基于细观机制的建筑用铸钢材料断裂模型与铸钢节点失效机理

The use of steel casting joints in construction is becoming increasingly more commonplace in China. However, it is almost impossible to avoid the occurrence of casting defects with a nominal length less than 1 mm, thus the fracture ductility of casting steel is unstable in magnitude, as the defects occur in a random manner, and is normally found to be poorer than that of rolling mild steel. Nowadays, all the existing fracture models of ductile metals cannot take the effect of casting porosity into account, such that a considerable margin of safety has to be guaranteed in the design of steel casting joints to prevent from possible material fracture, leading to bulky joints that are heavier than necessary. Therefore, this project aims at ascertaining the fracture mechanisms of casing steels containing casting defects, as well as establishing fracture models that reflect both the effect of casting defects and material stress states on the fracture ductility. The highlight of this project is that, by studying the spatial distribution characteristic of casting defects through X-ray radiographic technique, conducting coupon tests on casting steel in which the material is loaded under various stress states, carrying out SEM observations to investigate the meso-mechanism of casting steel fracture, and applying multi-scale computational methods, the whole study is carried out across different levels (meso- and macro-level) and thus the macroscopic fracture models can have very solid meso-mechanical foundation. Based on the fracture models, numerical studies are performed to investigate the plastic performance of several typical types of steel casing joints. The failure modes and the failure mechanisms will be uncovered, and the influence of crack initiation and propagation on the overall joint response will be revealed, so as to provide practical and reliable methods and necessary technical supports for design of large-scale complex steel casing joints.

建筑用铸钢节点已在我国越来越多的各类建筑钢结构中得到广泛采用。但铸造过程难免产生细观缺陷，导致铸钢材料延性相对较差且离散性大，故在已有韧性金属断裂模型无法考虑细观缺陷影响的情况下，结构设计中通常对铸钢节点采用非常大的安全系数，进而导致比实际所需更笨重的节点设计。基于此，本项目以国内工程常用铸钢材料为具体研究对象，以不同厚度铸件的细观缺陷空间分布为切入点，结合铸钢材料在多种应力状态下的力学实验、射线造影和扫描电镜等手段，揭示含细观缺陷的铸钢材料在复杂应力状态下的断裂机理，并采用材料多尺度计算方法构建考虑细观缺陷影响的建筑用铸钢材料的断裂模型，应用到几类典型铸钢节点的有限元分析中，研究节点进入塑性后的工作性能和在复杂受力条件下的失效机理，掌握节点发生局部断裂的模式与裂纹扩展对节点整体力学行为的影响，为大型铸钢节点的设计提供必要的手段与技术支撑，促进铸钢节点的广泛应用和技术进步。

建筑用铸件在铸造过程中难免产生细观缺陷，导致铸钢材料延性相对较差且离散性大，故在已有韧性金属断裂模型无法考虑细观缺陷影响的情况下，结构设计中通常对建筑用铸件采用非常大的安全系数。本项目以国内工程最常用的铸钢材料G20Mn5为具体研究对象，以不同厚度、深度、热处理方式和应力状态的铸钢材料的力学实验为切入点，结合采用超声波探伤获取的铸件细观缺陷空间分布，揭示含细观缺陷的铸钢材料在复杂应力状态下的断裂机理，构建考虑细观缺陷影响的建筑用铸钢材料的断裂模型。.本文的主要结论是：.1．铸造及热处理过程改变钢材的细观结构，故G20Mn5铸钢的延性和断裂特性受热处理方式、铸件厚度、距铸件表面深度和材料应力状态的影响。基于数理统计，将建筑用铸钢材料依据铸件厚度和距铸件表面距离分类；不同类别铸钢材料的断裂延性存在显著差异。.2．对建筑用铸钢件的设计有如下建议：厚度小于30mm的铸件，其材料具有良好延性，不会发生早于抗拉强度的脆性破坏，合理设计时可利用材料的抗拉强度；对于厚度为30-150mm的铸件，近表面材料（距表面深度不大于20mm）在发生断裂前具有相当程度的塑性变形能力，设计时可安全地利用其屈服强度，但铸件内部材料（距表面深度大于20mm）易发生脆性断裂，应严格遵循弹性设计。.3．对正火和调质状态的G20Mn5薄壁铸件及厚壁铸件的表面材料，提出了基于细观机制的断裂准则，可用于铸钢材料在多种复杂应力状态下的断裂模拟。.4．基于超声波探伤技术，系统地进行了G20Mn5材料缺陷分布的扫描试验，研究热处理方式、铸件厚度和距铸件表面深度对建筑用铸件缺陷的影响，建立铸造缺陷大小与间距分布的数学模型。总体上，正火与调质状态的G20Mn5铸件中铸造缺陷的大小与间距均服从对数正态分布，且调质状态铸件缺陷间距分布的离散性更大。