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。      

二维平纹机织复合材料弹性性能预测的域分解方法

为了预测二维平纹机织复合材料的弹性性能, 提出了基于有限元重合网格法的域分解方法。域分解方法与传统代表体元法的有限元建模不同, 前者不再建立精细的纤维与基体模型, 而是分别建立二维平纹机织复合材料单胞的整体域与纤维域, 整体域是真实基体体积与纤维体积的叠加, 两区域网格独立剖分, 互不影响。采用MSC. Nastran中的多节点约束在纤维节点与基体节点之间建立位移协调来模拟纤维和基体单元的位移函数关系, 实现了纤维域和基体域的耦合计算。研究表明, 域分解方法大大简化了机织复合材料细观力学建模的复杂性, 降低了建模时间, 采用域分解方法预测的二维平纹机织复合材料弹性常数与试验值吻合较好, 充分说明了该预测模型与方法的正确性。研究了不同纤维体积分数下, 域分解方法预测二维平纹机织复合材料的弹性常数的变化趋势, 结果表明, 随纤维体积分数增加, 模量呈上升趋势, 泊松比呈降低趋势。     

跨尺度预测非屈曲织物增强复合材料的刚度和强度

为了预测非屈曲织物增强复合材料的力学性能, 建立了纤维束的正六边形单胞和非屈曲织物复合材料的长方形单胞, 并重点推导了正六边单胞的方程边界条件。通过跨尺度逐级计算这两个单胞的有效弹性常数, 得到了非屈曲碳纤维织物增强环氧树脂基复合材料的宏观有效弹性性能和强度。对该非屈曲织物复合材料在拉伸载荷下的累计失效进行了有限元损伤分析。结果表明: 初始损伤发生在富树脂区或横向纤维束, 损伤在富树脂区与横向纤维束内逐步扩展, 最后向纵向纤维束扩展并迅速导致整体失效; 非屈曲织物增强复合材料的面内拉伸模量的计算预测值非常接近实验值, 面内拉伸强度计算值略小于实验值。     

基于多尺度方法的纤维织物增强柔性复合材料拉伸模量预测

以超高分子量聚乙烯（UHMW-PE）纤维织物增强-聚乙烯（PE）涂层柔性复合材料作为研究对象,首先,通过离子抛光仪对复合材料横截面进行处理;然后,使用SEM和光学显微镜测量复合材料细观结构,获得复合材料细观几何参数;最后,基于均匀化方法和连续介质假设,建立单胞力学模型,计算单胞的拉伸载荷-应变曲线,将理论值与实验值进行比较。结果表明:基于多尺度方法的复合材料单胞力学模型所得拉伸载荷-应变曲线与实验所得曲线能较好吻合,该理论模型能够较好地预报纤维织物增强柔性复合材料的拉伸模量。      

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。     

弯曲载荷下机织复合材料T型接头渐进失效分析

根据树脂传递模塑(RTM)成型的缎纹机织复合材料T型接头的结构特征和纤维布局特点, 基于ANSYS有限元数值分析平台, 建立符合其真实结构的几何模型和有限元分析模型。基于渐进失效强度预测方法的基本思想, 使用有限元计算软件ANSYS的参数化设计语言(APDL)开发相应的程序, 实现改进形式的Hashin失效准则。采用合适的最终失效评价方法, 建立二维机织结构复合材料T型接头受弯曲载荷时的渐进失效预测方法, 能够有效地模拟从初始加载到最终失效过程中机织复合材料T型接头结构的力学响应及损伤的萌生与发展, 并预测结构的静强度。      

缝合复合材料弹性性能分析的新方法——流场模型Ⅰ:缝合单层板弹性常数计算

为了求解缝合层合板中单层的弹性常数, 基于单层面内纤维走向与绕流流场中流线形状的相似性, 建立了与缝合单胞相对应的有限空间定常二维无粘性不可压理想流体的无旋绕流流场模型, 流场的几何边界与单胞的边界一致, 绕流物面与缝线截面形状一致。用该流场的速度变化描述单胞面内纤维体积含量的变化, 用流线形状描述缝线周围面内纤维的变形。以流场模型为基础, 用细观力学方法和均匀化方法求得缝合单层的弹性常数,结果与实验值吻合。最后用流场模型分析了缝合参数对缝合单层弹性常数的影响。      

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。      

2. 5维机织结构复合材料的几何模型

基于经纱矩形截面及纬纱双凸透镜截面假设 , 分析了 2. 5 维机织复合材料的细观几何结构 , 重点考虑了该结构表层经纱与内部经纱密度的不同及相同机织结构合成不同厚度和纤维束截面的情况 , 建立了 2. 5 维机织复合材料的单胞几何模型。该几何模型可以计算各种 2. 5 维机织结构单胞内各纱线系统的形态 , 包括取向角和纤维体积分数。通过对 8种结构 28 个试件纤维体积分数的测定 , 与计算预测结果的对比表明本文中建立的几何模型较好地反映了 2. 5维机织复合材料的内部结构。此外 , 利用本模型计算分析了 3 种不同结构的纤维体积分数和取向角。结果表明 : 单胞内经纬纱交织的次数是决定纤维体积含量的一个关键因素 ; 直联结构相比弯联结构 , 其经纱取向角明显降低。      

Numerical and experimental determination of in-plane elastic properties of 2/2 twill weave fabric composites

This paper presents the in-plane elastic properties of 2/2 twill weave, T300 carbon/epoxy, woven fabric composite plates, obtained by both finite element analysis and experiments. A micromechanical, three-dimensional (3D) finite element model of the single layer unit cell of a 2/2 twill weave fabric composite is built, and a homogenization process is implemented. A unit cell is chosen such that it encloses the characteristic periodic repeat pattern in the fabric weave. Detailed geometry together with construction procedures for this new model are developed by using ANSYS Parametric Design Language (APDL). In this respect, the scope for altering the weave and yarn parameters is facilitated. Standard tensile and rail shear tests with modifications are performed for this kind of woven fabric composite. Elastic mechanical properties determined by experiments are presented, and the finite element model is verified. Satisfactory correlation between the predicted and experimental results are obtained.     

Meshfree modeling and homogenization of 3D orthogonal woven composites

In this paper, evaluation of 3D orthogonal woven fabric composite elastic moduli is achieved by applying meshfree methods on the micromechanical model of the woven composites. A new, realistic and smooth fabric unit cell model of 3D orthogonal woven composite is presented. As an alternative to finite element method, meshfree methods show a notable advantage, which is the simplicity in meshing while modeling the matrix and different yarns. Radial basis function and moving kriging interpolation are used for the shape function constructions. The Galerkin method is employed in formulating the discretized system equations. The numerical results are compared with the finite element and the experimental results.     

A meso-scale finite element model for simulating free-edge effect in carbon/epoxy textile composite

Textile composites are well known for their excellent through thickness properties and impact resistance. In this study, a representative unit cell model of a triaxial braided composite is developed based on the composite fiber volume ratio, specimen thickness and microscopic image analysis. A meso-scale finite element (FE) mesh is generated based on the detailed unit cell dimensions and fiber bundle geometry parameters. The fiber bundles are modeled as unidirectional fiber reinforced composites. A micromechanical finite element model was developed to predict the elastic and strength material properties of each unidirectional composite by imposing correct boundary conditions that can simulate the actual deformation within the braided composite. These details are then applied in the meso-mechanical finite element model for a 0°/+60°/−60° triaxially braided T700s/E862 carbon/epoxy composite. Model correlations are conducted by comparing numerical predicted and experimental measured axial tension and transverse tension response of a straight-sided, single-layer (one ply thick) coupon. By applying a periodic boundary condition in the loading direction, the meso model captures the local damage initiation and global failure behavior, as well as the periodic free-edge warping effect. The failure mechanisms are studied using the field damage initiation contours and local stress history. The influence of free-edge effect on the failure behaviors is investigated. The numerical study results reveal that this meso model is capable of predicting free-edge effect and allows identification of its impact on the composite response.     

2.5D机织复合材料弹性性能的有限元分析

在已有研究的基础上,提出了一个新的2.5D机织复合材料有限元模型,该模型较为真实的模拟了织物内纤维束的轮廓结构和走向,模型中纤维束单元的材料属性根据其所处位置及纱线走向的不同对其分别进行定义。利用该模型,以机织结构和纤维束排列密度为参数,详细研究了其对2.5维机织复合材料弹性性能的影响情况,并对其影响特征进行了分析讨论。将计算结果与实验值和刚度平均法预测结果进行了对比分析。结果表明,有限元法预测结果介于实验值和刚度平均法预测值之间,非常接近于实验值,且优于刚度平均法预测的结果。     

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。      

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.     

A homogenization procedure for the numerical analysis of woven fabric composites

The paper presents a procedure for the numerical evaluation of the mechanical properties of woven fabric laminates. Woven fabrics usually present orthogonal interlaced yarns (warp and weft) and distribution of the fibers in the yarns and of the yarns in the composite may be considered regular. This allows us to apply the homogenization theory for periodic media both to the yarn and to the fabric. Three-dimensional finite element models are used in two steps to predict both the stiffness and the strength of woven fabric laminates. The model includes all the important parameters that influence the mechanical behavior: the lamina thickness, the yarn orientation, the fiber volume fraction and the mechanical characteristics of the components. The capabilities of the numerical model were verified studying the elastic behavior of a woven fabric laminate available in the literature and the ultimate strength of a glass fabric laminate experimentally investigated. The procedure, that can be implemented into commercial finite element codes, appears to be an efficient tool for the design of textile composites.     

Tensile Properties and Failure Mechanism of a New 3D Nonorthogonal Woven Composite Material

Tensile properties and failure mechanism of a newly developed three-dimensional (3D) woven composite material named 3D nonorthogonal woven composite are investigated in this paper. The microstructure of the composite is studied and the tensile properties are obtained by quasi-static tensile tests. The failure mechanism of specimen is discussed based on observation of the fracture surfaces via electron microscope. It is found that the specimens always split along the oblique yarns and produce typical v-shaped fracture surfaces. The representative volume cell (RVC) is established based on the microstructure. A finite element analysis is conducted with periodical boundary conditions. The finite element simulation results agree well with the experimental data. By analyzing deformation and stress distribution under different loading conditions, it is demonstrated that finite element model based on RVC is valid in predicting tensile properties of 3D nonorthogonal woven composites. Stress distribution shows that the oblique yarns and warp yarns oriented along the x direction carry primary load under x tension and that warp yarns bear primary load under y tension.