基于多尺度方法的三维编织C/C复合材料弯曲失效特性分析

为了预测三维编织C/C复合材料的弯曲失效行为,基于多尺度渐进展开理论,结合细观渐进损伤模型,建立了三维编织C/C复合材料宏细观多尺度分析模型。通过商业有限元软件ABAQUS用户子程序UMAT的二次开发,在宏观结构有限元分析中实时调用细观单胞模型进行细观渐进损伤分析,实现了宏细观尺度之间交互式信息传递和多尺度损伤模拟。利用上述模型对三点弯曲载荷下三维编织C/C复合材料梁的渐进损伤和失效过程进行了模拟,预测了梁的载荷-挠度曲线和弯曲强度,并与实验结果进行了对比分析,验证了基于多尺度方法的三维编织C/C复合材料弯曲强度预测模型的有效性,为此类材料及结构失效分析提供了一种手段。     

金属基纳米复合材料等效弹性模量的均匀化方法数值模拟

均匀化理论利用位移场双尺度渐近展开建立有限元列式，本文将其与有限元通用程序相结合，应用于金属基复合材料的弹性本构数值模拟。通过对不同尺度增强相金属基复合材料等效模量的数值模拟，考察了均匀化方法的适用情况。数值计算结果表明，对常规尺度增强相金属基复合材料，均匀化方法可以较准确地预测其等效弹性模量；对纳米增强相金属基复合材料，该方法仍可给出较好的预测，但存在某种程度的系统偏差。通过对纳米尺度增强机理的分析讨论，认为纳米增强相与基体材料问的界面效应可能有别于连续介质假设，指出可以考虑采用离散原子-连续介质耦合模型改进数值模拟结果。     

树脂基三维编织复合材料粘弹性性能的有限元分析

根据材料的细观结构,采用APDL语言分别建立了纤维束和三维编织复合材料两级单胞的参数化几何模型;推导了Prony级数表示的树脂粘弹性本构方程,对模型进行了组分材料参数设置;对纤维束单胞模型进行扫掠式网格划分,对三维编织复合材料单胞模型进行线-面-体式网格划分;对两级单胞模型均施加合理的边界条件,使单胞边界上的位移满足周期性和连续性。以有限元模型为基础,计算了三维编织复合材料的粘弹性能,并给出了材料粘弹性效应随工艺参数变化的规律。计算结果表明:三维编织复合材料编织方向的粘弹性效应随编织角的增大而增强,随纤维体积比的增大而减弱。该结果与已有实验结论一致。     

复合材料弹塑性多尺度分析模型与算法

对材料非线性多尺度分析的计算模型与算法进行研究.在构建周期分布单胞分析算法的基础上,发展针对复合材料结构材料非线性多尺度分析的一般有限元方法.方法的特点是将所建立的单胞分析过程作为有限元分析的子程序嵌入到总程序系统当中,完成对应的高斯点应力计算,因而使所发展的方法具有实现方便的特点.给出数值计算结果,验证了方法与所发展的多尺度有限元分析程序的正确与有效性.     

二维平纹编织复合材料压缩力学行为研究

通过对二维平纹编织复合材料压缩力学性能进行三维有限元计算,对复合材料在沿垂直于编织平面方向和纤维束方向压缩后的微观应力场进行了分析。对于沿垂直于编织平面方向的压缩(即横向压缩),发现在经纬向纤维束相互交替处基体材料的横向压应力最大,是其薄弱部位。分析表明,与0°/90°层合板相比,编织复合材料受到横向压缩载荷时,自由边界处层间剪应力较大,容易发生层间裂纹,与实验观察相吻合。通过改变纤维束截面尺寸和纤维束之间间隙大小研究了破坏面角度的变化。     

三维编织复合材料渐进损伤的非线性模型及强度分析

建立了考虑周期性位移边界条件的细观体胞模型,对三维编织复合材料的渐进损伤过程进行数值模拟。采用Eshelby-Mori—Tanaka方法计算含损伤裂纹的材料的剐度矩阵,并将有限元网格尺寸和单元裂纹尺寸引入损伤演化方程,有效地降低了模拟结果对有限元网格的依赖程度。通过计算得到了材料应力应变的非线性关系和失效时的极限强度,并分析了材料的破坏机理。结果表明,大编织角材料的破坏模式主要是基体失效与纤维横向拉剪破坏,模拟计算结果与文献中的实验值吻合较好。     

三维五向编织复合材料渐进损伤分析及强度预测

基于材料连续体细观结构单胞,提出了材料的三维渐进损伤分析模型,采用非线性有限元方法并结合均匀化平均思想,首次建立了三维五向编织复合材料的强度预测模型。经研究典型编织角材料在拉伸载荷作用下细观损伤的发生及演化过程,分析了材料的细观失效机理,获得了材料的宏观拉伸应力应变曲线和极限破坏强度,并详细探讨了主要工艺参数编织角对材料宏观力学性能的影响规律。     

多向编织复合材料的力学性能研究

综述并评价了关于二维和三维编织复合材料的有效弹性模量研究的代表性工作,并从材料设计的思想出发,宏观与微观相结合,材料科学与力学相结合,对多向编织复合材料的宏观力学性能与细观织物结构、组分性能的关系及编织复合材料非线性行为进行了详细的理论分析和研究     

基于降阶均匀化理论的混凝土双尺度力学性能分析

混凝土材料细观特性对宏观力学性能有着重要影响。为进一步分析混凝土细观特性对宏观力学行为的影响规律,将混凝土材料简化为由骨料、砂浆和界面三相组成,编制了随机凸多面体骨料生成、投放和网格剖分算法,建立可用于有限元计算的满足级配要求的随机细观模型。针对直接使用细观力学模型计算量较大的问题,采用降阶均匀化理论,对混凝土细观胞元模型进行预处理并编制了相应的双尺度计算程序。对不同强度混凝土进行了单轴静态压缩双尺度计算,与实验数据和细观力学模拟结果符合较好。研究表明,降阶均匀化理论在加快求解速度的同时具有较高的精度,可以用于混凝土的多尺度力学性能分析。     

周期性复合材料湿热力耦合问题的二阶渐近分析

渐近均匀化方法是一种很好的分析复合材料力学性能的方法。通过建立考虑材料湿热效应的双尺度渐近展开模型,研究了周期性复合材料在湿、热、力耦合作用下的结构响应。将带有小参数的位移渐近展开式代入湿热力耦合控制方程,利用等效积分"弱"变换推导出复合材料等效参数的单胞控制方程以及均匀化方程。为了提高计算精度,本文对均匀化解进行了一阶和二阶修正,利用FreeFem++软件进行了数值模拟。结果显示:双尺度二阶渐近分析能够高效、准确地计算出一系列多场耦合问题,为研究周期性复合材料的力学性能提供了新的方法。     

粘弹性树脂基三维编织复合材料的热膨胀性能研究

首先利用均匀化理论并结合有限元法研究了三维编织复合材料的粘弹性效应,根据松弛模量的计算结果研究了材料热膨胀系数随时间的变化关系,在此基础上,给出了编织结构和工艺参数(编织角、纤维体积比)对材料初始热膨胀系数的影响规律,计算结果与实验值吻合较好。本文工作为深入研究三维编织复合材料的热粘弹性能提供了基础。     

三维编织复合材料渐进损伤的非线性数值分析

基于考虑纤维束相互挤压的八边形纤维束截面单胞模型，引入周期性位移边界条件，采用 细观非线性有限元方法，建立了三维四向编织复合材料的渐进损伤拉伸强度模型. 该模型考 虑了增强体纤维束纵向非线性剪切应力-应变关系，采用Hashin型损伤失效准则定义了纤维 束的典型损伤类型，并根据纤维束和纯基体相应损伤类型所造成的材料性能退化，模拟了不 同编织角试件各类损伤产生、扩展及材料最终破坏的整个过程. 模型数值结果与实验数据吻 合较好，证明了该模型的合理有效性. 探讨了组分材料剪切非线性、损伤对材料宏观非线性 本构行为的影响，结果表明：随着编织角增大，纤维束剪切非线性效应和累积损伤对材料非 线性力学行为的影响明显增强.     

二维三轴编织复合材料的弹性性能分析

提出了二维三轴编织复合材料的几何模型,模型考虑了纤维束的弯曲扭转状态及空间交错特性等几何元素。基于体积平均法,建立了预测二维三轴编织复合材料弹性性能的理论分析模型;通过引入更普遍适用的周期性位移边界条件,结合二维三轴编织复合材料的细观实体结构,建立了分析其力学性能的有限元模型。两种模型预测结果均与试验结果吻合,证明了方法的合理有效性。分析了材料受载下的细观应力分布,并讨论了编织参数对材料性能的影响。研究表明,二维三轴编织复合材料轴向性能得到了增强,应力分布更均匀,编织角以及纤维体积含量对材料弹性性能影响较大。     

随机孔隙缺陷对3D编织复合材料力学性能的影响研究

3D编织复合材料经RTM工艺成型将难以避免产生孔隙缺陷,孔隙缺陷主要包括纤维束内干斑和富树脂区孔隙。经分析RTM工艺孔隙缺陷的产生机理,提出了随机孔隙缺陷分布模型,建立了基于细观胞元的含孔隙缺陷有限元分析模型,并应用概率统计方法,分析了缺陷的随机分布和孔隙率对材料等效弹性性能的影响,可为3D编织复合材料力学性能的设计提供依据。     

Nonlinear viscoelastic multi-scale repetitive unit cell model of 3D woven composites with damage evolution

Three-dimensional (3D) textile composites have great potential applications to aircrafts and high speed vehicles because of the high strength/weight ratios and the capabilities of manufacturing complex, net-shape preforms. This paper reports the nonlinear viscoelastic responses and damage mechanisms of one kind of 3D textile composites, named as 3D orthogonal woven composite (3DOWC) under quasi-static tensile loading based on a micro/meso-scale repetitive unit cells (RUCs) model. In the RUCs model, the resin is described with a nonlinear viscoelastic material and the fibers/tows with an elastic material. The damage initiation and propagation in resin are simulated by the post-damage constitutive models with maximum principal theory failure criteria. The fibers/tows impregnated with resin are defined by elastic transverse-isotropic material model with ultimate strengths failure of ‘expanded smeared crack’ both along and perpendicular to fibers/tows axis direction. The engineering parameters and ultimate strengths of homogenized fibers/tows filled with matrix in meso-RUCs model are transferred from the numerical analysis of the micro-RUCs. The results are compared with experimental and theoretical values of RUC deformation and damage initiation and propagation under monotonic axial loading. The methodology of establishing the nonlinear visco-elastic multi-scale model of 3D textile composites without introducing the real fabric architecture in finite element analyses is explained. With the multi-scale RUCs model, the mechanical behaviors of other kinds of 3D textile composites can also be predicted.     

三维编织CMC断裂韧性表征形式的试验研究

在试验的基础上 ,发现三维编织陶瓷基复合材料断裂试件的裂纹扩展沿着编织角方向进行 ,表现出一种非自相似的裂纹扩展模式 ,表明三维编织CMC的断裂是复合型断裂。利用材料的载荷 -位移曲线和声发射技术 ,分析了三维编织CMC在外载荷作用下的损伤行为和断裂机理 ,并且根据不同的外载荷类型 ,将三维编织CMC的断裂韧性表征分别界定在线弹性和弹塑性的两个领域里 ,初步确定三维编织CMC的断裂韧性表征形式     

Experimental and analytical characterization of multidimensionally braided graphite/epoxy composites

This paper presents results of an investigation of a novel, through-the-thickness fiber-reinforced composite material. The generic name for this composite technology is multidimensional (X-D) braiding. X-D braided composites consist of a net-shaped, densely braided fiber skeleton which is rigidized with a structural epoxy-resin system. This material is an alternative to the conventional laminated composite and has the potential for being more resistant to delamination and matrix cracking. This paper describes results of the mechanical characterization of one graphite fiber system which was braided into panels in which two braid parameters could be investigated. The variables investigated included the effect of edge condition and braid pattern on the tensile, compressive and flexural properties of the braided panels. These properties were obtained in the braid direction only. The cutting of the specimen edges substantially reduced both tensile and flexural strengths and moduli. Of the three braid patterns investigated, 1×1, 3×1, and 1×1×1/2 F, the 3×1 braid pattern showed superior tensile performance, while the 1×1×1/2 F braid pattern exhibited superior flexural properties. The development of an analytical method for modeling the tensile performance of the multidimensionally (X-D) braided composite is also presented. The fiber geometry in X-D braids was modeled based on the braid parameters used in the construction of these composites. By the nature of the symmetry of the resulting braided structure, an analytical model based on classical lamination theory was used to determine the extensional stiffness in the three principal geometric directions of a braided composite. These analytical results are shown to compare favorably with those obtained experimentally. Finally, to further validate the ability of this material to contain damage, multidimensionally braided and conventionally laminated panels were impacted and the resulting damage was nondestructively determined. The multidimensionally braided material was shown to reduce the area of damage caused by impact by a factor of three for the energy levels tested.     

DETERMINATION OF INTERNAL STRAIN IN 3-D BRAIDED COMPOSITES USING OPTIC FIBER STRAIN SENSORS

A reliable understanding of the properties of 3-D braided composites is of primary importance for proper utilization of these materials. A new method is introduced to study the mechanical performance of braided composite materials using embedded optic fiber sensors. Experimental research is performed to devise a method of incorporating optic fibers into a 3-D braided composite structure. The efficacy of this new testing method is evaluated on two counts. First, the optical performance of optic fibers is studied before and after incorporated into 3-D braided composites, as well as after completion of the manufacturing process for 3-D braided composites, to validate the ability of the optic fiber to survive the manufacturing process. On the other hand, the influence of incorporated optic fiber on the original braided composite is also researched by tension and compression experiments. Second, two kinds of optic fiber sensors are co-embedded into 3-D braided composites to evaluate their respective ability