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.     

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.     

三维四步方形编织结构的几何建模

研究三维复合材料的编织结构是分析这种材料力学性能的前提。从三维编织工艺和实际的编织过程出发,针对方形编织结构提出了一种单元几何模型。该模型以携纱器循环一周返回到起始位置所形成的纱线编织结构作为单元,保证了纤维束的连续性和材料整体结构的完整性。对每根纱线,选取它在编织体各个区域内合适的控制点,过这些控制点拟合成三次样条曲线,以此模拟纱线的空间结构中心线。最后得到纱线和编织体的结构。     

四步法三维矩形编织复合材料的细观结构模型

基于现有实验研究和编织工艺中携纱器的运动规律, 重点分析了材料内部区域纤维束的空间构型, 建立了材料的三维实体细观结构模型。该模型不仅体现了内部纱线因打紧工序而形成的紧密接触和截面变形, 而且考虑了内部和表面区域纱线因挤紧状态不同所造成的纱线填充因子变化。基于一种单胞取向平行于材料横截面边界方向的新划分方案, 解决了45°单胞划分方案的不足, 建立了便于力学性能分析的单胞几何模型, 并指出了编织工艺参数和模型宏细观结构参数的关系。模型数值结果与试件实测数据吻合, 表明了该模型的合理有效性,为材料后续力学性能分析奠定了基础。      

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。     

Effect of Yarn Distortion on the Mechanical Properties of Fiber-Bar Composites Reinforced by Three-Dimensional Weaving

A meso-structure model of fiber-bar composites reinforced by three-dimensional weaving (FBCR3DW) is proposed. Optical microscopy images of the preform structure revealed that the fibers along the circumference of the yarn cross-weave were twisted randomly due to alternating yarn winding on either side of the fiber bars during the manufacturing process. Sections of the cross-woven yarn were divided into five regions based on the twist characteristics. Stochastic function theory was used to describe the twist characteristics and to calculate the compliance tensor for each twisted yarn region. The twist characteristics and compliance tensor of each region were then introduced into a finite element model to calculate the elastic properties of the twisted yarn and FBCR3DW; unidirectional tensile stress–strain curves were generated based on the Tsai–Wu failure criterion. Several FBCR3DW specimens with randomly twisted yarns inside the weave structure were used in experimental tests. Our numerical results were in good agreement with the experimental values. Yarn distortion had a significant effect on the elastic properties and axial tensile strength of the yarn; specifically, the influence of yarn distortion on the transverse elastic modulus and transverse shear modulus of FBCR3DW was severe, whereas only a slight effect occurred with regard to the other elastic constants and unidirectional tensile properties. Thus, the proposed method provides an effective reference for modeling fiber composites with a weave structure.     

三维编织复合材料的弯曲和压缩性能探讨研究

对四步法三维编织结构增强树脂复合材料的弯曲,压缩性能进行了实验研究,获得了该材料的主要力学性能参数及变形,破坏规律,结果表明,三维编织复合材料具有良好的力学性能,编织结构及复合材料性能有较大的影响,这些结果为进一步研究编织复合材料的强度失效问题奠定了试验基础。     

二步法方型三维编织复合材料力学性能及影响因素

对具有不同细观结构参数的二步法方型三维编织复合材料试件进行了轴向拉伸、三点弯曲及轴向压缩的力学性能测试,获得了这种结构材料的基本力学性能数据。针对其细观结构特点,从宏观角度分析了编织纱的种类、编织纱与轴纱的线密度之比以及编织节距长度等参数对力学性能的影响。实验和分析结果表明:二步法方型三维编织复合材料的轴纱对轴向拉伸和压缩性能起主导作用;节距长度增大,编织纱越粗,轴向拉伸强度、拉伸模量以及轴向压缩强度和模量均有增大的趋势。      

三维全五向编织复合材料的细观结构分析

针对三维全五向编织复合材料,研究了其四步法编织工艺的实现过程,并详细描述了编织复合材料各控制区域内纱线的空间走向及运动规律,在此基础上建立了能反映三维全五向编织复合材料基本结构的几何单胞模型。通过分析编织物内纱线间的空间挤压关系,设定合理的假设,计算了全五向编织复合材料的纤维体积含量,并分析了纤维体积含量与编织工艺参数之间的关系,为进一步研究该材料的力学性能奠定了基础。     

Investigation on the Bearing Abilities of Three-Dimensional Full Five-Directional Braided Composites with Cut-Edge

The longitudinal tensile experiments of cut-edge effect on the mechanical performance of three-dimensional full five-directional (3DF5D) braided composites were conducted. The specimens involved two different braiding angles and two different cutting ways. Fracture appearance of specimens without cut-edge and cutting along width direction presented flush, while explosive for specimen with cut-edge along thickness direction. The fracture of axis yarns mainly contributed to the damage of specimens. Cut-edge had little influence on the stiffness of 3DF5D braided composites and had approximately 20 % reduction in tensile strength compared with specimens without cut-edge. The periodic boundary conditions under cut-edge and uncut-edge situations were applied to the RVC to simulate the mesoscopic damage mechanism using finite element method. The stress–strain curves and damage evolution nephogram were obtained. The variation of cut-edge effect with the number of inner cells was predicted by superimposing inner cells method, the addition of inner cells could strengthen the performance of 3DF5D braided composites with cut-edge. These results will play an important role in evaluating the mechanical properties of braided materials after cutting.     

A methodology for Cartesian braiding of three-dimensional shapes and special structures

Cartesian three-dimensional braiding as a method of preforming has been investigated. The design of complex and unusual 3-D braids was studied in three parts. These are the grouping of yarns, the fabrication of braids with a complex cross-section, and braids with surrogate material (replacing fibrous tows) added or removed. The grouping of yarns to potentially form hybrid composites was studied via an iterative simulation of the braiding process. Through further analysis of the braid cycles which produce specific yarn grouping, it was found that isolation/insertion rows and columns may be used to quarantine yarns within desired areas of preform cross-section and improve interlacing of the braided structure. In this study, the design of braided composite cross-sectional shape is accomplished through adaptation of the Universal Method. A computer algorithm has been developed which allows the desired cross-sectional shape to be specified and a braid plan for its fabrication automatically determined. A number of 3-D braids, the result of variations or extensions to Cartesian braiding, are also presented. These have been classified as those with axial and transverse yarn insertion, structures with voids, and fillers and fasteners. Braiding equipment has been developed to braid the designed structures. The machines have been used to fabricate four-step braids with transverse, fastener, and filler insertion, special hybrid structures from multiple row and column displacement and multi-step cycles, and uniquely shaped structures with constant and varying cross-sections along their lengths.     

三维六向编织复合材料弹性性能理论预测

在三维六向编织物纱线运动规律的基础上, 建立了单胞模型, 推导了编织参数之间的数学关系。基于该模型, 采用改进的刚度平均化方法, 导出了三维六向编织复合材料的工程弹性常数, 分析了编织角和纤维体积含量对弹性性能的影响。结果表明, 三维六向编织复合材料具有良好的力学性能, 由于面内纬纱的加入, 使面内的力学性能得到了提高。      

典型多向2.5D机织预制体近净形编织结构设计

针对多向异型复合材料构件用3D整体预制体,基于衬经2.5D机织结构,提出5种近净形转向仿形编织工艺,设计并制备了具有典型引纱加纱结构的板条状预制体试样。采用计算机断层扫描法(Micro-CT),观测各系统纱线横截面形态变化和纱线取向分布规律,发现引出加入的纱线沿织物厚度方向挤紧状态发生改变,其横截面从椭圆形变成梯形,又变为三角形,经纱被引出和加入会造成与其接触的纬纱横截面变化。结合复合材料构件的实际承载工况,对具有5种引纱加纱结构的复合材料试样进行了经向抗弯性能测试,结果表明,复合材料的弯曲强度和弯曲模量保持率分别达到82.6%~95.7%和89.1%~97.9%。可见,立足于满足复合材料力学性能要求,发展预制体的三维整体仿形编织技术,是实现复杂形状复合材料构件材料/结构一体化制造的有效途径。      

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。      

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。      

三维编织复合材料面内刚度和强度性能研究

以修正的经典层合板理论为基础, 分析三维编织复合材料的力学性能。在单胞的长度方向积分和平均, 预测编织结构复合材料的有效弹性模量; 采用蔡-胡多项式失效准则, 得到三维编织复合材料的强度性能。另外, 进行编织结构复合材料的力学性能实验, 探讨纺织工艺参数, 如纤维编织角、横向编织角、轴向纱数与编织纱数之比、纤维体积含量等对力学性能的影响, 理论预报和试验结果进行对比, 发现该力学模型能较好地预报三维编织复合材料的刚度和强度性能。      

Finite Element Analysis of Mechanical Properties of 3D Four-Directional Rectangular Braided Composites Part 1: Microgeometry and 3D Finite Element Model

Based on the microstructure of three-dimensional (3D) four-directional rectangular braided composites, a new parameterized 3D finite element model (FEM) is established. This model precisely simulates the spatial configuration of the braiding yarns and considers the cross-section deformation as well as the surface contact due to the mutual squeezing in the braiding process. Moreover, it is oriented in the same reference frame as the composites, which coincides with the actual configuration of 3D braided composites and facilitates the analysis of mechanical properties. In addition, the model investigates the relationships among the structural parameters, particularly the braiding angle and the interior braiding angle, which were not taken into account in the previous models. Based on the parameterized FEM, the structural geometry of the composites is analyzed and some conclusions are drawn herein. Good agreement has been obtained between the calculated and measured values of the geometric characteristics of braided composite samples.     

Prediction on viscoelastic properties of three-dimensionally braided composites by multi-scale model

Three-dimensional viscoelastic properties of four-step three-dimensionally (3D) braided composites are studied in this paper. Based on the three-cell division scheme, a multi-scale model for 3D braided composites is proposed. A periodic boundary condition is applied to characterize the periodic structure of 3D braided composites and yarns. Given the viscoelastic parameters of resin matrix and the elastic constants of fibers, the viscoelastic properties of yarns are obtained by the finite element method and Prony Series fitting. The three-dimensional viscoelastic constitutive relationship of interior cells is derived based upon the viscoelastic properties of yarns and resin matrix. Moreover, the viscoelasticity of 3D braided composites is studied by creep experiment. The viscoelastic deformation obtained from the multi-scale method agrees well with the experimental results. The influence of the two independent micro-structural parameters, braiding angles, and fiber volume fractions, on the viscoelastic properties of 3D braided composites is investigated in detail.     

Braiding Simulation and Prediction of Mechanical Properties

Rotary braiding is a cost effective method to manufacture near net shaped preforms that generally have a closed section and may have an arbitrary shape if braiding is performed over a shaped mandrel. The reinforcement architecture can be varied by the number and spacing of active bobbins, and by the speeds used to ‘take-up’ the braid and move the circumferential bobbins. Analytical methods are available that can reliably predict yarn paths and the final braid meso-structure for simple regular sections, and further analytical methods have been proposed to estimate composite braid elastic mechanical properties. A full simulation chain using the explicit Finite Element (FE) technique is presented for composite braid manufacture and mechanical stiffness prediction of the final composite. First simulation of the braiding process provides detailed information on yarns paths and braid meso-structure, from which Representative Volume Elements (RVE) of the braid may be constructed for analysis of stiffness properties. The techniques are general and can be applied to any braid geometry. A specific problem of meshing the yarn structure and interspersed resin volumes is overcome using conventional solid elements for the yarns and Smooth Particle Hydrodynamics for the resin, with link element to join the two constituents. Details of the background theory, braid simulation methods, meso- model analysis and validation again analytical and test measurements are presented.     

含界面相三维全五向编织复合材料拉伸行为模拟及失效机制研究

基于三维全五向（Q5D）编织复合材料的细观结构模型,通过引入界面相单元,建立了含界面相Q5D编织复合材料单轴拉伸损伤失效分析模型。应用Python语言实现对ABAQUS的二次开发,将Linde等提出的失效准则和Von-Mises应力准则分别用于纱线和基体的渐进损伤判断,并确定材料的整体失效模式;对于界面相,采用Quads准则进行损伤判断。利用周期性位移边界条件,对含界面相Q5D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,详细讨论了在纵向拉伸载荷作用下材料的细观损伤起始、扩展和最终失效的演化过程,分析了材料的细观损伤失效机制,预测了材料的极限破坏强度,并研究了界面相性能对材料整体力学行为的影响规律。研究结果表明,数值模拟结果与实验值吻合较好,验证了渐进损伤模型的有效性,为该类材料的力学分析和优化设计奠定了基础。