The analysis of failures in underground pipelines indicates a significant difference in the causes and mechanisms of such failures depending on the pipeline deformation pattern after the soil movement. Thus, in the pipelines of Siberia, the main cause of failure is corrosion processes on the outer surface of the pipe after its insulation destruction, whereas for pipelines laid in permafrost soils accidents are due to the destruction arising from various structural and process defects under conditions of significant strains. We show that cyclic movements of the pipeline bottom deflection point in the area of flooded thermokarst with its uneven freezing in the autumn-winter period can cause the pipe wall torsion effect, combined with loading by internal pressure and a significant bending of the pipeline axis.
The paper considers fatigue failure mechanisms of a main pipeline wall under conditions of a complex stress-strain state (SSS) with a detected torsion element in the zone of ring assembly weld at the main pipeline exits from thermokarst into design position. Fractures in the pipe wall arise from the fusion area of the ring assembly weld and has all the signs of fatigue failure. The fractographic analysis of the fracture surface established the presence of three stages of fracture development: the initiation, stable growth, and surface fracture transition into a through fracture with areas of high local plastic deformation. In all development areas of the above fatigue failure of a pipe wall made of steel of the K56 strength grade, significant differences are recorded in the structure of the fatigue fracture surface, when compared with fractures obtained on these steels under the conditions of cleavage failure. All stages of failure include cleavage fragments together with quasi-brittle and brittle fracture areas. This indicates a significant contribution to the destruction of torsional strain in the form of micro-shear deformations, against the background of a biaxial SSS of the pipe wall loaded with internal pressure. On impact samples with a KCV notch, we conducted a comparative analysis of the influence exerted by plastic tensile and torsion strains on the fracture resistance parameters of K56 strength grade pipe steels under the conditions of a biaxial SSS.
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