【作者】 董抒华；

【导师】 王成国；

【作者基本信息】 山东大学 ， 材料学， 2014， 博士

【摘要】 先进树脂基复合材料因其具有高比强度、高比模量、耐腐蚀、耐疲劳、可设计性强和可大面积整体成型等诸多优点而成为航空航天领域不可或缺的重要战略材料。然而,复合材料制造高成本限制了其进一步发展,为此低成本的树脂传递模塑(Resin Transfer Moulding, RTM)成型已成为航空工业的主流技术之一,但在其制品中经常会出现干斑、孔隙等浸润缺陷,这些缺陷会极大影响产品的质量和性能,而预制件渗透率的不均匀性是引起空隙或干斑的一个重要原因。纤维预制件的渗透率描述了预制件对树脂流动的阻碍程度,是RTM成型过程中树脂流动的一个重要参数。渗透率作为预制件的关键性能之一,与其结构密切相关,由于它与预制件多层次结构的复杂相关性而使得渗透率的预测一直面临着挑战；另外,树脂在纤维中的充分浸润流动是RTM复合材料制品性能的关键影响因素。因此,探讨预制件渗透率与其结构参数的相关性并对其精确预报,进而通过数值模拟预测RTM浸润工艺过程是目前亟待解决的关键问题。本文建立非弯折织物(Non-Crimp Fabric, NCF)预制件无缝线和有缝线两类单胞,针对纤维预制件具有双级多孔的特性,在基于双尺度多孔介质的束内／束间耦合流动基础上,数值模拟树脂在预制件细微观结构中的浸润耦合流动行为。采用Brinkman方程描述树脂在纤维束内孔隙的流动,采用Stokes方程描述树脂在纤维束间区域的流动,建立在预制件束内和束问树脂耦合流动的数学模型,利用体积平均化理论,求解速度场和压力场,通过Darcy定律开展NCF预制件面内渗透率的预测。探究了影响渗透率的关键结构参数,开展了不同纤维铺层方式对NCF预制件面内渗透率的影响研究,揭示了纤维预制件的多层次结构参数对面内渗透率的影响规律,在渗透率的预测方面取得了较大进展。对比了有缝线和无缝线两种类型NCF单胞的渗透率大小,重点分析了纤维束形状大小(包括纤维束宽度B、高度h和纤维束沿纤维方向截面椭圆长半轴c、束间距b)及纤维布不同铺层方式等结构因素对预制件面内渗透率的影响。通过引入Morris全局灵敏度分析方法量化了纤维预制件结构参数对渗透率的影响程度,为纤维预制件结构进一步优化提供了重要的理论依据。研究了纤维束在缝制分叉时等效直径、缝线的尺寸和分布、缝线偏离中心距离等因素对纤维预制件面内渗透率的影响。结果表明：没有缝线的预制件单胞面内渗透率大于有缝线的预制件渗透率。预制件渗透率随着纤维束宽度的增加而降低,随着纤维束的高度、纤维束截面边沿椭圆弧的长半轴和纤维束间距的提高而提高。当纤维束宽增加1.2倍时,无缝线的单胞渗透率降低了约52%,而有缝线的单胞渗透率降低了约54%；高度提高一倍时,对应无缝线单胞渗透率增加了约2倍,而有缝线单胞渗透率增加了1.73倍；长半轴增加3.2倍,对应无缝线单胞渗透率增加了30%,有缝线单胞渗透率增加了29%；纤维束间距增加4.7倍,对应无缝线单胞渗透率增加了11.3倍,而有缝线单胞渗透率增加了14倍。预制件渗透率对于不同参数的灵敏度顺序是：b>h>B>c。预制件渗透率对纤维束间距是最为敏感的,对于纤维束截面边沿椭圆弧的长半轴相对不敏感。NCF预制件因缝制时纤维束的分叉阻碍了流体的渗流速度,当流道内交叉束的等效直径与纤维束间距离比是0.7时,纤维束分叉情况下渗透率比没有分叉的预制件单胞渗透率降低约80%。预制件渗透率随着缝线中心位置逐渐偏离流道中心略有增加,随着缝线倾斜角度增加而略有降低。基于预制件纤维束间流道的压力降和其几何结构参数的关系,建立了纤维预制件面内渗透率与其结构相关联的解析模型；对于缝制的单胞,通过引入修正因子和缝线等效方法,建立了纤维预制件渗透率与关键参数的结构相关性解析模型。应用该模型预测的渗透率与有限元计算结果进行对比,结果表明所提出的解析模型在一定范围内能够精确预测纤维预制件面内渗透率,从而实现了多层次结构预制件渗透率的快速精确预测,为完善RTM成型充填理论、优化工艺参数和降低成本起到重要作用。针对北京航空材料研究院先进复合材料重点实验室提出的“离位”增韧技术,揭示了增韧层对预制件渗透率的影响规律和机理,重点分析了增韧层的厚度和渗透率以及不同纤维铺层角度对纤维预制件Z向渗透率的影响。结果表明：纤维预制件Z向渗透率随着增韧层厚度的增加而降低,随着增韧层渗透率的增加而增加；[0]2铺层的纤维预制件Z向等效渗透率远大于其他铺层角度的预制件渗透率,在[0/30]、[0/45]、[0/60]和[0/90]铺层中,[0/45]铺层预制件渗透率最大,[0/30]铺层预制件渗透率最小。纤维体积分数、流道比表面积和流道分布结构决定着渗透率的大小；所提出的数值模拟模型能够精确预测Z向渗透率,作为Z向流动RTM成型模拟的关键参数,它的精确预测对进一步RTM成型的设计和优化提供了重要指导。针对树脂在多尺度纤维预制件中的流动特点,引入了不饱和因子修正Darcy定律,建立了描述树脂在纤维预制件中非稳态流动的偏微分方程(Partial Differential Equation, PDE),利用COMSOL软件作为求解器求解RTM成型时树脂流动的控制方程,研究了树脂流动前沿演化规律,将其与解析解和实验结果进行对比,结果表明该模型能够精确预测树脂在纤维预制件中的非稳态流动。针对“离位”增韧技术和Z-RTM成型技术,研究了RTM非稳态浸润过程注射口压力与时间关系的曲线,并与实验结果进行了对比。模拟了树脂在层间未增韧和“离位”增韧纤维预制件束内和束间的流动,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。通过分析发现增韧层使得流动前锋更加平滑,从而减少了成型制品的缺陷。研究了不同成型工艺参数(包括树脂黏度、注射压力和不同渗透率等)对流动前沿的影响规律,从而为实际生产提供了一定的指导作用。更多还原

【Abstract】 Advanced resin matrix composites are indispensable strategic materials in the field of aerospace due to their advantages of high specific strength, high specific modulus, corrosion resistance, fatigue resistance, easy design and large area molding. But its further development is restricted by the high cost of the composite manufacturing. Resin transfer molding (RTM), as a typical representative in the advanced composites molding process, has become one of mainstream technologies to realize low-cost. However, the products in RTM often have the defects such as dry spots or voids, which will affect the product quality and performance. The permeability inhomogeneity is one of important reasons of dry spots or voids. The permeability of the fiber preform, which represents the ease of flowing through a porous medium, is an important parameter influencing the resin impregnation process. The permeability is one of preform’s key properties, which is closely related to its structure. Many challeges are faced for permeability prediction due to its complex correlation with multi-level structure of preform. Besides, the sufficient infiltration flow of the resin in the fiber is a key factor directly affecting the performance of RTM products. Therefore, it is an urgent issue to discuss the permeability’s correlation with structural parameters of the preform and further accurate prediction of RTM process by numerical simulation.In this paper, two types of cells with or without stitch of Non-Crimp Fabric (NCF) are built. Aimed at the dual porous charateristic of the fiber preform and based on intra-tow and inter-tow coupling flow of dual scale porous medium, the infiltration coupling flow behaviour of the resin in the meso-level and micro-level of the preform is numerically simulated. Brinkman equation is used to describe intra-tow flow, while Stokes equation is used to discribe inter-tow flow. The mathematical models of the resin flow in the intra-tow and inter-tow of the preform are built. The velocity field and the pressure field are solved by the volume averaging theory and the prediction for NCF’s in-plane permeability is developed combined with Darcy’s law.The key structural parameters affecting permeability are discussed. The effect of the different NCF’s ply orientation on the in-plane permeability is investigated. The influencing law of multi-level structural parameters of the preform on the in-plane permeability is revealed, which result in the breakthrough in the permeability prediction. The permeability of two cell of the preform with or without stitch is compared. The effect of the structural factors including the width (B), the height (h), the semi-major axis length of the ellipse section of fiber bundle (c) and the distance between fiber bundles (b) and fiber ply orientation on the in-plane permeability of the preform is investaged. The influencing degree of different structural parameters on the permeability is quantized by Morris sensitivity analysis method, which proves an important theoretical basis for further optimization and design of the fiber preform structure. The effect of crossing equivalent radius between the fiber bundle and flow channel of the cell with stitch, size and distribution of the stitch, off-center distance of stitch on preform permeability is focused. The results show that in-plane permeability of the preform cell without stitch is larger than that of the preform cell with stitch. The preform permeability decreases with the increase of the fiber bundle width, increases with the increase of the fiber bundle height and the semi-major axis length of the ellipse section of fiber bundle and the distance between fiber bundles. The permeability of the cell without stitch decreases about52percent and the permeability of the cell with stitch decreases about54percent when the fiber bundle width inreases by1.2times. The permeability of the cell without stitch increases about two times and the permeability of the cell with stitch increases about1.7times when the fiber bundle height doubles. The permeability of the cell without stitch increases by30%and the permeability of the cell with stitch increases by29%when the semi-major axis length of the ellipse section of fiber bundle increases by3.2times. The permeability of the cell without stitch increases by11.3times and the permeability of the cell with stitch increases by14times when the distance between fiber bundles increases by4.7times. The order of permeability sensitivity degree corresponding to different structural parameters is as follows:b>h>B>c. The in-plane permeability is most sensitive to the distance between fiber bundles, while the semi-major axis length of the ellipse section of fiber bundle is relatively unsensitive. The fiber bundle crossing greatly hampers the flow velocity. Compared with the preform permeability without crossing, the preform permeability with crossing lowers by80%when the ratio of equivalent diameter of crossing bundle of flow channel to distance between the bundles is0.7. The preform permeability increases slightly with the increase of off-center distance, while decreases slightly with the increase of inclination angle of the stitch.The analytical model of the in-plane permeability of the fabric correlated with its structure is built on basis of the relation between the pressure drop and geometric parameters of the flow channel between the flow channels. The analytical model correlated with stitched fiber preform structure is built by the introduction of the modifying factor and equivalent channel method for the stitched fabric cell. The predicted permeability by the proposed model is compared with the results by the finite element method. The results show that the proposed mathematical model can accurately predict in-plane permeability of the fiber preform, which makes it possible to fast and accurately predict the permeability of the preform with multi-level structure. It play an important role in promoting RTM molding mold filling theory, improving composite material technology, optimizing the process parameters and reducing costs.Aiming at ex-situ toughening technology proposed by National Key Laboratory of Advanced Composites of Beijing Institute of Aeronautical Materials, the influencing law and the mechanism of the interlaminar toughening layer on preform permeability are investigated. The effect of the toughening layer thickness and permeability and fiber ply orientation on the Z-direction permeability of the preform is analyzed emphatically. The results show that the Z-direction permeability of the preform decreases with the increase of toughening layer thickness, while increases with the increase of the toughening layer’s permeability. The Z-direction permeability of the preform with [0]2ply orientation is larger than the permeability corresponding to other ply orientations. Among [0/30],[0/45],[0/60] and [0/90] ply orientations, the Z-direction permeability of the preform with [0/45] ply orientation is the largest, while [0/30] ply orientation is the lowest. Fiber volume fraction, specific surface areas of the flow channel and flow channel structure determine the preform permeability. The proposed numerical simulation model can accurately predict Z-direction permeability. It is a key parameter of RTM process flow simulation along the Z-direction of the preform, which will provides an important guiding role for further RTM process design and optimization.Aimed at the characteristics of the resin flow through multi-scale fiber preform, Darcy’s law is modified by the unsaturated factor. The partial differential equation (PDE) describing the resin unsteady flow in the fiber preform is established. The control equation of resin flow in the preform for RTM is solved by COMSOL software as a solver. The resin flow front evolution is investigated, which is compared with analytical solution and experimental result. The results show that the model can accurately predict resin unsteady flow in fiber preform. Aimed at ex-situ toughening technology and Z-direction RTM process, the inlet pressure and time relation curve of untoughening and toughening unsteady infiltration process is investigated and compared with experimental results. The resin flow intra-tow and inter-tow of the preform with toughening layer and untoughening layer is simulated. The infiltration visualization of resin flow through meso-scale and micro-scale fiber preform is realized, which provides an important supplement for prediction of macro-flow in fiber preform and guidance for actual process. The toughening layer makes flow front smoother, which reduces defect of the product. The influencing rule of the process parameters (including the resin viscosity, injection pressure and preform permeability) on the flow front is studied, which makes the model become a powerful tool to guide RTM mould design and process design, and will prove technical support for the actual production.更多还原