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。     

纤维缠绕复合材料起伏区域残余应力

针对纤维交叉起伏区域残余应力,建立一种细观分析模型。基于热传导方程、固化反应动力学模型和复合材料层合理论,采用有限元方法和细观分析模型,对缠绕复合材料在固化工艺过程中的残余应力分布及其变化规律进行数值模拟。通过算例,研究纤维起伏区域残余应力的分布特点,结果表明:起伏纤维束不同位置处残余应力差别很大,层合区纤维束呈现拉应力状态,起伏区纤维束呈现压应力状态,富树脂区出现最大压应力;残余应力沿纤维束起伏方向呈现V型变化趋势,在纤维束上不同位置出现拉、压不同的应力状态,起伏角度最大处出现最大压应力。     

三维机织复合材料多尺度黏弹性分析

建立了一种三维机织复合材料多尺度的黏弹性分析模型。首先构造了微观尺度纱线束胞元和细观尺度复合材料周期结构胞元两级有限元模型, 由微观尺度胞元分析得到纱线束的弹性常数, 再代入细观尺度胞元计算出复合材料的平均弹性常数。两级胞元模型均施加周期边界条件, 保证了胞元边界上位移和应力满足周期性和连续性。随后分别建立了树脂基体和浸润树脂纱线束的蠕变模型, 用实验标定树脂的蠕变参数, 代入微观尺度胞元进行蠕变计算来修正纱线束蠕变模型的参数。最后将树脂和纱线束的蠕变本构关系应用于细观尺度胞元, 得到材料宏观平均的应力-应变响应, 模拟了三维机织复合材料的蠕变实验曲线。本文模型对于该种复合材料弹性常数和蠕变性能的预测, 均与实验吻合。      

三维机织复合材料残余应力/应变多尺度分析及工艺参数优化

为预测三维机织复合材料工艺引入的残余应力/应变,提出工艺制度优化方案,建立了一种工艺过程分析的多尺度模型。通过建立纤维尺度及纱线尺度代表体元(RVE),计算了成型过程中纤维纱线及三维机织复合材料的模量演化历程。考虑固化过程中树脂的化学收缩效应,在纱线尺度上开展热-化学-力学耦合分析,预测了细观残余应力-应变及其演化规律。采用三维机织技术,实现了光纤布拉格光栅传感器(FBG)在三维机织预制体中的预埋,并对其树脂传递模塑(RTM)成型过程中的温度、应变历程进行监测,试验结果验证了有限元模型的准确性。采用基于空间信息、误差信息和优化结果的三种序列采样方法,建立了工艺过程分析代理模型,并开展工艺参数优化设计,结果显示采用优化后的工艺参数,残余应变降低了15.4%,工艺周期缩短了10.6%。      

残余应力对复合材料弹2塑性变形的影响

从细观力学的角度给出了分析残余应力对一般复合材料塑性性能影响的一种解析方法, 该方法基于应力二阶矩的割线模量法及Ponte Castaneda 和W illis 给出的弹性细观模型。有残余应力时, 所提的细观解析模型能够同时考虑纤维形状, 体积百分比, 纤维取向及纤维的分布对复合材料变形的影响。计算结果表明, 残余应力的存在会引起复合材料拉压变形的不对称, 材料宏观的拉压硬化曲线又与复合材料的细观结构参数密切相关。对单向复合材料, 本文作者对其等效割线热膨胀系数, 拉压应力-应变曲线的有限元分析结果与给出的细观解析模型定量吻合。      

基于结构参数的平纹机织复合材料等效弹性性能预测

经向纤维束与纬向纤维束纵横交错引起的纤维弯曲(也称为波纹)是平纹机织复合材料固有特征。首先,提出了一种精确描述平纹机织复合材料单胞3D结构特征的数学表达式。其次,基于经典层合板理论和等应力假设,考虑平纹机织复合材料厚度方向非对称引起的弯曲-拉伸耦合效应及单胞结构特征,建立了含结构参数的平纹机织复合材料等效弹性性能多参数解析模型。通过数个典型算例验证了建立的多参数解析模型,结果表明：该多参数解析模型预测值与相关文献中有限元模型预测值、解析模型预测值、实验值等均吻合较好;该多参数解析模型预测值尤其是Z向弹性性能预测值,比文献中解析模型预测值更接近于实验值。在此基础上,进一步探讨了纤维束波纹比(包括纤维束波动方向波纹比与纤维束横截面波纹比)、经向与纬向纤维束构成的预成形体厚度、纤维束中弯曲部分的长度、相邻纤维束之间间距等结构参数对平纹机织复合材料弹性性能影响。该多参数解析模型建模方法为研究纺织复合材料力学性能提供了参考。     

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。      

三维四向编织复合材料界面损伤机理数值分析

考虑界面脱粘表面压应力下摩擦力对材料界面力学性能的影响,建立损伤-摩擦相结合的界面本构模型,编写用户材料子程序VUMAT,实现其在有限元软件ABAQUS中的嵌入。基于周期性胞元分析思想,在单胞模型中纤维束/基体、纤维束/纤维束分界面引入界面单元,结合损伤-摩擦相结合的界面本构模型,建立含界面相三维四向编织复合材料的细观有限元模型。模拟典型载荷下界面损伤的起始和扩展过程,分析界面应力传递和界面破坏机理,研究界面性能对复合材料宏细观力学性能的影响规律,为实现三维四向编织复合材料界面性能优化设计和控制提供参考。      

含缺陷平纹机织复合材料拉伸力学行为数值模拟

基于平纹机织复合材料的细观结构单胞模型, 考虑其制备过程中产生的孔隙缺陷为随机分布的特征, 通过引入两参数Weibull分布函数, 应用Python语言实现了ABAQUS的二次开发, 并采用Linde等提出的失效准则, 建立了含孔隙缺陷平纹机织复合材料的渐进损伤模型, 利用有限元数值方法模拟了其拉伸应力-应变行为, 针对该模型, 讨论了孔隙缺陷对材料拉伸应力-应变行为的影响, 并阐述了该平纹机织复合材料单胞模型在经向拉伸载荷作用下其纤维束的损伤及演化过程。结果表明, 该模型给出的数值模拟结果与实验数据吻合较好, 证明了模型的有效性, 为该类材料的优化设计及其力学性能分析提供了一种有效方法。      

含纤维束截面形状变化的三维编织复合材料细观模型及刚度预报

基于实验观察和理论研究, 重点分析了材料内部区域纤维束的空间构型, 建立了一个新的三维实体细观结构模型, 并指出了编织工艺参数和模型细观结构参数之间的关系。该模型较真实地反映了纤维束之间的相互挤压变形方式, 纤维束横截面积沿纤维束轴向不断变化, 更符合三维四向编织复合材料的实际结构。基于刚度体积平均及柔度体积平均混合思想, 建立了相应的刚度预报模型。用该模型计算编织复合材料几何特性及工程弹性常数的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, 为进一步研究三维编织复合材料的拉伸强度及破坏机制提供了基础。     

碳纤维织物力学性能和冲压成形实验研究

在实际成形过程中,碳纤维复合材料往往处于复杂的应力状态,开展近于真实载荷环境下的力学试验分析,能够更准确地认识实际应用中材料的成形性能和变形机理.为获得碳纤维织物的基本力学特性,设计了平纹碳纤维织物拉伸试样及成形试样,进行了单轴拉伸、双轴拉伸、镜框剪切试验和方盒冲压成形实验研究,对比了不同双拉比及纱线取向对力学性能及成形性能的影响.研究结果表明:碳纤维织物具有高度的非线性、各向异性和双拉耦合特性,即经纬向纤维的力学性能会相互影响;剪切变形是成形过程中的主要变形模式,当剪切角达到临界锁死角时,织物发生起皱现象;同种织物不同纱线取向试样表现出不同的成形性能,因此可以根据零件几何形状选择合适纤维取向的织物,从而减少缺陷,优化成形零件的力学性能.研究结果为后续建立碳纤维织物本构模型和成形仿真奠定了基础.     

Estimate Interface Shear Stress of Woven Ceramic Matrix Composites from Hysteresis Loops

An approach to estimate the fiber/matrix interface shear stress of woven ceramic matrix composites during fatigue loading has been developed in this paper. Based on the analysis of the microstructure, the woven ceramic matrix composites were divided into four elements of 0^o warp yarns, 90^o weft yarns, matrix outside of the yarns and the open porosity. When matrix cracking and fiber/matrix interface debonding occur upon first loading to the peak stress, it is assumed that fiber slipping relative to matrix in the interface debonded region of the 0^o warp yarns is the mainly reason for the occurrence of the hysteresis loops of woven ceramic matrix composiets during unloading and subsequent reloading. The unloading interface reverse slip length and reloading interface new slip length are determined by the interface slip mechanisms. The hysteresis loops of three different cases have been derived. The hysteresis loss energy for the strain energy lost per volume during corresponding cycle is formulated in terms of the fiber/matrix interface shear stress. By comparing the experimental hysteresis loss energy with the computational values, the fiber/matrix interface shear stress of woven ceramic matrix composites corresponding to different cycles can then be derived. The theoretical results have been compared with experimental data of two different woven ceramic composites.     

Micro-CT analysis of internal structure of sheared textile composite reinforcement

Simple shear is a deformation mechanism typical for a woven fabric during draping. The mesoscopic internal structure of the fabric differs between the non-deformed state and a sheared state. This paper presents an analysis of the internal structure of woven fabrics in a sheared state based on micro-CT (X-ray micro computed tomography) imaging of the internal structure of woven fabrics in a sheared state. Two methods for the analysis of the fabric geometry are used: automatic mapping of the local fibre directions based of the micro-CT image analysis and manual measurements of the yarns cross section shape, size and middle line coordinate of yarns on the micro-CT images. Changes of the fibre orientations within the yarns and of the yarn geometrical parameters in a carbon fibre twill woven fabric before and after shear deformation are quantified.     

三维机织复合材料在拉压循环载荷下的疲劳性能

为研究三维机织复合材料在拉伸-压缩循环载荷下的疲劳性能,对材料进行了应力比R=-1的疲劳试验。在不同的载荷水平下,分别进行了纬向和经向两类拉压疲劳试验。试验获得了试样在疲劳载荷下的滞回曲线和全过程中剩余刚度比随寿命的变化曲线。结果表明,在拉伸-压缩循环载荷下,三维机织复合材料的疲劳损伤过程主要包含3个阶段,分别发生基体破坏、纱线横向裂纹扩展和纱线的最终断裂。基体的破碎和开胶、垂直于载荷方向排布的纱线撕裂和沿载荷方向排布的纱线断裂是试样内部的主要失效模式。试验还获得了纬向和经向拉压疲劳的拟合S-N曲线,可应用于工程中对该型材料进行疲劳寿命估算。该型材料的疲劳寿命在低应力区和高应力区均显示出较小的分散性,双对数坐标系下的拟合S-N曲线具有较好的线性度。     

三维机织结构的几何模型

根据三维机织结构中纱线系统的组成和相应纱线的几何形态,建立了具有普适意义的几何模型,获得了组成三维机织结构各纱线系统在一个结构单元内的纱线长度和取向角,进而计算出纤维体积分数。随后,选择了基于11种不同接结组织的三维机织复合材料试样,测试了试样的纤维体积分数,所得的测试结果与模型输出的预测值有很好的一致性。利用所建立的模型还定量讨论了接结组织对纤维体积分数和取向角的影响。结果表明:分层接结可以提供比正交接结高的纤维体积分数;而正交接结中接结经具有较大的取向,有利于增强三维机织复合材料在厚度方向上的力学性能。     

A micromechanical compaction model for woven fabric preforms. Part I: Single layer

A micromechanical model was developed to investigate elastic compression behaviour during compaction of a single layer of woven fabric preform. The compaction model describes two important deformation mechanisms at different hierarchical levels, addressing micro-deformation of yarn cross-section compaction and macro-deformation of yarn bending accompanied by yarn waveform flattening. The stress carried by the fabric is decomposed into two parts in relation to two distinct mechanisms, coupled through the requirement of deformation compatibility. With this micromechanical model the effects of microstructures of single layer woven fabric on their compaction behaviour are evaluated. It is shown that both the macro-bending stiffness of fibre and the initial fibre packing ratio of yarn affect the compaction behaviour of single layer fabric preform. The prediction is correlated with experimental data available, and satisfactory agreement is observed.     

Modeling and analyses of thermo-elastic properties of radially grown carbon nanotubes-based woven fabric hybrid composite materials

Carbon nanotubes (CNTs) may be useful due to their sound thermo-mechanical properties. The present work deals with the evaluation and analysis of elastic properties and coefficient of thermal expansions (CTEs) of a CNTs-based 2D plane woven fabric composite material system where the CNTs are radially grown on the surface of the carbon fiber. Detailed constructional features of the proposed trans-scale composite material system is explained in detail. The mathematical modeling for the material properties of each constituent or building block of the hybrid composite is developed. The mathematical model for the thermo-elastic properties of yarns is also formulated based on strength of material method whereas the thermo-elastic properties of representative unit cell is based on the unit cell method. Various numerical results have been obtained by varying the CNTs content, carbon fiber contents, and temperatures. Effects of different geometrical parameters (such as yarn thickness, yarn width, and the ratio of gap length to yarn width of the yarn) on the elastic properties as well as the CTEs are also investigated.     

碳纤维-玻璃纤维混杂增强环氧树脂三维编织复合材料薄壁圆管压溃吸能特性与损伤机制

基于三维编织成型及真空辅助树脂传递成型技术,制备了编织纱和轴纱不同混杂方式(编织纱/轴纱:碳纤维-碳纤维(CF-CF)、碳纤维-玻璃纤维(CF-GF);玻璃纤维-碳纤维(GF-CF))增强环氧树脂(EP)的三类三维编织复合材料薄壁圆管,通过准静态轴向压溃及详细的破坏断面观察,研究了纤维混杂方式对薄壁圆管的能量吸收性能和破坏模式的影响。研究发现:CF-CF/EP样品的比能量吸收值分别比GF-CF/EP大36%,比CF-GF/EP大12%。编织纱为碳纤维时(CF-CF/EP及CF-GF/EP),圆管的破坏模式均为折叠破坏模式,编织纱采用碳纤维能有效地遏制中央裂纹的轴向扩展,折叠变形的三维结构内部发生了较多细小的微观破坏。而编织纱为玻璃纤维的GF-CF/EP,破坏模式则为开花内外弯曲式,中央裂纹产生,三维结构呈现分层并向圆管内外弯曲。      

Modeling and numerical simulation of the mechanical behavior of woven SiC/SiC regarding a three-dimensional unit cell

The mechanical behavior of a two-dimensional woven SiC/SiC ceramic matrix composite (CMC) under tensile loading is modeled by regarding a three-dimensional unit cell of the composite which takes the waveness of the fiber yarns in two directions into account. The numerical evaluation of the model is accomplished by means of the finite element method (FEM). Because of the small diameter (15 μm) of the fibers in the bundles, these fibers are not considered as single entities in the finite element mesh. Instead the mesh is constructed on the yarn scale considering the fiber bundles as homogeneous with ‘fiber bundle' properties. The brittle cracking of interyarn matrix as well as transverse cracking of the fiber bundles are considered by defining a fracture criterion for these components. The increasing degradation of the fiber bundles in fiber direction during progressive loading is described by three damage variables. Damage and cracks are modeled by reducing the elastic coefficients of the finite elements. The scattering of the strength values of the single components is regarded by using Weibull distribution. Before mechanical loading, the structure is subjected to thermal loading to consider the residual thermal stresses in the structure resulting from the cooling down process after fabrication of the composite. The effect of the scattering of the strength values on the behavior of the unit cell will be examined.