【作者】 方国东；

【导师】 梁军；

【作者基本信息】 哈尔滨工业大学 ， 工程力学， 2010， 博士

【摘要】 三维编织复合材料是为了满足在航空航天领域中结构部件具有高比刚度、高比强度并且能够抵抗多向应力以及热应力的需求下设计和研制的复合材料。随着编织工艺和成型工艺的发展,三维编织复合材料在大尺寸自动化编织和整体成型方面有着独特的优势。并且三维编织复合材料具有良好的抗冲击性能、高的损伤容限以及材料结构可设计性等特点,使三维编织复合材料成为较有潜力的第三代纤维增强复合材料。三维编织复合材料结构部件能够广泛应用的关键是对其结构热力学性能(强度、损伤容限和可靠性)进行正确的分析及评价,但三维编织复合材料的力学性能依赖于复杂的细观结构以及细观材料等诸多影响因素,从而本文结合宏微观实验,从细观角度出发,根据三维编织复合材料细观结构周期性特点构造具有代表性的细观几何结构(代表体积单胞),利用细观损伤模型,对三维编织复合材料的宏观有效性能、损伤和失效进行了分析。本文详细综述和分析了国内外三维编织复合材料的研究现状,就编织复合材料的几何结构、力学模型和实验等方面进行展开,同时描述了当前表征复合材料非线性力学性能的不同的损伤模型,给出了研究三维编织复合材料力学性能的基本思路:从三维编织复合材料宏细微观不同层次进行分析,作为一个多尺度问题进行处理。针对三维四向编织复合材料的细观编织结构特点(内部编织结构和表面编织结构不同),为了尽量减少三维四向编织复合材料表面编织结构的影响,采用不同厚度和直径的三维编织试件对三维四向编织复合材料进行拉伸和压缩试验研究,并且通过拉伸和压缩试件的宏细观破坏截面观察,确定三维四向编织复合材料的宏细观破坏模式,为细观损伤模型的建立提供指导。由于三维四向编织复合材料内部纤维束相互挤压,使纤维束的表面纤维出现弯曲和扭曲现象。本文把代表体积单胞内部每根纤维束的表面分成六个区域,利用随机函数理论对扭曲纤维束表面的不同区域进行有效性能预报,结合有限元方法,施加周期边界条件,考虑扭曲纤维束对三维四向编织复合材料宏观力学性能的影响。同时利用体积平均化方法,建立考虑纤维束弯曲的代表体积单胞模型,对内部纤维束弯曲的三维四向编织复合材料的宏观力学性能进行研究。为了研究三维四向编织复合材料积累损伤破坏的过程,建立了与局部的应变、单元的特征长度和材料的断裂能释放率相关的损伤演化模型,利用Murakami-Ohno损伤模型实现细观组分材料刚度的折减,从而对在不同载荷作用下的不同编织角(30°和45°)的编织复合材料进行积累损伤分析。在细观损伤模型中考虑了细观组分材料不同的细观破坏模式:纤维束纵向拉伸剪切和压缩剪切、横向拉伸剪切和压缩剪切以及基体拉压损伤。在单轴纵向拉伸载荷作用下,利用界面单元又考虑了三维四向编织复合材料内部纤维束/纤维束和纤维束/基体界面损伤模式,并且讨论了不同界面强度对该编织材料单轴拉伸性能的影响。为了提高数值模拟的收敛速度,对损伤模型中的损伤变量进行粘性规则化,并且讨论了粘性系数对数值结果的影响。在单轴纵向压缩载荷作用下,引入了基体理想塑性模型,分析了编织复合材料内部扭曲纤维束表面纤维初始未对齐角和纤维束纵向剪切非线性对该编织复合材料单轴纵向压缩性能的影响。在纵向单轴拉伸和压缩模拟过程中,对编织复合材料不同组分材料的积累损伤过程进行分析,探讨了细观组分材料不同的损伤机理,通过与宏观实验进行对比,验证了数值模拟的可靠性。通过对三维四向编织复合材料代表体积单胞施加复杂载荷条件,对编织复合材料在复杂载荷条件下的强度进行预报,得到了编织复合材料的破坏包络线,并且构造了三维编织复合材料宏观破坏准则,为编织复合材料结构部件的强度校核和结构优化设计提供强度判据。该研究路线同样适合于其他形式的编织复合材料,为三维编织复合材料在工程中广泛应用提供理论依据。更多还原

【Abstract】 In order to meet the requirement of the structural components in aeronautic andastronautic fields, three dimensional (3D) braided composites have been designed anddeveloped, which can resist the multi-directional stress and thermal stress and have highstiffness and strength at low density. With the development of braid processes and mold-ing techniques, it has predominant advantage to form the 3D braided composites withlarge sizes in an automatic braid process and an integral braid manner. In addition, comb-ing the excellent resistance impact, high damage tolerance and material structure redesigncharacteristics, 3D braided composites have been become the third generation fiber re-inforced composites. It is the key to study the thermal mechanical properties, such asstrength, damage tolerance and reliability analysis, for 3D braided composites which canbe widely used in structural components. However, the mechanical properties of the 3Dbraided composites are determined the complicated mesoscopic structures and materialbehavior of constituents. In this dissertation, the macroscopic and microscopic experi-ments for the 3D braided composites are conducted. Based on the periodic braid structureof the 3D braided composites, the representative volume cell (RVC) is established. Themacroscopic mechanical properties, damage and strength of the braided composites areanalyzed by using mesoscopic damage theory.The domestic and foreign researching development of the 3D braided composites,including the geometrical structure, mechanical models, experimental studies and dam-age models for investigating the nonlinear properties of composites have been discussedand analyzed. The general routine to evaluate the mechanical properties of 3D four-stepbraided composites has been provided. 3D braided composites which can be divided intothree levels: macroscopic, mesocopic and microscopic, which can be regarded as multi-scale problems.Based on the micro-braid structural characteristics of the 3D four-step braided com-posites (internal and surface of the braided composites with different braid structures),the different thickness and diameter of the specimens are chosen to conduct the tensileand compressive tests in order to minimize the effect of surface braid structure on themechanical properties of the braided composites. The macro and micro failure modes of the braided composites can be obtained by observing the fractographs of the failurecross-section, which is the experimental basis to establish damage model.Owing to the squeezing of braid yarn each other within the 3D four-step braidedcomposites, the surface of these braid yarns exhibits the undulation and twisted phe-nomenon. In the present dissertation, the surface of each braid yarn within an RVC isdivided into six regions. The effective properties of each region are obtained by stochas-tic function theory. The RVC is applied the periodic boundary conditions by using finiteelement method to calculate the effect of the surface twisted fiber of braid yarn on themechanical properties of the braided composites. Meanwhile, the fiber undulation withinRVC is considered. The effect of fiber undulation on the macroscopic mechanical prop-erties has been studied by using volume homogenization method.In order to investigate the progressive damage and failure process, the damage modelof composites has been established, in which some mescoscopic damage modes, such aslongitudinal and transverse tensile-shear, compressive-shear modes of braid yarn and ten-sile, compressive modes of matrix, are considered. The damage evolution model has beenbuilt, which are depended on the fracture energy of the constituents, characteristic lengthof element and local strain. The Murakami-Ohno damage model has been used to com-plete the stiffness reduction of the constituents of the braided composites. Therefore, theprogressive damage analysis for the braided composites under different loading are con-ducted. In order to improve the numerical convergence, an artificial Duvaut-Lions viscousmodel is used in the damage model. In this dissertation, when the braided composites aresubjected to the uniaxial tensile loading, the interface damage between yarn/yarn andyarn/matrix are considered in the damage model further. The effect of interfacial strengthand viscous parameters on the macroscopic stress-strain curves of the braided compositesis discussed. When the braided composites are under only uniaxial compressive loading,the damage of matrix is neglected but regarded as perfect plastic material. The effect offiber misalignment angle of surface fiber of braid yarn on the mechanical properties of thebraided composites is considered. In addition, the longitudinal shear nonlinear of braidyarn in the numerical model is included. The effect of braid angle (30°and 45°) on themacroscopic stress-strain curves and damage development of the constituents is analyzedfor the braided composites under the longitudinal tensile and compressive loading. Thedamage mechanisms of the constituents are also discussed. Compared with experimental results, the reliability of the numerical results is verified.By applying complicated boundary conditions on the RVC of the braided compos-ites, the strength of the braided composites under different boundary conditions has beenobtained. The macroscopic failure criterion of the braided composites is constructed,which can provide a theoretical criterion to check the strength of structural componentsmade by the braided composites and can be used to the structural optimization design.The researching method can also fit for other multi-directional braided composites, andprovide a theoretical basis for the braided composites widely used in engineering appli-cation.更多还原