角度对织物预成型体厚度方向上渗透性能的影响.实验结果表明,在一定的定型剂质量分数下,测试流体在垂直方向上的流动基本为稳态流动,大体受Darcy定律控制.随着织物层数的增加,流体流速减小,预成型体厚度方向渗透率逐渐增加.随定型剂质量分数的增加,厚度方向渗透率表现出先增大而后又减小的趋势,而且织物的铺层角度对预成型织物厚度方向渗透率也具有较大影响.     

Permeability of natural fiber reinforcement for liquid composite molding processes

We investigate the influence of liquid type on the saturated permeability of natural fabrics in liquid composite molding processes. The permeability of flax woven fabric was characterized with two different liquids which have different viscosity, wettability, and sorption characteristics with flax fiber. From the experimental data, it was observed that the saturated permeability values were different for the liquid type. The fiber swell during the mold filling process and the corresponding change of fabric microstructure were assumed to be the main reason for this dependency of saturated permeability on the liquid type. The fiber swell due to the liquid sorption was characterized as a function of time, and the corresponding change of fiber diameter was investigated. The effective fiber volume fraction of wet natural fabric was defined in terms of fiber swelling ratio. The predictions by the classical Kozeny–Carman model and by the modified Kozeny–Carman model with two model constants were compared with the experimental data. It was shown that the modified Kozeny–Carman equation considering fiber swell could predict very well the saturated permeability of natural fabrics regardless of liquid type.     

Effect of Consolidation Method on the Mechanical Properties of Nonwoven Fabric Reinforced Composites

Nonwoven fabrics provide low cost reinforcement for composites. The consolidation behavior of E-glass nonwoven preforms and the mechanical properties of the resulting composites are studied. The reinforcement structures include stitch-bonded chopped-strand mat, powder-bonded chopped-strand mat, and continuous-strand mat. A woven fabric is also included for comparison. The bulkiness of the fabric influences their processibility in the hand lay-up and resin transfer molding (RTM) processes. Mechanical properties of specimens from the hand lay-up and the RTM processes are evaluated in tensile, compressive, and flexural tests. The test results and failure mechanisms are reported, and the effect of consolidation condition on the mechanical properties are analyzed. Theoretical analysis of the elastic properties of nonwoven composites are discussed. The tensile moduli from the experiment for such composites are compared with theoretical predictions and good agreement is observed.     

层间纳米纤维膜对玻纤预制体渗流特性的影响

为了研究层间纳米纤维膜对玻纤织物渗流特性的影响,使用超景深三维显微镜表征了纳米纤维含量对玻纤织物微观结构的影响,采用径向法测量了纳米纤维膜夹层玻纤织物预制体的渗透率,重点分析了纳米纤维含量对玻纤织物预制体渗流模式的影响。结果表明:玻璃纤维束间的毫米尺度区域被纳米纤维膜填充而离散成微米尺度区域;预制体孔隙率及渗透率值均随着纳米纤维含量的增加而减小;随着纳米纤维含量的增加,复合预制体表现出的各向异性程度逐渐减小;树脂宏观流动前沿内部分饱和区域面积比例随纳米纤维含量的增加而增大;相同纳米纤维含量预制体的部分饱和区域面积比例随注入时间的增加呈先增大后减小趋势。     

Lattice Boltzmann method based computation of the permeability of the orthogonal plain-weave fabric preforms

Changes in the permeability tensor of fabric preforms caused by various modes of fabric distortion and fabric-layers shifting and compacting is one of the key factors controlling resin flow during the infiltration stage of the common polymer-matrix composite liquid-molding processes. While direct measurements of the fabric permeability tensor generally yield the most reliable results, a large number of fabric architectures used and numerous deformation and layers rearrangement modes necessitates the development and the use of computational models for prediction of the preform permeability tensor. The Lattice Boltzmann method is used in the present work to study the effect of the mold walls, the compaction pressure, the fabric-tows shearing and the fabric-layers shifting on the permeability tensor of preforms based on orthogonal balanced plain-weave fabrics. The model predictions are compared with their respective experimental counterparts available in the literature and a reasonably good agreement is found between the corresponding sets of results.     

“离位”增韧预成型体压缩特性

针对"离位"增韧预成型体的液态成型工艺性,研究了两种不同结构形式增韧层"离位"增韧预成型体的厚度压缩特性。分别采用多孔薄膜结构增韧层、高孔隙率无纺布结构增韧层与碳纤维织物交替铺层制备"离位"增韧预成型体,采用力学试验机测试其厚度压缩特性。实验结果表明,预成型体压缩过程中,在相同压力水平下,增韧层的引入会降低预成型体的纤维体积分数;不同压力水平下,预成型体的压缩行为与增韧层结构形式有关。此外,采用织物预成型体厚度压缩本构模型,对"离位"增韧预成型体的压缩厚度进行了预测,通过模型预测值与实验值的比较,确定了模型中的经验指数k=2时,两者吻合较好。     

Effective permeabilities of multilayer fabric preforms in liquid composite moulding

A homogenisation method was developed to predict effective permeabilities of multilayer fabric preforms by considering interlayer continuity and coupling between in-layer flow and trans-layer flow. A simplified approach was addressed first, from which the common approaches of arithmetic mean formation and harmonic mean formation can be deduced to define the effective permeability by neglecting coupling between in-layer flow and trans-layer flow. For multilayer preforms in liquid composite moulding, the permeability predicted by such an approach, however, is generally different from the actual one because of neglecting effects of micro-structure between layers and synchronisation between in-layer flow and interlayer flow. A conceptual model for interlayer flow, based on the hydraulic radius theory, was then proposed to quantitatively characterise the effect of interlayer micro-structure on the effective permeability of multilayer fabric preforms. The predictions show good agreement with experimental results available in literature.     

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.     

Continuous transverse permeability of fibrous media

The compaction of composite preforms and the flow of resin through the fibrous network take place simultaneously during the Resin Film Infusion process. Therefore there is a coupled loading of the porous reinforcements. A new experimental device to impose combinations of hydraulic and mechanical loadings (Hydro-Mechanical loadings) to fibrous preforms is used to evaluate the transverse permeability in a continuous manner (under Hydro-Mechanical conditions) for a flax mat, a flax non-crimped fabric, a carbon plain weave and a glass satin weave. For a 0.5 mm/min compression speed the continuous technique gives values similar to the “classical” technique for the four composite reinforcements of very different nature. This suggests that it is possible to use the continuous technique to evaluate the transverse permeability behaviour of fibrous reinforcements (permeability vs. fibre volume) with a time reduction of about 8–10. Increasing the compression speed gives slowly decreasing continuous transverse permeability values.     

Evaluation of through-thickness permeability and the capillary effect in vacuum assisted liquid molding process

Through-thickness penetration under vacuum assistance is crucial for resin film infusion (RFI) and vacuum assistant resin transfer molding (VARTM) process. In this paper, values of the through-thickness unsaturated permeability (TTUP) and capillary pressure (P_c) are estimated based on the infiltration velocity in preforms of carbon fiber fabric and glass fiber fabric, respectively, measured by a specially designed apparatus. It reveals that, for the through-thickness permeation, the P_c values generally decrease with increasing fiber content. Relatively accurate TTUP can be obtained by counting P_c into the permeation dynamics. If P_c is neglected, liquids with good-wettability, such as silicone oil, tend to result in larger TTUPs. The corrected TTUPs show good agreement according to Carman–Kozeny, Gutowski modified Carman–Kozeny equation, and Gebart model, respectively. The resultant permeability resistance parameters of the preforms indicate that the penetration in carbon fabric bed is slower than in glass fabric bed. However, for fiber volume fraction more than 60%, the corrected TTUPs show no significant difference for all the preforms.     

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。      

Thermoplastic composite from innovative flat knitted 3D multi-layer spacer fabric using hybrid yarn and the study of 2D mechanical properties

Due to their improved mechanical properties, 3D multi-layer spacer fabrics could be used for lightweight applications such as textile-based sandwich preforms. Modern flat knitting machines using high performance yarns are able to knit complex 3D multi-layer spacer fabrics consisting of individual surface and connecting layers. This paper reports on the development of 3D flat knitted spacer fabric for 3D thermoplastic composites using hybrid yarns made of glass (GF) and polypropylene (PP) filaments. Moreover, mechanical properties of reinforcement yarns, 2D knit fabrics and 2D composites manufactured using various integration methods of reinforcement yarns were also studied. The integration of reinforcement yarns as biaxial inlays (warp and weft yarns) is found to be the best solution for knitting, whereas the tuck stitches show optimal results.     

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。      

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.     

A micromechanical compaction model for woven fabric preforms. Part II: Multilayer

With nesting between adjacent layers and inter-layer packing, the microstructure and the compaction behaviour of a multilayer woven fabric preform are much more complicated than those of a single layer fabric preform. A micromechanical model, based on the hierarchical structure characteristics of woven fabric preforms, was developed to investigate the elastic compaction behaviour of multilayer plain weave fabric preforms. The compaction mechanisms of fabrics at different hierarchical levels including deformation and compaction of yarn cross-section, flattening of yarn waveform, nesting between adjacent layers and inter-layer packing, are considered in an integrated approach in this predictive model. Effects of structural elements at different hierarchical levels on compaction behaviour of multilayer plain weave fabric preforms are investigated in detail. Both the number of layers and shifting are shown to have significant effects on compaction behavior, while the effect of nesting increases as the number of layers increases. The predictions by this model are correlated well with the experimental data.     

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.