含局部脆性区的钢焊接接头开裂与止裂机理的理论与模拟研究

During the welding process of steel structures, the steel’s excellent combination of high tensile strength and good fracture toughness generally deteriorates as a result of the presence of local brittle zones (LBZs). The LBZs exhibit significantly lower resistance to fracture initiation, which is the main reason to reduce the weldments’ safety. Currently, there are still some doubts and contradictions on the effect of the LBZs on the toughness of weldments. Hence, the revelation of the influence of the LBZs on the toughness of weldments is significant. This project aims to investigate the effect of the LBZs in the weld joints on the toughness of weldments through the combination of theoretical analysis and simulation by elasto-plastic finite element method and molecular dynamics. The micromechanics model based on the experimental testing will be established to study the correlation between the brittle fracture of LBZs and their microstructure characteristics. The micro-mechanisms of the fracture initiation and arrest in the LBZs will be revealed. Besides, the CTOD fracture toughness testing will be modeled and simulated by the elasto-plastic finite element method and molecular dynamics. The correlation between the critical CTOD values of LBZs and base materials and the plastic zone sizes will be used to determine whether a crack extends or arrests at the brittle-ductile interface. The experimental tests will also be conducted to validate the theoretical analysis and numerical simulation. On one hand, the study can provide the scientific evidence for the toughness improvement of weldments. On the other hand, the basic theory for the actual structural integrity prediction and assessment of the weldments will be provided as well.

钢结构在焊接过程中由于局部脆性区（LBZs）的出现，严重削弱了钢材本具有的良好拉伸强度和断裂韧性。LBZs的抗开裂性能很低，是降低焊接结构安全性的主要因素。目前，LBZs对焊接结构韧性的影响机制仍存在疑点和矛盾。因此，探明其影响机制是提高焊接结构安全性的关键科学问题之一。本项目拟采用理论分析与弹塑性有限元、分子动力学模拟相结合的方法，重点研究钢焊接接头LBZs对焊接结构韧性的影响规律。从微观力学角度出发，基于试验观测建立相应模型，分析LBZs的脆性断裂与其微观结构特征的内在关联，揭示LBZs开裂与止裂的微观机理；结合有限元和分子动力学方法，模拟焊接接头的CTOD断裂试验过程，探讨LBZs和基体的临界CTOD值与塑性区尺寸的内在关联，预测裂纹在脆韧界面的扩展或止裂行为，并将试验结果与理论、模拟结果进行对比验证。为提高焊接结构的韧性提供科学依据，为预测和评估焊接结构整体性和安全性提供理论基础。

钢焊接接头局部脆性区由于存在一些不利微观结构，其韧性值很低，是焊接结构的一个严重问题。在这些不利微观结构中，马氏体-奥氏体岛状组织是典型的硬脆相，被认为是微裂纹萌生的起始点，是造成局部脆性区低断裂韧性的重要因素之一。硬脆相与基体之间的界面以及粗晶区晶界的三叉节点都是裂纹萌生的有利位置。本项目通过理论建模分析，研究了粗晶区临近三晶交点的晶界硬脆相附近微裂纹的形核与扩展趋势。结果表明，硬脆相与基体界面处的开裂是导致钢焊接接头局部脆性区解理断裂的主要因素。大多数裂纹在硬脆相边界形核且向硬脆相内部扩展（块状的马氏体-奥氏体岛状组织倾向于自身开裂形核微裂纹），这与试验中观测到的现象一致。研究了椭圆形硬脆相附近任意角度裂纹的弹塑性断裂行为。运用位错分布法模拟椭圆硬脆相附近的微裂纹，采用广义Irwin修正模型对裂纹尖端的塑性区尺寸进行修正。通过求解奇异积分方程得到了裂纹尖端的塑性区尺寸、等效应力强度因子和裂纹尖端张开位移。 研究发现椭圆硬脆相的形状和材料参数对裂纹扩展有显著影响。特别地，当硬脆相接近圆形、裂纹倾角接近90度时，裂纹扩展可以被屏蔽。研究了纳米变形孪晶、非晶等微观结构形核的位错机制，并通过分析孪晶、非晶等微观结构对裂纹尖端位错发射的影响规律，讨论了这些微观结构对裂纹扩展以及材料断裂韧性的影响。该研究为提高材料韧性提供科学依据，为预测和评估焊接结构整体性和安全性提供理论基础。