考虑混凝土微观结构特性的多尺度PD-FEM混合模型及破坏过程研究

The safety of hydraulic concrete structures attract many concerns in both of the academic and engineering fields. The failure mechanisms of crack initiation and propagation in hydraulic concrete are not clearly. It is very difficult to analyze and simulate the whole process of local micro crack propagation to the whole structure failure. Within the framework of the continuum mechanics, the traditional methods of numerical analysis are difficult to simulate the failure of the concrete material in the microstructures, due to the transition between the continual and discontinue states. The peridynamics method can solve such a problem. However, the existing bond-based peridynamics models cannot exactly describe the deformation and failure behavior of concrete under the complex stress state, which computational efficiency is very low.. The proposed study is divided into the following three tasks: (1) Characterizing the microstructures of concrete material through experiments and statistical analysis. (2) Define the state of all material points, obtain the pairwise function through the calculating the state operator. By establishing the contact function between material points in overlapping regions and defining the interface element, develop the PD-FEM mixed model, and mesh adaptivity numerical solution, in order to improve the computational efficiency. (3) Establishing the multi-level and multi-scale damage index from material, element to structure. Conducting and numerical simulations on the whole process of microdefects growth, damage accumulation, crack initiation and propagation, local fracture till the failure of the structure in concrete material and structures.. According to these results, the whole process of damage accumulation and progressive failure in concrete materials and structures can be simulated. The failure mechanism of concrete material and structures can be explored and provides a basis of practical guidelines for the development of high performance concrete and safety evaluation of hydraulic concrete structures.

水工混凝土结构的安全是学术界和工程领域关注的热点。混凝土裂纹萌生和扩展机制尚不明晰，从局部微小裂纹扩展至结构整体失效全过程的分析和模拟存在困难。基于连续介质理论的数值方法采用局部微分方程求解破坏问题时面临瓶颈，而基于非局部积分思想的近场动力学(Peridynamics,PD)方法具有这方面优势。现有“键型”PD模型难以精确反映混凝土在复杂应力状态下的变形及破坏行为。本项目引入基于“状态”的近场动力学方法，在准确把握混凝土微观结构特性的基础上，定义各组分物质点的“状态”算子。通过构造重叠区域物质点间的接触函数以及引入界面单元等手段，构建近场动力学和有限元结合的PD-FEM混合模型。建立材料、单元和结构多层次、多尺度损伤指标，模拟混凝土内部损伤累积、裂纹萌生和扩展、结构局部断裂直至整体失稳的全过程，揭示其破坏机制，为高性能混凝土的研发和复杂环境下水工混凝土结构的安全评估提供科学依据。

混凝土结构的损伤过程和破坏机制一直是学术界和工程界极为关注的热点和难点。基于连续介质力学理论框架下的传统数值方法在模拟材料和结构的损伤累积和渐进破坏时面临网格重构等诸多困难，基于非局部积分思想的近场动力学理论和方法，通过空间积分型支配方程的离散和求解，彻底避开传统方法中采用局部微分方程求解不连续问题时的奇异性，显示出其优势。按照研究计划，本项目紧密围绕三个关键科学问题开展了深入研究，具体研究内容如下：1.混凝土非均质细观近场动力学本构模型的构建；2.基于近场动力学方法的钢筋混凝土损伤模型研究；3.多尺度PD-FEM混合模型的研究及其应用；4.混凝土结构损伤累积和渐进破坏全过程的连续模拟。. 项目研究过程中，顺利解决了三个关键科学问题。具体研究成果有，采用DIP技术对混凝土微观结构特征进行提取，建立了基于近场动力学的混凝土微观结构模型，构建能准确描述混凝土结构特征的多尺度PD-FEM混合模型。考虑混凝土准脆性特征及材料特性，发展了适用于混凝土材料的PD模型。定义了拉压异性的临界伸长率，损伤变量及破坏准则，发展适用于结构破坏分析的近场动力学数值算法，描述了混凝土渐进破坏的过程，揭示混凝土材料和结构破坏的内在机制。提出了基于近场动力学方法并考虑拉压异性和断裂特征的钢筋混凝土损伤模型，采用分级加载、动态松弛、平衡收敛准则以及刚性体冲击接触算法对钢筋混凝土结构进行破坏分析。结合本项目所得到的理论研究成果，研究了混凝土框架结构、重力坝的受力和变形特点以及破坏特征，模拟了其失稳破坏的过程，实现对混凝土结构损伤累积和渐进破坏全过程的连续数值仿真。确定破坏过程各阶段的损伤指标，提出结构安全性评估和剩余寿命预测方法，显示了近场动力学理论和方法在混凝土材料和结构破坏分析方面的强大优势，为近场动力学方法应用到实际的工程结构做出积极探索。研究成果对发展新型不连续介质力学问题的力学模型与多尺度数值计算方法也具有重要的意义。