Along curved sections of underground pipelines, in the parent metal of pipes and in butt welds, there may be annular defects. When making a decision on the need for and priority of the repair of a section with an annular defect, the assessment of the hazard associated with such defects is especially important. The hazard of a defect is mainly determined by the amount of damage that can be caused if the defect opens up. The amount of damage, among other factors, significantly depends on the opening size of a through crack. The maximum opening occurs when a crack propagates over the entire cross section (the so-called "guillotine" failure). Experience shows that if a pipeline fails due to its annular defect or if an underground pipeline loaded mainly by bending is defect-free, a through crack most commonly occurs with a small opening (a leak).
The paper describes the conditions for the stable growth of through annular cracks in an underground pipeline along its sections with non-design axis curvature in case of the pipeline deformation loading mainly by bending. These conditions are analyzed depending on the initial crack half-length, critical crack tip opening, geometric parameters of pipes and mechanical properties of pipeline metal, bending radius of the pipeline axis, and mechanical properties of the soil. The growth pattern of a through annular crack is described using the concept of plastic crack tip opening displacement and its critical value. The paper shows that under conditions of deformation loading of an underground pipeline mainly by bending, a through annular crack, which develops from a surface defect due to its growth until breakage of the bridge between the defect tip and the pipeline surface, does not always propagate unstably around the circumference and results in a rupture of the pipeline at its full cross section. To confirm the adequacy of the model used to describe the deformation of a pipeline with an annular through crack in the limit state in the absence of crack growth, we have performed computer simulation of the elastic-plastic stress-strain state of a pipeline with an annular through cut. Examples of calculation using the model proposed are given.
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