非弯折织物渗透率的数值预测

采用多孔介质模型替代纤维束区域的方法,数值计算预制件单胞模型的渗透率。平行和垂直方向纤维束分别用平行和垂直多孔介质区域替代,多孔介质阻力系数由最小二乘法拟合得出。首先验证了四方排列平行及垂直方向纤维单胞模型流场计算方法的可靠性;其次验证了纤维单胞多孔介质模型的可靠性;然后通过预制件单胞模型的研究,发现考虑束内渗透率的误差远小于不考虑束内的误差,说明考虑束内流动对准确模拟渗透率是有必要的;最后研究了预制件结构参数对渗透率和孔隙率的影响。纤维束宽度增大,预制件渗透率减小,孔隙率减小。纤维束高度增大,预制件渗透率增大,孔隙率不变。纤维束间距增大,预制件渗透率增大,孔隙率增大。     

纤维束及非弯折织物渗透率预测公式的研究

根据数值模拟结果,对Gebart公式进行了修正,研究了NCF单胞渗透率与结构参数的关系,得到了NCF单胞渗透率预测公式。对于纤维束,平行方向的数值结果及修正公式在较高孔隙率下比Gebart公式更接近于实验结果;对NCF单胞模型渗透率计算,与实验结果误差为5%;对5种纤维束孔隙率的NCF单胞分别进行正交试验,得到渗透率的数值结果,拟合出了预测公式;NCF渗透率预测公式结果与数值结果对四方和六方排列的平均误差为4.60%和4.57%,说明了预测公式的准确性。     

网状增强相纤维棒几何建模及弹性性能研究

以网状增强相纤维棒为研究对象,采用单胞几何模型分析法将该纤维棒的整体结构分为内部单胞、外部单胞等类型,并分别建立了内单胞、外单胞的力学模型。研究结果表明:基于等应变-应力混合平均思想并结合桥联模型,可计算出复合材料内外不同结构的有效工程弹性常数。该方法可有效分析不同结构组成的纤维复合材料内部纤维束的排列方式、外部增强相各参数[如编织角、纤维束截面和纤维体积分数等]之间的关系、纤维体积分数对纤维复合材料工程弹性常数的影响规律等,通过对比仿真结果验证了该方法在预测不同结构复合材料弹性性能方面是可行的。     

三维织造复合材料预制体纤维体积含量的预测及实验验证

为确定三维织造复合材料预制体的纤维体积含量,建立四边形和三角形预制体的细观结构模型,将预制体划分为内胞、边胞、角胞等单胞形式,确定单胞类型系数并根据单胞排布方式将预制体用矩阵表示,将计算的纤维体积含量理论值与称重法得到的实测值进行比较。结果表明:预制体纤维体积含量的理论计算模型较为准确,误差控制在4%以内;纤维体积含量随Z向纤维股数的增大而增大,随规格的增大而减小,当规格增大到一定值时,纤维体积含量趋近相同;纤维体积含量实测值随织造层数和水平面内纤维股数的增大而减小。     

纤维曲率对界面过渡区初始微观结构影响的计算机模拟

借助计算机模拟技术,以单粒径球形粒子体系在不同尺度的圆截面纤维周围堆积时固相体积分数分布为例,研究了纤维直径与粒子直径比值变化对纤维-浆体界面过渡区初始微观结构的影响。将模拟计算结果与单粒径球形粒子体系在平集料表面的固相体积分数分布曲线进行对比。研究了纤维曲率对多尺度球形粒子体系在纤维表面固相体积分数分布曲线的影响,并将该模拟计算结果与多尺度球形粒子体系在平集料表面的固相体积分数分布曲线进行对比。结果发现：无论是对单粒径球形粒子体系而言,还是对多尺度球形粒子体系而言,初始界面过渡区的微观结构与纤维的曲率无关。球形粒子沿不同曲率纤维周围的固相体积分数分布曲线的形式与在平集料表面周围的粒子固相体积分数曲线的分布形式相同,曲线第一个峰值点的位置(即初始界面过渡区厚度的位置)也都出现在模型水泥平均质量半径的位置附近,水胶比的降低,会使初始界面过渡区的厚度略有减小。因此,可以用平集料表面-浆体结构取代圆截面纤维-浆体结构来研究各种因素对初始界面过渡区微观结构的影响。     

RFI成型工艺中渗透率的研究及树脂流变模型的建立

渗透率测量是树脂膜熔渗(RFI)工艺在复合材料设计和优化中最关键的条件。随着纤维体积分数的增加,渗透率非线性降低,可见纤维体积分数的变化对整个树脂体系浸渍纤维预制体有很大的影响。在黏度实验的基础上,对用于RFI工艺的环氧树脂体系的化学流变特性进行研究,并根据双阿累尼乌斯方程建立树脂体系的流变模型,为合理地制定RFI工艺参数、保证产品质量和实现工艺参数的全局优化提供科学依据。     

三维机织复合材料单胞模型各向异性的有限元分析

使用三维绘图软件Pro/E绘制出三维浅交弯联机织复合材料数字化结构模型,借助大型有限元分析软件ANSYS模拟单胞模型承受不同方向压缩载荷作用下的力学性能。探究在不同方向的压缩载荷作用下复合材料单胞模型的应力分布情况,并借此分析复合材料单胞模型的各向性能;以承受X方向压缩载荷的单胞模型为例,分析复合材料中纤维与树脂的受力情况。结果表明:三维机织复合材料受到压缩载荷时,表现出明显的各向异性,表现为X方向压缩性能最好,Z方向压缩性能最差;纤维作为主要承载体,承担较多载荷作用,树脂作为次要承载体,承担较少载荷作用。     

椭圆边非弯折织物渗透率预测公式的研究

将NCF的纤维束两侧近似为半椭圆,对NCF单胞设计五因素五水平正交试验,利用CFD方法得到渗透率的数值结果,进而拟合出预测渗透率的椭圆边公式。对于纤维束四方排列的NCF单胞,椭圆边公式的计算结果和数值结果平均误差为1.91%;对于六方排列,平均误差为1.90%。对NCF渗透率的预测结果和实验结果的相对误差为0.22%,小于矩形边公式的误差3.96%。当束间间距w1与椭圆边的半长轴c1之比小于0.8时,两预测公式值之比大于1.1。说明椭圆边公式对NCF材料渗透率的预测更加准确,适用范围更广。     

基于随机算法的纤维微观结构渗透率预报及树脂流动数值模拟研究

树脂基纤维增强复合材料的液体成型工艺制件的质量与制备过程中树脂流动浸渍情况紧密相关,微观尺度纤维结构和微观渗透率作为介观尺度和宏观尺度的基石,对树脂的流动浸渍具有直接影响,然而微观尺度难以有效开展试验,数值模拟成为更有效的研究手段。本文采用Monte Carlo随机算法建立了微观尺度纤维束内单丝随机排布结构和纤维束内、束外介观尺度结构模型;基于有限元方法对纤维束内结构进行渗透率预报,并创造性地采用Phase Field相场方法考虑表面张力和毛细力进行树脂流动数值模拟。结果表明:本文所建立的纤维单丝随机结构渗透率预报方法较传统经验公式预报精度显著提高;本文模拟的微观纤维束内孔隙的形成过程,对从微观尺度系统阐述树脂流动浸渍过程和微观气泡的产生机理具有重要意义。     

RTM工艺中玻纤增强材料渗透率的测量与分析

通过对树脂传递模塑成型工艺中广泛使用的纤维增强材料——玻纤连续毡渗透率的测量和分析，建立了该增强材料在模具中的纤维体积含量与渗透率之间的关系，考察了纤维增强材料的渗透率与注模时间的关系，分析对比了纤维增强材料的结构型式对注模时间和纤维浸透性的影响。结果表明；随着纤维体积含量的增大，渗透率迅速下降，对于玻纤连续毡，其渗透率ｋ与纤维体积含量ｖｆ的关系可以表示为一个多项式；在恒定的压力下，渗透率大，注模需要的时间越短，体积含量相同的玻纤连续毡和玻纤方格布比较，玻纤连续毡的渗透率约大一倍，而所需注模时间约为玻纤方格布的１／２；玻纤方格布中存在渗透率相差特别大的两大区域是造成其浸透性差的主要原因。     

The effect of nesting on the in‐plane permeability of unidirectional fabrics in resin transfer molding

The nesting of layers has great effect on the permeability, which is a key parameter in resin transfer molding. In this article, two mathematical models were developed to predict the in‐plane permeability of unidirectional fabrics with minimum and maximum nesting, respectively. For different zones of characteristic yarn arrangement in the unit cell, the local permeability was modeled as a function of geometrical yarn parameters. The global permeability was then modeled as a mixture of permeabilities of different zones with the electrical resistance analogy. A reasonably good agreement was found between the model predictions and experimental results. We also found that at the same fiber volume fraction, the results for Ky were two times larger with minimum nesting than with maximum nesting, whereas the results for Kx were a little lower with minimum nesting than maximum nesting. In addition, the differences between minimum and maximum nesting decreased with increasing fiber volume fraction. POLYM. COMPOS., 37:1695–1704, 2016. © 2014 Society of Plastics Engineers     

Preform permeability predictions by self-consistent method and finite element simulation

The self-consistent method and finite element simulations have been used to estimate the permeability of an aligned fiber bundle. The self-consistent method gives out formulas for both longitudinal and transverse permeabilities as functions of the fiber volume fraction. The finite element simulation presents the solutions of various periodic fiber packings. The results for square and hexagonal fiber packings are found coincident with the literature results. It is shown that the permeability is not only related to the fiber volume fraction of porosity, but is also greatly influenced by the packing structure or micro-level disturbance. A unified empirical model is proposed, which uses as variables ultimate fiber volume fraction in addition to fiber volume fraction. The model predications agree with numerical simulation results in different cases.     

Determination of corner and edge permeability in resin transfer molding

Resin flow in resin transfer molding may be strongly influenced by small heterogeneities in the fiber preform, such as narrow channels formed by bending and fitting into a mold. An evaluation of preform permeability in these regions of nonuniform packing is presented. The study incorporates an empirical and analytic approach to iteratively determine the permeability in these regions. Flowfront data were recorded for high fiber volume fraction woven fiber preforms. Edge gap width, injection pressure, and fiber volume fractions were varied. The flow front data were compared with numerical simulations to estimate enhanced permeability at the edges. The results indicate that, for high fiber volume fraction preforms, flow in the channel dominates for relatively small (≈ 2 mm) gaps.     

Effects of liquid absorption and swelling on the permeability of natural fiber fabrics in liquid composite moulding

Liquid composite moulding (LCM) has been successfully used to manufacture natural fiber reinforced composites. Effects of liquid absorption and swelling on the permeability of natural fiber fabrics in LCM were investigated. It shows that the saturated permeability measured in the corn syrup is 23.1%–69.5% smaller than that in the motor oil because of reduced porosity and flow paths caused by liquid absorption and swelling. Based on traditional models by considering the effects of liquid absorption and swelling of jute fibers, a mathematic model was developed to predict the variable permeability during the mold filling. Experimental results demonstrate excellent agreement with the prediction. The effects of flow rate and fiber volume fraction on permeability were also analyzed. It is found that liquid absorption and swelling may dominate the permeation when the flow rate is low while the flow rate may contribute more when it is large. Furthermore, fiber volume fraction is more sensitive to permeability in the syrup than is the case in the motor oil because of the absorption and swelling of natural fibers. POLYM. COMPOS., 38:996–1004, 2017. © 2015 Society of Plastics Engineers     

乙醇对简单节杆菌TCCC11037生长和细胞性质的影响

研究了乙醇体积分数对简单节杆菌TCCC 11037 生长和细胞性质的影响.结果表明,随着乙醇体积分数的增大,菌体的延滞期延长,生物量和比生长速率下降,当乙醇体积分数为6%时几乎不能观察到明显的菌体生长;菌体形态随着乙醇体积分数的增大出现较明显的改变,当乙醇体积分数为8%时,细胞完整性明显破坏,胞浆大量外泄;菌体的耗糖能...     

Estimation of the resin flow permeability of fiber tow preforms using the self-consistent method

This paper presents a model using the self-consistent method for estimating the resin flow permeability of fiber tow preforms. The permeable resin is assumed to be a Newtonian fluid, such as an uncured and low viscosity epoxy resin. Permeability in both longitudinal and transverse directions is considered. In the discussion, a basic cell insertion is constructed to include a representative fiber tow surrounded with fluid. The insertion is placed into a homogeneous medium with an unknown permeability. Stokes flow is applied to the fluid region, and Darcy flow is considered at the tow region and in the outside homogeneous medium. Boundary and interface conditions as well as consistent conditions, including the total amount of the flow and the dissipation energy, are applied accordingly. Permeability is solved from these considerations. The prediction expresions using parameters of average fiber volume fraction, tow volume fraction, and tow size are derived. The results show that the main flow paths are the openings or gaps between neighboring tows in both longitudinal and transverse directions. The observation provides a scientific base for the simplification of the problem treatment.     

Random discontinuous carbon fiber preforms: Experimental permeability characterization and local modeling

Injection experiments indicate that for random discontinuous carbon fiber preforms, increasingly uneven flow fronts develop with increasing fiber bundle length and filament count. While at high propensity for fiber bundle splitting, the preform permeability increases continuously with increasing fiber length, no trend can be identified at low propensity. No clear influence of the virgin bundle filament count on the preform permeability was observed. Types of sizing used on the fibers and bundle cross‐sectional shapes may vary and affect the intrinsic filamentization behavior, thus dominating the preform permeability. In a model for local preform permeability, interbundle voids, distributed randomly across the preform thickness, are approximated via a regular void structure. Simulated filling patterns are qualitatively similar to those observed experimentally, showing more pronounced features than those derived from a model based on local through‐thickness homogenization of the filament distribution. A model based on an alternating arrangement of fiber bundles and voids allows prediction of global preform permeability values from series of injection simulations, showing quantitatively better agreement with corresponding experimental results than the homogenization model. For global permeability, agreement between simulated and experimental mean values improves with increasing fiber volume fraction, whereas calculated coefficients of variation show no strong dependence on the fiber volume fraction. POLYM. COMPOS., 2010. © 2009 Society of Plastics Engineers     

热压工艺密实过程数值模拟

本文针对热压成型工艺,利用多孔介质有效应力原理和体积平均方法,建立了三维流动-热-化学耦合的数值模型.该模型将密实过程中的纤维层视为可变形的多孔介质,从而能反映纤维层变形的影响.在此基础上,本文采用有限元方法,结合ALE移动网格方法处理动边界,对热压工艺密实过程进行了数值分析.计算结果表明,与非耦合的经典模型相比,该模型给出的结果与实验吻合得更好.     

The dry-to-wet transition of fiber networks—Return to mechanical stability

In this article, we provide a comprehensive experimental, numerical, and theoretical explanation of the dry-to-wet transition of nonbonded fiber networks made of natural fibers. Given that the main functionality of many common products consisting of fluff pulp fiber networks requires absorption of liquids, we focus on understanding the solid volume fraction transition from a dry to a wet state as a crucial component for controlling properties such as permeability and capillary pressure, on which product function eventually depends. From studying the wetting of fluff pulp fiber networks with a distribution of fiber lengths, we show that the change in the solid volume fraction going from dry to wet state is driven by the disappearance of fiber–fiber adhesion. The mechanically stable state to which the network transitions is independent of its prior dry solid volume fraction and predetermined primarily by the fiber aspect ratio.