Fracture in Wrinkled Linepipe Under Monotonic Loading

Buried pipelines may undergo large deformations due to geotechnical movements, temperature effects, and fluid pressure which result in axial loads, shear loads, moments and various complex combinations of these loads and load histories. Consequently, localized buckles and wrinkles may form in the pipe wall and, subsequently, fractures may develop at wrinkle locations. Previous research showed that large deformation due to monotonically applied symmetric loading can produce accordion type wrinkle geometry but fracture is not normally produced unless strain reversals at the wrinkle location occur due to the application of variable loads. Recently, a field fracture that developed in an NPS10 pipe was brought to University of Alberta Structures Laboratory for investigation. From the description of the load history at the failure location, and the inspection of the deformed geometry and fracture surface, it was recognized that no significant strain reversals had occurred in this particular wrinkled pipe segment. Examination implied that the final failure was a “tearing” failure resulting from monotonic application of a longitudinal load not aligned with the axis of the pipe. However, the true load history that caused the pipe to fracture at the wrinkle location was unknown. To verify the nature of the failure mechanism, and determine the characteristics contributing to its formation, it was felt necessary to attempt to reproduce similar failures in a wrinkle by subjecting it simultaneously to monotonically applied axial load and shear. Two full-scale tests applying axial load, and a shear load with different boundary conditions were carried out on NPS12 pipe. The second test produced a deformed geometry and a fracture configuration very similar to the one that developed in the field. This paper describes the test procedure for these two tests and the mechanics of the fracture. It is concluded that, with the appropriate load history and boundary conditions, fractures can be triggered at wrinkle locations by monotonic loading histories.