针刺碳/碳复合材料多尺度耦合的氧化与力学性能研究

The proposed research project is to investigate the oxidation and relevant mechanical properties of needle punched carbon/carbon composites (NP-C/Cs) using a coupled multi-scale strategy. Technologies of Micro-CT image identification and meso-structure reconstruction specific for NP-C/Cs are going to be developed. Using these technologies and combining with the microscopic observation by scanning electron microscope (SEM), the micro- and meso-scale morphology features of this composite are characterized. Based upon research works above, unit cell models and mathematical homogenizations for both oxidation behaviors and mechanical properties of NP-C/Cs will be pursued at micro-scale (individual fibers surrounded in matrix) and meso-scale (representative architectures constructed with nonwoven cloth, chopped strand mate and needle fiber bundle) respectively. The further objective is to deliver macroscopic oxidation and mechanical models applicable to the analysis of engineering structures. As a part of the proposed project, a series experiments on oxidation behaviors (e.g. temperature dependent mass loss rate, surface roughness) and consequent mechanical properties (oxidation caused degradations of moduli, ductility, static and fatigue strengths) of NP-C/Cs will be designed and conducted. The test data are used to validate the proposed multi-scale models. The main innovative elements in the proposed project are as follows. 1. Coupled multi-scale studies on either oxidation or mechnical properties for NP-C/Cs are proposed for the first time; 2. Mixture element developted here for meso-scale FE model reconstrucion can compromise precision and computing cost; 3. Propability distribution functions, instead of constants, will be employed in meso-sacle structrure modeling and macro-scale oxidational and mechanical properties representations. By this method, the inherent dispersibility of this composite can be more comprehensively characterized than by the exsiting models; 4. The fatigue performence degradation is modeled at perspective of changes of surfacial roughness and property caused by oxidation. This approach is based on sounder fracture mechanics theory.

拟采用多尺度耦合的方法研究针刺碳/碳复合材料（NP-C/Cs）的氧化和力学性能。发展针对NP-C/Cs细观结构特点的Micro-CT图像识别与重构技术，结合表面和断面的微观形貌观测，表征其微、细观结构；在此基础上逐级建立纤维单丝微观尺度、纤维束（针孔）细观尺度、均质化宏观尺度NP-C/Cs的氧化和力学模型；开展NP-C/Cs的氧化及氧化前后力学性能对比试验，为各尺度理论模型提供试验依据，积累基本数据。主要创新有:1.针对NP-C/Cs的多尺度耦合的氧化和力学性能研究均是首次提出；2.本研究提出的混合单元模型可以平衡NP-C/Cs细观结构重构中精度与效率之间的矛盾；3.采用概率分布函数来表征NP-C/Cs的细观结构和宏观力学性能固有的分散性，比现有的单一参数模型更进一步；4.从材料表面层性状改变的新角度研究氧化对NP-C/Cs疲劳性能的影响，具有断裂力学理论依据。

本项目对于针刺碳/碳复合材料（NP C/C）从细观结构重构、氧化和力学性能试验及多尺度分析等几个方面进行了研究。具体工作和成果可概括为如下四个方面：.一．细观结构模型重构. 针对NP C/C细观结构Micro-CT图像中组分材料灰度值对比度弱的特点，提出了一种投票窗算法将渐变灰度变换为阶跃灰度，结合中值滤波、开运算等算法，成功提取了其纤维束轮廓线和轴线等关键细观结构参数，并据此统计出针刺弯曲纤维束的曲率分布。.二．多尺度氧化模型.在复合材料微观氧化模型方面，将现有的两组分模型发展至三相组分模型；采用分区建模策略将稳态解推广至瞬态解；首次建立了纤维束侧向氧化模型；同时还提出了两种数值模拟方法。建立了NP C/C细观氧化理论和数值模型。.三．多尺度力学模型.建立了NP C/C一种曲梁/弹簧单元组合的细观力学模型，模型可有效描述其细观结构和缺陷特征并降低计算成本，采用该模型分析了针刺缺陷随机性造成的材料性能分散性。通过引入材料细观相的氧化体积损失，模拟了NP C/C 力学性能的氧化衰减。建立了一种NP C/C 协同多尺度渐进损伤分析模型，用多项式函数构造了细观单胞内的局部位移场，并引入阶跃函数描述针孔处应力的不连续性，用较少的计算量获得较精细的局部分析，可实现对复合材料复杂细结构和损伤过程高效的多尺度模拟。.四．氧化试验.对两种密度的NP C/C 进行了几种温度下的氧化失重试验，并观测了氧化表面微、细观尺度的形貌。进行了氧化剩余强度试验，得到了温度、材料密度与氧化率的关系。初步获得了氧化剩余性能与质量损失建间的定量联系，试验数据和观察也证明了上述多尺度理论模型的可靠性。.本项目的研究深化了对NP C/C性能的认识，积累了数据，丰富和精细化了预测方法，有助于更准确预测其性能，助力工程应用。所建立的模型也为具有其他细观结构和材料体系的复合材料的多场、多尺度分析提供参考。