Fracture is a critical failure mode that deserves attention during the design of pipelines used in the offshore industry. The metals used in these structures can withstand large plastic deformations before a fracture occurs; the ductile fracture assessment becomes an essential task for safe design and operation. To characterize the fracture toughness of pipeline steels, small-scale fracture specimens such as three-point bending SE(B), single edge notched tensile SENT, and compact tension C(T) are commonly used. Employing classical fracture mechanics parameters such as stress intensity factor K, J-integral, and crack tip opening displacement CTOD, the toughness properties are transferred from small specimens to large structures. Although some laboratories can carry out experimental tests on full-scale pipelines, these procedures require a more significant amount of resources, making them expensive and consequently unviable in many situations. In the last decades, computational damage models have been used as an alternative to studying ductile fracture in metals. Based on micromechanical and phenomenological approaches, these models could satisfactorily predict the formation and propagation of ductile cracks in different materials and geometries. A numerical investigation of ductile fracture in pipelines using a phenomenological computational damage model will be presented in this study. The damage model will be applied using the Finite Element Method (FEM) for an ASTM A285 grade C steel. The damage model includes a damage initiation criterion dependent on the stress triaxiality and the Lode angle parameter coupled with a post-initiation damage evolution law. A series of experimental tests were performed on round bars with smooth and notched cross-section, flat notched specimens under axial tensile loads, and fracture toughness tests in deeply cracked bending samples SE(B) to calibrate the elastoplastic response and the damage parameters. The model was verified using fracture toughness tests of SE(B) sample with shallow crack-size ratio, clamped SENT geometry, and C(T) specimens having different thicknesses. The calibrated numerical model was applied to execute a parametric study in pipes with circumferential surface cracks submitted to tensile and internal pressure loads simultaneously. The effects of the variation of geometric parameters and the ratio between loads on the strain capacity of the pipes were investigated. The varied geometric parameters were the depth and length of the crack and the pipe's thickness. The effects of longitudinal misalignment between welded pipelines were also investigated.

Numerical Investigation of Ductile Fracture in Pipelines Under Complex Loading Using a Phenomenological Damage Model

Paper Proceedings: https://asmedigitalcollection.asme.org/PVP/PVP2021/volume/85314

ASME 2021 Pressure Vessels & Piping Conference (PVP2021)

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