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。      

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%。      

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。      

真空导入模塑工艺中织物预成型体的可压缩性

采用真空加载方法研究了循环加载、织物形态、纤维种类、织物层数、铺层方式和混杂方式等参数对真空导入模塑工艺(VIMP)中纤维织物预成型体压缩行为的影响。结果表明: 预成型体纤维体积分数随着压缩循环加载次数的增加而逐渐增大, 但增幅呈现逐渐减小的趋势; 在相同的压缩载荷下, 预成型体的纤维体积分数随着织物层数的增加而增大, 但增幅很小, 对于VIMP制备复合材料构件基本可以忽略; 纤维预成型体在压缩载荷下的响应方式与织物形态、纤维种类、铺层方式和混杂方式等因素密切相关, 单向铺层比正交铺层更容易压缩而获得较高的纤维体积分数, 夹芯混杂比层间混杂方式更容易压缩。      

VARI工艺成型纤维增强树脂复合材料层合板厚度和纤维体积分数的影响因素

分析了影响真空辅助成型技术（VARI）工艺成型复合材料的纤维体积分数和厚度均匀性的关键因素,即VARI成型工艺的树脂流动控制形式、纤维预制体状态、织物状态、树脂黏度,通过试验分析了各因素对VARI成型复合材料厚度和纤维体积分数的影响。试验结果表明,采用HFVI（high fiber-volume vacuum infusion）工艺、BA9914树脂及真空处理后的U3160单向机织物成型的纤维增强树脂复合材料层合板,其纤维体积分数和厚度均匀性能够接近预浸料/热压罐成型的复合材料制件的水平。     

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.     

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。      

真空辅助成型工艺中预成型体的厚度变化与过流控制

采用无接触式电涡流位移传感系统, 对真空辅助成型工艺中预成型体的厚度变化进行了实时监测。揭示了该成型工艺过程中预成型体的厚度变化规律, 并考察了树脂过流控制时间对制件厚度与纤维体积含量的影响。结果表明, 在整个工艺过程中预成型体的厚度变化可分为三个阶段: 在树脂浸入后, 预成型体厚度迅速增加; 在树脂过流控制阶段, 预成型体厚度变化较小且保持在较高水平; 在树脂管关闭后, 预成型体厚度迅速下降并逐渐趋于稳定。制件厚度与树脂过流控制时间的变化关系类似于正弦曲线, 在树脂过流控制时间约为10 min时, 纤维体积分数最低, 较无过流控制降低1.7%; 在树脂过流控制时间约为40 min时, 纤维体积分数最高, 较无过流控制提高1.6%。     

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。     

真空导入模塑工艺纤维预成型体厚度变化和流体压力变化研究进展

纤维预成型体厚度控制问题是真空导入模塑工艺(Vacuum infusion molding process,VIMP)面临的主要挑战之一。综述了国内外关于纤维预成型体压实回弹特性和渗透率特性的研究进展,介绍了VIMP工艺在一维线性流动和二维径向流动时厚度变化的理论模型,分析了流体压力特性方程的求解、流体压力场的分布和流体压力对厚度的影响,指出了Correia推导过程中存在的问题并进行了修正,总结了厚度变化对VIMP工艺及复合材料制品的影响,并对VIMP工艺厚度变化的理论研究和工艺控制进行了展望。     

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.     

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.     

基于“离位”增韧技术Z向注射RTM成型的浸润研究

针对"离位"增韧技术和Z-RTM成型技术,引入饱和度参数修正Darcy定律,建立描述树脂在纤维预制件中非稳态流动的偏微分方程,研究恒流注射过程中体积流量、树脂黏度和纤维预制件渗透率等工艺参数对非稳态浸润过程注入压力的影响,模拟树脂在层间未增韧和增韧纤维预制件束内和束间的流动。结果表明:数值模拟结果具有可靠性;随着注射时间的增加,纤维预制件内部各点的压力增加;随着体积流量、树脂黏度的增加,注入压力线性增加,而随着纤维渗透率的增加,注入压力减少,符合Darcy定律;实现了树脂在纤维预制件细微观层次浸润的可视化,这种可视化结果为预测树脂在预制件中的宏观流动提供了重要补充,并为实际工艺提供了一定指导作用。     

Mechanical properties of composite structures fabricated with the vacuum induced preform relaxation process

A new technique called vacuum induced preform relaxation (VIPR) can be used to improve the processing of composite parts manufactured using vacuum resin infusion methods. The VIPR process is a method for manipulating and guiding the resin filling pattern during a vacuum assisted resin transfer molding (VARTM) manufacturing process with a relatively small external vacuum chamber. This VIPR chamber can be sealed against the flexible molding surface of a VARTM mold and used to create vacuum above the preform. This causes the compressive forces compacting the fabric to decrease allowing the resin to flow faster in the effected region under the chamber. Thus the chamber can influence the resin flow pattern as well as avoid the formation of voids due to merging flow fronts. When the regulated vacuum in the chamber is applied it temporarily decreases the fiber volume fraction of the preform. It is important to investigate if this relaxation has a permanent adverse effect on the mechanical properties of the composite. The results of these tests strongly suggest that the use of the VIPR process does not compromise the mechanical properties of composite structures.     

A nonlinear compaction model for fibrous preforms

Based on the mechanics of porous media and physical insight gained from experimental observation, a model for predicting the nonlinear compaction of fibrous preforms in the resin transfer molding process is developed. A key physical constant — namely, preform bulk compressibility — is proposed to establish the relationship between the applied pressure and the preform bulk volume. The preform bulk compressibility is a function of fiber volume fraction and five parameters — the initial fiber volume fraction, the final (maximum attainable) fiber volume fraction, the initial pore volume compressibility, the fiber compressibility, and an empirical index. Results of compaction experiments on plain-woven fabric preforms and unidirectional non-woven materials support the validity of the model. Excellent agreement between theory and experiments has been obtained. The present model provides for fibrous preforms a nonlinear constitutive law whose coefficients can be physically interpreted.     

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.     

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.