考虑损伤演化的核石墨接触结构失效机理及强度分析

Line contact of cylinders and point contact of spheres can be seen in the nuclear graphite structures of high temperature gas-cooled reactors. The strength analyses of the contact structures are of great importance for the safety assessment of the whole reactor. However, there is still a lack of in-depth understanding of the failure processes of these contact structures, which makes the accurate strength analyses impossible. Firstly, a high-precision temporal mesh-based digital image correlation (Mesh-DIC) algorithm will be developed in this study to measure the full-field deformation of nuclear graphite specimens. Then the damage evolution of nuclear graphite under complex stress states will be inversely characterized using the finite element model updating (FEMU) technique. Secondly, with the experimental results as constraint conditions, a numerical model for progressive damage analyses of nuclear graphite contact structures will be created. The numerical model will incorporate the inversely identified damage constitutive equations and a smoothed floating node method (S-FNM) which will be developed based on the smoothed finite element method for explicit crack propagation simulation. Finally, based on the simulation results for the whole damage and failure processes of the nuclear graphite contact structures, the relationship between the macroscopic nonlinear mechanical behavior and the microscopic damage will be established. Thereafter the failure mechanisms of nuclear graphite contact structures will be revealed and theoretical models for contact strength analyses will be proposed. This study is expected to clarify the failure processes and mechanisms of contact structures of quasi-brittle materials and provide the theoretical basis for related engineering design.

高温气冷反应堆的核石墨结构中存在着柱体的线接触和球体的点接触结构，这些接触结构的强度分析对整个反应堆的安全性评估至关重要，但目前对核石墨接触结构的失效过程还缺乏足够的认识，因而也无法准确分析其强度。本研究拟首先发展高精度时间序列网格化数字图像相关算法实现核石墨试件全场变形测量，利用有限元模型修正技术反演识别核石墨材料在复杂应力状态下的损伤演化规律。然后以核石墨接触实验结果为约束条件，引入反演得到的损伤本构关系并发展基于光滑有限元的光滑浮动节点法显式模拟裂纹扩展，建立核石墨接触结构的渐近损伤失效数值模型。最后基于核石墨接触结构变形失效全过程的分析数据确定宏观非线性力学行为和细观失效破坏之间的对应关系，揭示核石墨接触结构的失效机理并提出接触强度分析的理论模型。本研究有望阐明准脆性材料接触失效的过程和机理，并为相关工程设计提供理论依据。

核石墨是由混合物经过复杂工艺制成的具有大量微缺陷的准脆性复合材料，是高温气冷反应堆中最重要的结构和功能材料。核石墨接触结构是反应堆中容易发生失效破坏的危险点，因此，其强度评估是反应堆的结构设计和后期运行安全评估的关键，这需要深入理解并定量表征核石墨接触结构的失效破坏过程。然而，核石墨材料的特殊性、接触结构的复杂性以及现有实验方法的局限性，导致核石墨接触结构失效破坏过程及破坏机理的研究十分困难。本项目发展了考虑位移连续性及基于可信赖点的网格化数字图像相关算法，实现了核石墨试件高精度全场变形测量；利用全场变形信息、有限元模型修正技术及人工神经网络技术反演识别核石墨材料在简单和复杂应力条件下的损伤演化规律；发展了一系列适用于不同加载条件的核石墨材料/结构关键力学参数测量的实验方法和技术，系统研究了不同加载条件下核石墨材料的失效破坏过程及强度准则；以核石墨线接触和点接触实验结果为约束条件，引入复杂应力状态下核石墨损伤演化模型及多个强度准则，建立了核石墨接触结构的渐近性损伤失效数值模型；基于核石墨接触结构变形失效全过程的分析数据建立了核石墨接触结构中细观损伤演化、局部破坏扩展与宏观力学响应的对应关系，揭示了核石墨接触结构的失效机理并提出了其接触强度评估方法。本项目研究工作阐明了准脆性核石墨材料接触失效的过程和机理，为高温气冷反应堆的结构设计和后期运行安全评估提供了理论依据。