Fracture Criteria for XFEM for pipeline steels

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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.