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。      

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。      

Analysis of Transversal Permeability for Different Types of Glass Fiber Reinforcement

In some of the new processes to produce elements in composite materials, the innovation is due to the fact that the resin flow is perpendicular to the plane of the reinforcement during the impregnation.So a very important parameter is the transversal permeability of the reinforcing materials used in these processes. In fact for a high value of this parameter it is possible to increase the product quality and to reduce the process time.In this work, using Darcy's law and a particular permeameter, the transversal permeability of preforms constituted by fabric and mat layers of fiber glass has been valued for different injection pressures. In particular, the permeability of two types of preform, consisting respectively of only mat layers and of only fabric layers, have been determined by tests and calculated by Kozeny–Carman equation. The research has been also focalised on the evaluation of the permeability of preforms constituted by a mix of the two different types of reinforcement. Furthermore an analytical model has been studied to calculate the permeability of preforms consisting of layers of different reinforcement types, starting from the permeability data characterizing the single reinforcement type and determined by the tests or by Kozeny–Carman equation.     

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.     

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.     

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%。      

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。     

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.     

Influence of fabrics’ design parameters on the morphology and 3D permeability tensor of quasi-unidirectional non-crimp fabrics

This research addresses the effects of quasi-UD non-crimp fabric (NCF) design parameters on the fabric architecture and on the permeability tensor. These fabrics are designed for the Liquid Resin Infusion (LRI) of large and thick composite parts. Three fabrics’ parameters intended to bring a flow enhancement to the NCF are investigated: the stitch spacing, the stitch pattern and the weft tow lineal weight. Image analysis is undertaken to characterize the morphology of non-crimp fabric composite. A new continuous permeability measurement method based on compressive tests is proposed to relate the permeability of the quasi-UD NCF to the design parameters during the infusion process. The latter are proven to influence significantly both the fabric architecture and the permeability tensor coefficients.     

Experimental determination of the permeability of engineering textiles: Benchmark II

In this second international permeability benchmark, the in-plane permeability values of a carbon fabric were studied by twelve research groups worldwide. One participant also investigated the deformation of the tested carbon fabric. The aim of this work was to obtain comparable results in order to make a step toward standardization of permeability measurements. Unidirectional injections were thus conducted to determine the unsaturated in-plane permeability tensor of the fabric. Procedures used by participants were specified in the guidelines defined for this benchmark. Participants were asked to use the same values for parameters such as fiber volume fraction, injection pressure and fluid viscosity to minimize sources of scatter. The comparison of the results from each participant was encouraging. The scatter between data obtained while respecting the guidelines was below 25%. However, a higher dispersion was observed when some parameters differed from the recommendations of this exercise.     

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.     

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.     

Dry fiber automated placement of carbon fibrous preforms

The superior material properties of carbon fiber-reinforced composites make them especially attractive for applications in aeronautics and aerospace industries. Cost reduction and time saving are continuously driving industry, leading to new industrial challenges which include manufacturing composite structures with optimal mechanical performances using the potential of advanced processes using robotics.To produce complex part shapes, technologies implying fabric draping in a mold imply large waste amount, fabric structure variability and uncertainties concerning local fiber volume fraction amount and thus final mechanical properties. To overcome such issues and comply with cost and time efficiency, automated dry fiber placement for preform manufacturing is proposed. This approach allows to integrate many functions in a complex part thank to the ability of the robot to steer fiber tows at specific locations. The final composite part is obtained by injecting the produced preform with resin using RTM (Resin Transfer Molding) or infusion process.The presented project aims to define the influence of the process driving parameters during fiber placement on the final preform properties range. Preforms were produced using a lab-scale automated placement demonstrator. Three preforms configurations were tested to highlight the influence of the preform structure on permeability and mechanical parameters through characterization of the compression behavior and permeability of the produced preforms. Choice of configuration will affect mechanical properties on the manufactured preforms, whereas creation of open channels to enhance the flow propagation during manufacturing does not necessarily increase the preform permeability.     

基于新型测试装置的网孔板层开孔率对纤维厚度方向渗透率的影响

纤维预成型体厚度方向渗透率的准确表征是液体模塑成型工艺制备大厚度复合材料计算机优化的关键。网孔板层是布置在试样上下表面、用于控制纤维厚度并防止纤维变形的刚性构件，其开孔率是纤维厚度方向渗透率K_z测量中的重要表征参数。本文首先通过实验对用于控制纤维预成型体厚度的垫圈进行校正；然后在排除纤维体积分数误差的前提下，通过金属网增大网孔板层的开孔率，测量缎纹织物SW220C-100b在不同压力和纤维体积分数下的K_z，提出有效计算面积的概念并得到其变化规律；最后，基于测试结果，研究注射压力和纤维体积分数对K_z的影响。结果表明:有效计算面积随着注射压力和纤维体积分数的增加而减小；纤维体积分数的增大会减小织物的K_z值，网孔板层开孔率的增大会减小注射压力对K_z的影响。      

“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。     

Assessment of the mechanical properties of a new 3D woven fibre composite material

Fully interlaced 3D fabric is produced by a new weaving technology, and it is here utilised to produce woven carbon fibre preforms, which are then used as reinforcement in composite materials. The purpose of this study is to assess the mechanical performance of this new type of composite material. A prototype loom was used to weave preforms with a rectangular cross-section where all warp and weft yarns were fully interlaced in plain weave. Tensile, compressive, out-of-plane, shear and flexural properties of the composite flat beam specimens were tested. The in-plane stiffness and strength were found to be lower, while the out-of-plane properties were higher compared to conventional 2D laminates. In terms of strength, it was not possible to quantify the difference, since the specimens with 3D woven material exhibited other failure modes than those tested for.     

基于数字图像技术的纤维织物面内渗透率表征

材料渗透率的表征受其结构空间离散性和求解方式准确性的严重影响。基于数字图像技术,评估了纤维织物渗透率的空间分布,并探讨了阶梯铺层对灌注工艺的影响。首先,从恒压单向注射实验的视频流中动态提取了流动前沿的流速分布和流动前沿角,通过织物渗透率与结构的关系仅一次实验便可求得纤维织物的面内局部渗透率分布;其次,利用正态分布函数拟合,建立了基于数字图像技术的纤维织物面内主方向渗透率张量的求解方法,并利用该方法研究了编织形式对渗透率的影响;最后,研究了阶梯铺层和恒定铺层对灌注过程的影响。结果表明:建立的基于数字图像技术的渗透率表征方法可以通过一次实验同时获取面内主方向上的渗透率及其空间离散型;在恒定铺层下缎纹织物渗透率随着纤维层数增大而增大,从厚铺层向薄铺层的灌注方式可以达到最优的灌注时间。      

Numerical prediction of in-plane permeability for multilayer woven fabrics with manufacture-induced deformation

A unit cell based Computational Fluid Dynamics model is presented for predicting permeability of multilayer fabric structures. In Liquid Composites Moulding processes, fabric lay-ups undergo significant manufacture-induced deformation, combining compression, shear, and inter-layer nesting. Starting from the configuration of un-deformed fabric, the deformation is simulated geometrically by accounting for self-imposed kinematic constraints of interweaving yarns. The geometrical modelling approach is implemented in the open-source software TexGen. The permeability tensor is retrieved from flow analysis in ANSYS/CFX, based on TexGen voxel models. Using only measured geometrical data for un-deformed fabrics, deformed plain weave fabric and twill weave fabric lay-ups were modelled and their permeability tensors predicted. Comparison with experimental data demonstrates the generally good accuracy of predictions derived from the proposed numerical method.     

树脂传递模塑成型工艺中嵌套效应引起渗透率变异的实验与数值模拟

树脂传递模塑成型工艺(RTM)中最重要的变形模式之一是厚度方向压缩。厚度方向压缩减小了织物预成型体的厚度,使织物预成型体局部结构形式发生改变从而引起嵌套效应。嵌套效应不仅会减少织物预成型体的厚度,增加纤维的体积分数并改变孔隙率,而且相邻织物层嵌套效应具有一定的空间分散性,从而使得织物预成型体渗透率具有变异性。本文针对低黏度树脂设计了一种实验装置用以测量局部渗透率的空间分散性,随后建立了随机嵌套单胞模型,利用ANSYS/CFX有限元软件实现了单胞填充浸润的数值模拟,通过流量分析获得局部渗透率,并研究了渗透率的统计分布。通过实验结果与数值模拟结果相对比,验证数值模拟结果的可靠性。最后,基于渗透率的统计分布建立了随机渗透率场,并进行填充浸润的数值模拟,通过与传统恒定渗透率的方法进行比较,证明该方法具有更高的先进性。研究结果可以对未来RTM工艺的稳健性优化提供依据。