Three-Dimensional Mixed-Mode Fracture Mechanics Methodologies for Structural Integrity Assessments of Welded Structures

高性能焊接工程结构的安全运行，如核压力容器和管道、油气管道、航空航天和海洋工程等，具有重要的实际意义。 在使用寿命期间，焊缝中可能出现缺陷，其形式为表面裂纹、角裂纹和嵌入裂纹。 因此，为了确保安全操作和促进这些结构的寿命延长，具有先进的工程工具来量化这种焊件缺陷的行为并提供对疲劳和断裂失效的安全裕度的可靠估计是至关重要的。 在目前的工程实践中，基于断裂力学的方法已被广泛用于缺陷焊接结构的安全评估。 在断裂力学框架内，处理材料不均匀性、失配和裂纹前沿约束以及考虑焊接残余应力影响的特殊技术现已得到很好的发展。 然而，到目前为止，大多数发展是在模式I（开放模式）加载下的缺陷。 工程焊接结构的服役条件复杂，一般涉及拉伸、弯曲、扭转和剪切载荷。 因此，三维裂纹前缘周围的局部载荷条件包括所有三种模式，即，模式I（打开）、模式II（面内剪切）和模式III（面外剪切）。 为了对这些病例进行适当的安全性评估，必须考虑混合模式效应。 目前，还没有特定的工具来正确地进行评估的三维缺陷的焊接件下的混合模式加载。 拟议研究的目的是通过开发和验证先进的工程工具来填补这一知识空白，这些工具用于在复杂的混合模式服务载荷条件下对具有各种3D缺陷的焊接结构的结构完整性进行可靠评估。 建议的工作包括以下四个方面的进展：1）。在混合模式条件下焊接件中裂纹前沿场的表征，包括材料不均匀性、强度失配和残余应力等不同特征的影响; 2）.利用细观力学模型研究了复合型载荷下焊接裂纹的破坏过程，建立了相应的断裂失效准则; 3）.对具有不同焊件特征的混合模式断裂和疲劳试样进行实验测试，以校准和验证失效标准;以及4）.通过应用新开发的断裂失效准则，对焊接结构部件中的三维缺陷进行失效评估和疲劳裂纹扩展预测。 本文的研究成果将为全面了解复合加载下焊接件的韧性断裂和疲劳扩展行为提供理论依据，对高性能焊接工程结构的设计、安全运行具有重要的工程意义。

Safe operation of high-performance welded engineering structures, such as those in nuclear pressure vessel and piping, gas/oil pipelines, aerospace and offshore industries, is of significant practical importance. During the service life span, flaws can occur in the welds and are in the form of surface, corner and embedded cracks. Therefore, to ensure safety operation and to facilitate the life extensions of these structures, it is critically important to have advanced engineering tools to quantify the behaviors of such weldment flaws and to provide reliable estimates of the safety margins against fatigue and fracture failures. In current engineering practice, fracture mechanics based methodologies have been widely used for the safety assessment of defective welded structures. Within the fracture mechanics framework, special techniques for treating the material inhomogeneity, mismatch and crack front constraint, and for accounting for the effects of welding residual stresses, have now been well developed. However, most developments thus far are for flaws under mode I (opening mode) loading. The service conditions of engineering welded structures are complex, generally involving tension, bending, torsion and shear loading. As a result, the local loading conditions around 3D crack fronts consist of all three modes, i.e., mode I (opening), mode II (in-plane shear) and mode III (out-of-plane shear). To properly carry out the safety assessment for these cases, mixed-mode effects must be considered. Presently, specific tools are not available to properly carry out assessment of 3D flaws in weldments under mixed-mode loading. The objective of the proposed research is to fill this knowledge gap by developing and validating advanced engineering tools for the reliable assessment of structural integrity of welded structures with various 3D defects under complex mixed-mode service loading conditions. The proposed work includes advances in the following four areas: 1). characterization of crack front fields in weldments under mixed-mode conditions incorporating the effects of different features including material inhomogeneity, strength mismatch and residual stress; 2). study of failure processes of cracks in weldment using micromechanics-based models under mixed mode loadings to establish corresponding fracture failure criteria; 3). experimental testing of mixed-mode fracture and fatigue specimens with different weldment features to calibrate and validate the failure criteria; and 4). failure assessment and fatigue crack growth prediction of 3D defects in welded structural components by applying the newly developed fracture failure criteria. The outcome of the proposed research will provide comprehensive understanding of mixed-mode loading on ductile fracture and fatigue propagation behavior of weldments, and be of significant engineering importance to the design, safe operation of high-performance welded engineering structures.