Deformation and Ductile Cracking Behavior of X80 Grade Induction Bends

Permanent ground movement is expected in seismic areas and in permafrost regions, and pipelines buried in those areas need to be designed to have sufficient deformability. Especially, bends need to have superior deformability, because it was pointed out in the recent earthquake event that deformation tends to concentrate in the connection region of pipelines. Severe deformation can lead to a fracture of the pipe wall and this may cause explosion of the pipeline or leakage of the gas, which need to be prevented in the areas with high population density. In spite of the importance of deformability for pipe bends, there are only a few reports on this issue. Furthermore, those investigations are limited for up to X65 grade induction pipe bends. In this study, two types of API X80 grade induction pipe bends, 610mmOD × 11.0mmWT and 610mmOD × 16.6mmWT, bending radius of three times the pipe diameter and bending angle of 90 degree for both, were manufactured using longitudinally submerged arc welded pipes as mother pipes. And large scale bending test using X80 grade pipe bend was conducted by applying closing displacement on the tangents under the internal pressure of 12MPa by water. Bending load was continuously applied up to the maximum load point, and then prescribed displacement was applied until twice the maximum load point. Local deformation was shown in the middle of the bend portion, however, no cracking was observed. Furthermore, EF analysis of bending test was performed for precise estimation of stress/strain response of pipe bend, and analytical results were compared with experimental data. These bending tests proved that large deformability could be expected on the X80 grade pipe bends even under the high internal pressure. In order to investigate ductile cracking behavior of the X80 grade induction pipe bend, notched round bar tensile tests were also conducted, and the criterion for ductile cracking was compared with X65 grade bend material. Relation between equivalent plastic strain and stress triaxiality at a ductile crack initiation point was determined by FE analysis, and this analysis proved that X80 grade bend material has enough resistance to ductile cracking compared to X65 grade bend. This result also corresponds to the results of the bend test, which is showing enough deformability of the X80 grade induction bends.