基于多轴延性断裂模型的多裂纹干涉与合并研究

A proper defect assessment for structures containing multiple cracks is of great importance to preventing the catastrophic failure. The aim of the proposal is to establish a multi-axial ductile fracture model of physical significance and applicable to engineering, and to search for the possible pattern behind the interaction and coalescence of multiple cracks for metal structures. The following three points will be focused in the study: (1) Using finite element method and digital image correlation technique, design a novel double-notched specimen of which the fracture strain can be accurately measured and the wide-range multi-axial stress states can be identified, and then conduct the multi-axial ductile fracture tests; (2) Based on microscopic examination and mesomechanics analysis, describe the effect of stress triaxiality and Lode parameter on the deformation mechanism of micro cavity, explore the physical relationship between equivalent fracture strain and multi-axial stress, and establish a mesomechanics-based fracture criterion in a wide-range stress triaxiality; (3) Based on the proposed ductile fracture model, develop the numerical method to simulate the propagation behavior of multiple cracks, and establish the crack combination rules under ductile fracture conditions. The findings of this project are believed to provide some important insights on the improvement of the defect assessment for structures containing multiple cracks.

对多裂纹结构准确地进行缺陷评定，是预防突发性灾难事故发生的重要保证。本项目力图建立物理意义明确且适合工程应用的多轴延性断裂模型，据此探索金属结构多裂纹干涉与合并的规律：（1）运用有限元分析和数字图像相关技术，设计能方便测量断裂应变和表征宽范围多轴应力状态的新型双缺口试样，开展多轴延性断裂试验；（2）通过显微观察和细观力学分析，合理阐释应力三轴度和Lode参数对于微孔洞变形机制的影响规律，实现等效断裂应变与多轴应力的物理关联，构建应力三轴度范围宽泛的细观断裂判据；（3）基于新的延性断裂模型发展多裂纹扩展的数值模拟方法，结合数字图像相关技术，建立延性断裂机制下的多裂纹合并准则。研究成果可望为多裂纹结构缺陷评定技术水平的提升提供科学支持。

面向预防突发性灾难事故发生的需要，针对多裂纹结构剩余寿命预测困难之挑战，项目组致力于多轴断裂试验技术、裂纹扩展预测方法和多裂纹结构缺陷评定三个方面的研究，较好地完成了项目的预期计划。主要研究工作及成果包括：（1）设计了新型的拉伸剪切缺口试样和拉扭缺口薄壁管试样，建立了宽范围应力三轴度作用下的断裂试验方法；（2）澄清了应力水平对金属蠕变延性的影响机理，发现Wen-Tu多轴延性因子模型可以很好地描述应力三轴度对金属蠕变延性的影响规律；（3）开展了基于应变梯度塑性理论的自增强和安定性分析工作，发现了安定极限解随着应变硬化水平的提高和尺寸的减小而增大；（4）基于多轴应力晶界孔洞化模型，建立了蠕变疲劳裂纹扩展分析方法，较好地预测了高温循环载荷及环境因素作用下的裂纹扩展行为。（5）通过有限元技术成功实现对多个裂纹扩展、干涉和合并全过程的仿真，指出了现有评定规范中的不保守性，提出了新的基于寿命的共面多裂纹合并准则。研究成果为建立多裂纹结构的安全评定方法提供了科学支持。项目实施以来共发表学术论文8篇（其中SCI收录4篇），在国际学术会议上作邀请报告3次，申请发明专利4项、软件著作权1项。项目负责人获中国机械工程学会压力容器分会优秀博士学位论文卓越奖等奖励，并培养毕业硕士生2名。