三维多向编织复合材料非线性本构行为的细观数值模拟

针对三维多向编织复合材料, 在已建立的单胞几何模型及材料力学性能细观计算力学分析方法的基础上,引入M urakam i 的几何损伤理论模拟纤维束的细观损伤行为, 建立了预报该类材料非线性本构行为数值模拟及细观损伤机理的有限元分析方法。结合实例预报了碳/环氧四向编织复合材料本构的非线性行为, 并与实验结果进行了对比。      

二维编织陶瓷基复合材料偏轴拉伸力学性能预测

基于二维编织C/SiC复合材料的细观结构,建立了碳纤维丝/热解碳界面/SiC基体和纤维束/表层SiC基体两种尺度下的细观单胞模型,通过有限元方法计算碳纤维丝/热解碳界面/SiC基体模型的等效弹性常数和强度,然后代入纤维束/表层SiC基体模型中计算,并引入Tsai-Wu失效准则,考虑不同失效模式的损伤,建立了二维编织C/SiC复合材料的渐进损伤模型,模拟了其偏轴拉伸应力-应变行为。针对该模型,阐述了二维编织C/SiC复合材料单胞模型在复杂应力状态下其纤维束的损伤过程。数值模拟结果与实验数据吻合较好,验证了模型的有效性,为该种材料的力学性能分析提供了一种有效方法。     

三维六向编织复合材料渐进损伤模拟及强度预测

基于三维六向编织复合材料的细观结构,假设第六向纱线的截面形状为菱形,建立了三维六向编织复合材料的渐进损伤有限元模型。采用Linde等提出的失效准则,引入周期性位移边界条件,对三维六向编织复合材料的纵向拉伸应力-应变行为进行了渐进损伤数值模拟,讨论了单胞模型在纵向拉伸载荷作用下的细观损伤起始、扩展和最终失效的演化过程,并预测了材料的拉伸强度。在此基础上,进一步研究了编织角、纤维体积分数和编织纱水平取向角等参数对材料纵向拉伸力学性能的影响规律。研究结果表明,三维六向编织复合材料的轴向纱线拉伸断裂是导致其破坏的最主要因素。所得数值结果与现有试验值吻合较好,验证了该模型的有效性,为更深入研究此类材料的力学性能奠定了基础。     

考虑纤维束间粘接层的三维四向编织复合材料弹性性能数值预测

考虑了相邻纤维束之间的界面粘接效应, 建立了考虑纤维束间粘接层的三维四向编织复合材料单胞有限元模型, 较为真实地模拟了该材料的细观结构, 讨论了相应的边界条件和约束条件, 并采用有限元方法计算了该材料的等效弹性性能参数, 计算结果与实验值符合较好。研究了等效弹性性能参数随不同编织角及体积分数的变化关系, 得到了体胞的细观应力场, 为强度分析提供数据。      

2.5D机织复合材料的渐进损伤与失效模拟

为了研究典型2.5D机织复合材料的压缩性能,开展了复合材料单胞结构的经向和纬向压缩实验,并通过对材料编织结构的细观表征,建立了细观尺度的单胞有限元模型来模拟压缩载荷下单胞内部的变形及渐进失效过程。结果表明,2.5D机织复合材料在受压时表现出明显的非线性力学响应,材料沿经向的压缩模量和强度均高于纬向;经向压缩时材料的主要破坏模式有经纱的横向开裂、纤维束间的界面分层破坏、纬纱的压溃及基体的开裂,纬向压缩时出现的主要破坏模式是纬纱的压溃、纬纱纤维束的断裂及基体开裂;通过对比试验与有限元结果,认为所建立的细观有限元模型能够准确预测材料单胞在压缩载荷下的应力-应变响应,并且能够模拟编织结构中的损伤起始和演化过程。      

基于面-内胞模型的三维四向编织复合材料渐进损伤数值模拟

为了准确预测三维四向编织复合材料的纵向拉伸力学性能,对编织复合材料的面胞和内胞细观实体模型进行参数化建模,面胞模型考虑了纱线空间轨迹的偏移和横截面的挤压变形。用体素网格离散模型并施加合适的边界条件,将各组分材料的损伤模型编入到有限元分析软件ABAQUS用户定义材料子程序UMAT中。分别对内编织角为30°和45°的三维四向碳纤维/环氧树脂编织复合材料的面-内胞模型进行数值分析,经体积加权平均获得不同厚度编织复合材料试件的纵向拉伸模量和强度,通过统计具有相同破坏模式的积分点数量研究复合材料的渐进损伤过程。结果表明:基于面-内胞模型预测三维四向编织复合材料的纵向拉伸力学性能与试验值吻合良好,损伤分析结果合理地反映了面胞和内胞的渐进损伤过程。      

三维四向编织复合材料有效性能的预报

根据三维四向编织复合材料中纤维束的空间几何结构特征, 建立了比较合理的三胞模型。模型中考虑了3种单胞各自纤维束的空间结构和弯曲, 同时引入纤维束填充因子来描述各类单胞中纤维束的不同截面形状对材料弹性常数的影响。基于刚度体平均方法, 建立了相应的刚度预报模型, 得到了三维四向编织复合材料的工程弹性常数。用细观力学方法分析了工艺参数和尺寸效应对材料有效性能的影响规律。不同尺寸试件的数值预报结果和实验结果吻合较好。      

考虑界面层和孔隙的SiCf/SiCm复合材料热膨胀性能研究

以三维四向编织SiCf/SiCm复合材料为对象,建立基于周期性边界条件、包含界面层和孔隙的复合材料单胞有限元模型,模型细观结构与工业CT扫描结果一致。计算了材料各个方向的热膨胀系数,发现界面层对纤维束热膨胀系数的影响不可忽略,基体孔隙位置的随机分布对热膨胀系数计算结果没有影响,孔隙率的增加会引起系数的显著减小,对胞元的热应力分析表明纤维束上的热应力水平大于基体。通过自由膨胀加温试验对材料纵向热膨胀系数进行了测定,在室温至1100℃区间内热膨胀性能稳定,试验结果与预测值符合较好。可为含界面层和基体孔隙的三维编织复合材料及其他多孔复合材料热膨胀性能研究提供理论基础。     

二维二轴1×1编织复合材料细观结构模型及力学性能有限元分析

考虑纤维束相互挤压及横截面形状变化, 采用纤维束截面六边形假设, 建立了二维二轴1×1编织复合材料的参数化单胞结构模型。通过引入周期性位移边界条件, 基于细观有限元方法, 对编织材料的弹性性能进行预测, 讨论了编织角及纤维体积含量对面内弹性常数的影响, 并分析了典型载荷下单胞细观应力场分布。研究表明: 单胞结构模型有效反映了纤维束的空间构型和交织特征, 实现了不同编织工艺参数下模型的快速建立; 基于单胞有限元模型的弹性性能预测结果与试验结果较为吻合; 模型给出了单胞合理的应力场分布, 为二维编织复合材料的结构优化和损伤预测奠定基础。      

圆管状立体机织复合材料的多尺度分析

采用多尺度耦合的数值模型研究了圆管状立体机织复合材料的力学性能。建立了反映纤维束中纤维/基体二相材料的微观尺度单胞和反映周期性编织结构的细观尺度扇形单胞,并重点讨论了扇形单胞的周期性边界条件。通过逐级计算微观单胞、细观单胞的平均弹性常数,得到了圆管状立体机织复合材料的刚度参数,实现了由组分材料性能及编织参数预测圆管的宏观弹性性能,模型预测刚度与试验结果吻合。另一方面,研究了从大到小各尺度耦合的应力分析,对于圆管环向应力非周期分布的情况,建立了嵌入细观单胞的环状模型,进行了复杂荷载下从宏观圆管结构、到细观纤维束尺度、再到微观纤维尺度之间的逐级应力分析。     

Meso-Scale Damage Simulation of 3D Braided Composites under Quasi-Static Axial Tension

The microstructure of 3D braided composites is composed of three phases: braiding yarn, matrix and interface. In this paper, a representative unit-cell (RUC) model including these three phases is established. Coupling with the periodical boundary condition, the damage behavior of 3D braided composites under quasi-static axial tension is simulated by using finite element method based on this RUC model. An anisotropic damage model based on Murakami damage theory is proposed to predict the damage evolution of yarns and matrix; a damage-friction combination interface constitutive model is adopted to predict the interface debonding behavior. A user material subroutine (VUMAT) involving these damage models is developed and implemented in the finite element software ABAQUS/Explicit. The whole process of damage evolution of 3D braided composites under quasi-static axial tension with typical braiding angles is simulated, and the damage mechanisms are revealed in detail in the simulation process. The tensile strength properties of the braided composites are predicted from the calculated stress-strain curves. Numerical results agree with the available experiment data and thus validates the proposed damage analysis model. The effects of certain material parameters on the predicted stress-strain responses are also discussed by numerical parameter study.     

The effect of yarn distortion on the mechanical properties of 3D four-directional braided composites

A representative volume cell (RVC) is chosen to analyze the mechanical properties of 3D (3 dimensions) four-directional braided composites. Owing to braid yarns (an assembly of fibers) squeezing against each other in the braided composites, the braid yarns are distorted. Based on geometrical characteristics of the braided composites, cross-section of each braid yarn is supposed to be an octagon and divided into seven regions in the RVC. The distortion characteristics of yarns are considered in each region. Elastic properties of each region obtained by stochastic function theory are introduced into finite element model to calculate the mechanical properties of the RVC. The influences of yarn distortion on the stiffness and strength of the braided composites are obtained and discussed.     

三维四向编织陶瓷基复合材料改进模型及刚度预报

基于对三维四向编织陶瓷基复合材料CT扫描结果的观察和理论分析, 参考现有交织模型, 建立了改进的胞元三维实体模型, 较为真实地反映了材料内部的细观结构。模型内部纤维束横截面沿纤维束轴向不断发生形状和面积的周期性变化, 纤维束横截面呈平行四边形、五边形交替变化, 不同纤维束轴线间呈交织关系, 接近材料内部纤维束间打紧后的挤压变形规律。通过测算平均纱线填充因子并配合有限元法获得了纤维束及材料的弹性性能, 与试验结果符合较好。有限元仿真显示在材料单胞内, 纤维束承担主要载荷, 纤维束与基体的某些交界处往往会出现应力集中现象, 可能是发生裂纹扩展及局部破坏的主要区域。该细观应力场的获得也为分析材料破坏机理和强度提供了基础。      

Effect of Interface Properties on Mechanical Behavior of 3D Four-Directional Braided Composites with Large Braid Angle Subjected to Uniaxial Tension

A Representative Volume Cell (RVC) chosen to epitomize the entire three dimensional four-directional braided composites is investigated to evaluate the mechanical behavior of the material by computational micromechanics. In addition to including several damage modes of braid yarn and matrix within the braided composites, the numerical model also takes into account interface damage mode by using a Cohesive Zone Model (CZM). A parametrical study is conducted to evaluate the influence of interface properties on the macro stress-strain curve and the interaction of different failure modes of the braided composites under uniaxial tensile loading. The interface damage evolution of the braided composites with large braid angle is also provided further. Preliminary results indicate that the interface damage, which is one of the key factors to cause the nonlinearity of the stress-strain relationship, can decrease the elastic modulus but not obviously control the ultimate strength of the braided composites with large braid angle.     

Meso-Scale Finite Element Simulations of 3D Braided Textile Composites: Effects of Force Loading Modes

Meso-scale finite element method (FEM) is considered as the most effective and economical numerical method to investigate the mechanical behavior of braided textile composites. Applying the periodic boundary conditions on the unit-cell model is a critical step for yielding accurate mechanical response. However, the force loading mode has not been employed in the available meso-scale finite element analysis (FEA) works. In the present work, a meso-scale FEA is conducted to predict the mechanical properties and simulate the progressive damage of 3D braided composites under external loadings. For the same unit-cell model with displacement and force loading modes, the stress distribution, predicted stiffness and strength properties and damage evolution process subjected to typical loading conditions are then analyzed and compared. The obtained numerical results show that the predicted elastic properties are exactly the same, and the strength and damage evolution process are very close under these two loading modes, which validates the feasibility and effectiveness of the force loading mode. This comparison study provides a suitable reference for selecting the loading modes in the unit-cell based mechanical behavior analysis of other textile composites.     

三维全五向编织复合材料弹性性能及热物理性能的有限元分析

在三维全五向(Q5D)编织复合材料细观结构模型的基础上, 建立了其单胞参数化有限元模型。通过施加合理的边界条件, 计算得到了Q5D编织复合材料的弹性常数、 热传导系数和热膨胀系数, 所得结果与现有的实验数据吻合较好。在此基础上, 深入研究了纤维体积分数、 编织角等工艺参数对材料弹性性能和热物理性能的影响规律, 并将计算结果与三维四向(4D)和三维五向(5D)编织复合材料的相应结果进行了对比。结果表明, Q5D编织复合材料具有较好的力学性能和纵向导热性能, 其零膨胀结构的可设计性更强, 为进一步研究此种结构材料的强度问题和热力耦合问题奠定了基础。     

Investigation on the compressive properties of the three dimensional four-directional braided composites

The compressive mechanical properties of three dimensional (3D) braided composites are of key concern for design in actual engineering application. A representative volume cell (RVC) is chosen to study the uniaxial compressive mechanical properties of the braided composites with different braid angles by combing damage theory and finite element method. The fiber misalignment and longitudinal shear nonlinearity of braid yarn are considered in the computation model. And their influences on the compressive behavior of the braided composites are also evaluated. The damage development of constituents within the braided composites are obtained and analyzed. The main damage and failure modes and their interaction of braid yarn are provided as well. The numerical results are found that the compressive mechanical behavior of the braided composites with lower braid angle is sensitive to the fiber initial imperfection of braid yarn. The strength of the braided composites with different braid angle is controlled by the different microscopic failure modes.     

Finite Element Analysis of Thermo-Mechanical Properties of 3D Braided Composites

This paper presents a modified finite element model (FEM) to investigate the thermo-mechanical properties of three-dimensional (3D) braided composite. The effective coefficients of thermal expansion (CTE) and the meso-scale mechanical response of 3D braided composites are predicted. The effects of the braiding angle and fiber volume fraction on the effective CTE are evaluated. The results are compared to the experimental data available in the literature to demonstrate the accuracy and reliability of the present method. The tensile stress distributions of the representative volume element (RVE) are also outlined. It is found that the stress of the braiding yarn has a significant increase with temperature rise; on the other hand, the temperature change has an insignificant effect on the stress of the matrix. In addition, a rapid decrease in the tensile strength of 3D braided composites is observed with the increase in temperature. It is revealed that the thermal conditions have a significant effect on the strength of 3D braided composites. The present method provides an effective tool to predict the stresses of 3D braided composites under thermo-mechanical loading.