Fracture Criteria for XFEM for pipeline steels

管道裂缝是管道完整性的主要威胁之一，因为裂缝的存在会导致管道失去控制能力。裂缝既可以存在于纵向上，也可以存在于周向上，并且可能与其他完整性威胁有关，例如凹痕或腐蚀。有一些分析模型可以预测纵向裂缝的破裂压力和周向裂缝的拉伸应变能力，但它们仅限于单独的裂缝和特定的管道等级。例如，所建立的周向缺陷方程大多是针对高强度钢管道开发的，不适用于低强度钢管道。为了在没有方程的情况下预测管道的行为，操作人员要么依靠经验（使完整性决策不一致），要么依靠数值工具。然而，目前基于传统有限元分析方法（FEM）的数值工具非常耗时，通常需要在分析过程中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，其中裂纹扩展可以通过有限元分析网格进行，从而减轻了重新网格划分的要求。然而，目前的XFEM裂纹扩展准则尚未对管道钢进行校准。该研究项目旨在开发可在基于XFEM的方法中用于预测管道裂纹扩展的校准损伤力学准则。在这种情况下，损伤是在一个临界应变值开始的，而损伤是根据损伤区域内的耗散能量而演变的。该准则考虑了裂纹尖端的约束条件，适用于不同等级的管道钢。损伤的起裂和演化是应力三轴性和Lode角的函数。该准则将使用ABAQUS用户子程序UDMGINI和ABAQUS中可用的各种损伤演化规律来实现。校准是通过重复实验拉伸和断裂韧性测试来完成的，例如单边缘缺口张力（SENT）、单边缘缺口弯曲（SENB）和致密张力（CT）测试。开发的准则将为预测与裂纹尖端约束相关的脆性或延性断裂提供显著的数值便利，并提高准确性，而不受试样几何形状、预裂尺寸和加载条件的影响。这个项目有各种长期目标。首先是提供能够预测裂缝管道在各种载荷条件下的行为的数值工具，包括地面移动和内部压力。开发的校准XFEM模型的方法也可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.