Fracture Criteria for XFEM for pipeline steels

管道中的裂纹构成了其主要的完整性威胁之一，因为它们的存在可能导致遏制能力的丧失。裂纹可能存在于纵向或周向方向，并且可能与其他完整性威胁（如凹痕或腐蚀）相关。有一些分析模型来预测纵向裂纹的爆破压力和周向裂纹的拉伸应变能力，但它们仅限于裂纹和特定的管道等级。例如，针对周向缺陷开发的方程主要针对高强度钢管开发，不适用于较低等级。为了在方程不可用的情况下预测管道的行为，运营商要么依赖经验（使完整性决策不一致），要么依赖数值工具。然而，基于传统有限元分析方法（FEM）的当前数值工具非常耗时，通常需要在分析期间重新划分网格以考虑裂纹扩展。最近开发的扩展有限元分析方法（XFEM）提供了一种更稳健的方法，其中裂纹可以通过有限元分析网格进行传播，从而减轻了重新划分网格的要求。然而，在XFEM裂纹扩展的现有标准还没有校准管线钢。 本研究项目旨在开发校准的损伤力学标准，可以在基于XFEM的方法预测管道中的裂纹扩展。在这种情况下，损伤开始在一个临界应变值，而损伤的发展，根据损伤区内的耗散能量。该准则考虑了不同载荷条件下裂纹尖端的约束，适用于不同级别的管线钢。损伤的形成和演化是应力三轴度和矿脉角的函数。该标准将使用ABAQUS用户子程序UDMGINI和ABAQUS中可用的各种损伤演化定律来实现。校准是通过再现实验拉伸和断裂韧性测试，如单边缺口拉伸（SENT），单边缺口弯曲（SENB），和紧凑的拉伸（CT）测试。所开发的标准将提供显着的数值便利性，并提高预测与裂纹尖端约束相关的脆性或韧性断裂的准确性，而与试样几何形状、预裂尺寸和载荷条件无关。 该项目有多个长期目标。首先是提供能够预测各种载荷条件下，包括地面运动和内部压力的开裂管道的行为的数值工具。校准XFEM模型的开发方法也可以应用于预测裂纹和未裂纹的金属和铝结构的行为。

Cracks in pipelines constitute one of their major integrity threats as their existence can lead to loss of containment capability. Cracks can exist in either the longitudinal or circumferential direction and can be associated with other integrity threats such as dents or corrosion. There are some analytical models to predict burst pressure for longitudinal cracks and tensile strain capacity for circumferential cracks, but they are limited to cracks alone and specific pipeline grades. For example, the equations developed for circumferential flaws are mostly developed for high strength steel pipelines and are not applicable to lower grades. In order to predict the behaviour of a pipeline in situations where equations are not available, operators either rely on experience (making the integrity decisions inconsistent), or on numerical tools. Current numerical tools based on traditional Finite Element analysis Method (FEM), however, are extremely time consuming often requiring remeshing during the analysis to account for crack propagation. The more recently developed eXtended Finite Element analysis Method (XFEM) provides a more robust approach in which crack propagation can occur through the finite element analysis mesh, thus alleviating the requirement for remeshing. However, the current criteria for crack propagation in XFEM have not been calibrated for pipeline steels. This research project aims at developing calibrated damage mechanics criteria that can be implemented in the XFEM based approach for predicting crack propagation in pipelines. In this context, damage is initiated at a critical strain value while damage evolves according to dissipated energy within the damaged zone. The criteria consider the constraints at the crack-tip specific to various loading conditions and applicable to different grades of pipeline steel. Damage initiation and evolution are proposed to be functions of stress triaxiality and Lode angle. The criterion will be implemented using the ABAQUS user subroutine, UDMGINI, and the various damage evolution laws available within ABAQUS. The calibration is performed by reproducing experimental tensile and fracture toughness tests, such as single edge notch tension (SENT), single edge notch bending (SENB), and compact tension (CT) tests. The developed criteria will provide significant numerical convenience and improve accuracy in predicting brittle or ductile fractures related to the crack-tip constraints, independent of the specimen geometry, pre-cracking size, and loading conditions. There are various long term objectives for this project. The first is to provide numerical tools capable of predicting the behaviour of cracked pipelines under various loading conditions including ground movement and internal pressure. The developed methodology of calibrating the XFEM models can also be applied to predict the behaviour of cracked and uncracked metallic and aluminum structures.