三维整体编织复合材料多通管件多尺度细观结构和疲劳强度

Three-dimensional (3-D) braided multi-way tube is a multi-way seamless connected tube structure manufactured under an integrated braided technique. The braided multi-way tube has not the possible structure defects which induced from the connections among branch tubes. Compared with multi-way metal tubes, the braided multi-way tube has higher fatigue strength / weight ratio. To find the fatigue strength of the braided multi-way tube is a new topic both for the mechanics of composite materials and the strength theory of mechanical structures. In this project, a multi-scale structural model which covers from fiber, yarns, unit-cells and macro-structures will be established for characterizing the geometrical structures of the two-step tri-directional 3-D integrated braided multi-way tube. A series of fatigue tests will be conducted to compare the fatigue damage differences between multi-way and unidirectional tube, and to analyze the fatigue damage morphology and damage mechanisms. The influences of microstructure and geometrical shape on the fatigue strength will also be investigated. A user-defined-material subroutine which includes the critical damage area failure criterion and the constitutive equation of the braided composite material will be written. The fatigue damage will be calculated at the yarn level and unit-cell level. From the numerical simulations, the fatigue damage mechanisms of the braided multi-way tube would be revealed and the design method for the high fatigue strength multi-way tube would be proposed. With the investigations of this project, the fatigue damage mechanisms of the two-step tri-directional 3-D integrated braided would be understood. The design methods of the high fatigue strength braided multi-way tube would also be developed. This investigation will also provide a new object in the mechanics of composite materials and the strength theory of mechanical structures.

三维编织复合材料多通管件是整体编织技术制造的多通管状无缝连接件，避免了各支管因接缝可能导致的结构缺陷，具备金属管件没有的高疲劳强度/重量比优势，其疲劳强度研究是复合材料疲劳力学和机械结构强度理论共同面临的新课题。本项目将建立涵盖纤维、纱线、单胞到宏观结构的二步法三向三维编织复合材料多尺度细观结构模型，研究整体编织多通管件多尺度细观结构特征；开展系列疲劳实验，比较多通管件与单通管件疲劳破坏差异，分析多通管件疲劳破坏形态和损伤机理，探索细观结构和几何外形对疲劳强度的影响；编制临界失效面积破坏准则和材料本构的用户子程序，在纱线和单胞尺度计算疲劳破坏，揭示编织复合材料多通管件疲劳破坏机理，提出设计高疲劳强度多通管件的方法。该项目将在研究二步法三向三维编织复合材料多尺度疲劳破坏机理基础上，应用于整体编织复合材料多通管件疲劳强度设计，为复合材料疲劳力学和机械结构疲劳强度设计增添新研究内容。

三维编织复合材料具有非均质、各向异性特点，使三维编织复合材料管件设计比金属管件更复杂。研究三维整体编织复合材料多通管件疲劳损伤机制并应用于多通管件设计，是比金属材料设计更为复杂的课题。项目围绕三维编织复合材料多通管件疲劳强度问题，结合编织复合材料平面试件、单通管件和多通管件制造、静态力学性质测试、平面试件疲劳性质测试和多尺度细观结构模型等内容，研究三维纺织结构复合材料多尺度细观结构与刚度降解规律和强度分布间关系，得到不同应力水平下循环加载剩余强度－加载循环数曲线（S-N曲线）和在纤维束层面、单胞层面和连续介质层面的疲劳机理，形成三维编织、机织、针织和混编复合材料多尺度几何结构和动态强度关系等成果。.主要发现：.（1）研究三维编织复合材料和管件疲劳寿命。对比载荷-挠度曲线和刚度降解曲线，发现四步法三维编织复合材料在疲劳加载过程中会出现明显的“三阶段”形态，三个阶段中损伤机理呈现不同特点。.（2）研究三维编织复合材料和管件在准静态载荷以及循环交变载荷条件下的力学响应、疲劳性能、结构效应以及损伤机理，得到复合材料在整个疲劳加载过程中承载应力变化规律以及损伤扩展过程。.（3）研究三维角联锁机织复合材料在三点弯曲交变循环应力加载下的疲劳性能、结构效应以及破坏机理。得到对应于不同应力水平下的疲劳寿命(S-N)曲线、在弯曲循环加载过程中的应力-挠度曲线、刚度退化曲线、挠度变形曲线等。.（4）研究三维正交机织物复合材料低周疲劳性质，建立细观/中观/宏观跨尺度几何单胞模型预测复合材料低周疲劳力学行为和损伤演化。.（5）基于以上研究内容，编制系列用户子程序库，自定义复合材料属性，用于连续介质层面动态强度计算。.项目执行期间共有研究生10名获博士学位，其中2名留学生；12人获硕士学位，其中2名留学生。发表标注论文被SCI收录40篇。项目于负责人2014年获评教育部长江学者特聘教授。