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、天然气/石油管道、航空航天和海洋工业等，具有重要的现实意义。在使用寿命期间，焊缝可能会出现缺陷，其形式为表面、角落和预埋裂纹。因此，为了确保安全运行并延长这些结构的使用寿命，拥有先进的工程工具来量化这些焊件缺陷的行为，并提供可靠的疲劳和断裂失效安全裕度估计是至关重要的。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。在断裂力学的框架内，处理材料不均匀性、失配性和裂纹前缘约束以及计算焊接残余应力影响的特殊技术已经得到了很好的发展。然而，到目前为止，大多数开发都是针对模式I（开放模式）加载下的缺陷。工程焊接结构的使用条件复杂，一般涉及拉伸、弯曲、扭转和剪切载荷。结果表明，三维裂缝前缘附近局部加载条件为ⅰ型（张开）、ⅱ型（面内剪切）和ⅲ型（面外剪切）三种模式。为了正确地对这些情况进行安全评估，必须考虑混合模态效应。目前还没有专门的工具能够正确地对混合模式载荷下的焊接件进行三维缺陷评估。所提出的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可靠地评估具有各种三维缺陷的焊接结构的结构完整性。提出的工作包括以下四个方面的进展：1)考虑材料不均匀性、强度失配和残余应力等不同特征影响的混合模式条件下焊接件裂纹前场表征；2)利用基于细观力学的模型研究混合模式载荷下焊接件裂纹的破坏过程，建立相应的断裂破坏准则；3)对不同焊件特征的混合模式断裂和疲劳试样进行试验测试，对失效准则进行校准和验证；4)应用新制定的断裂失效准则对焊接结构件三维缺陷进行失效评估和疲劳裂纹扩展预测。该研究成果将全面了解混合模态载荷对焊接件韧性断裂和疲劳扩展行为的影响，对高性能焊接工程结构的设计和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.