Rupture of Induction Bend Pipe Under Opening Mode Bending

For installing pipelines in a seismic area prone to soil liquefaction, it is essential to ensure that pipeline deformation does not surpass set limits. Because pipe deformation may exceed the maximum bending moment owing to soil liquefaction during landslides, a failure limit is set for the pipes. This problem becomes more severe in the cases of high-grade pipes or pipes experiencing high internal pressure because the deformability, which is defined as the deformation up to failure, tends to be reduced in these cases. This study focuses on the opening mode of induction bend pipes; this mode represents the deformation outside a bend pipe. Bending experiments performed on induction bend pipes in previous studies were referred to in this study, and the manner of failure of these pipes was investigated. Rupture occurred at the intrados of the bend pipes and was accompanied by the local reduction of wall thickness, i.e., necking that indicates strain localization. An evaluation method that can predict the rupture in this mode is proposed based on finite element analysis (FEA). The true stress-strain relationship after uniform elongation is determined by the inverse calibration method. A method that can avoid the effects of mesh division on the simulation of strain localization behavior is proposed. Based on these arrangements, FEA for simulating the deformation behavior of the bend pipes was conducted. It was clarified that the yield surface shape of the pipe material severely affects the rupture. The von Mises yield criterion, which is commonly used in cases of elastic-plastic FEA, could not predict the rupture and overestimated the deformability. In contrast, a yield surface obtained by performing tensile tests on a biaxial specimen could predict the rupture. That is, to predict the rupture by FEA, yield surface shapes should be defined on the basis of the actual properties of the pipe material.