This research program focused on the behaviour of low cycle fatigue (LCF) of wrinkled pipes, and was designed to develop the LCF life prediction models for the wrinkled pipes. It consisted of three phases of work, which are strip tests, full-scale pipe tests, and finite element analysis (FEA). In strip tests, 39 strip specimens were tested by a complete-reversed stroke-controlled method to investigate the effects of bend angle, bend radius, and stroke range on the low-cycle fatigue (LCF) life. Also, the LCF behaviour was explored by viewing the spectra of key variables and their corresponding hysteresis loops. The failure mechanism was discussed by examining the fracture surfaces. Two LCF life prediction models, life-based and deterioration rate-based, were developed and their prediction results were evaluated. In full-scale pipe tests, two specimens were tested according to a complicated loading procedure. The loading was a combination of axial load, bending moment, and internal pressure; and it consisted of monotonic loading stage and cyclic loading stage. Based on those two tests, the global and local behaviour were investigated, the failure mechanism was studied and the application of the developed LCF life prediction models was discussed. In FEA, three numerical models were developed and they were the strip model, the half-pipe model and the full-scale pipe model. In the strip model, the residual stresses and strains were analyzed and discussed. In the half-pipe model, the effects of pipe geometry, internal pressure, and global deformation on the wrinkle geometry were studied and discussed. In the full-scale pipe model, the full-scale pipe tests were simulated and both the global behaviour and local behaviour were discussed. From this research program, some important conclusions were obtained. The wrinkle geometry is found to be greatly related to the pipe geometry, internal pressure, and global deformation. The global deformation has become localized after the wrinkle is fully developed. The opening deformation cycle is more detrimental to wrinkled pipes than the closing deformation cycle. The test results also show that the seam weld governs the failure of wrinkled pipes if the pipes are subjected to cyclic axial deformation. The LCF life prediction models developed from this research program demonstrate good prediction capacity when they are applied to both strip tests and full-scale pipe tests. / Structural Engineering

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京都大学 / 0048 / 新制・課程博士 / 博士(工学) / 甲第22755号 / 工博第4754号 / 新制||工||1743(附属図書館) / 京都大学大学院工学研究科都市社会工学専攻 / (主査)教授 清野 純史, 教授 高橋 良和, 准教授 古川 愛子 / 学位規則第4条第1項該当 / Doctor of Philosophy (Engineering) / Kyoto University / DFAM

Buried pipeline networks play a vital role in the transportation of oil and natural gas from centers of production to centers of consumption. A common manufacturing technique for such pipes is the UOE process, where a flat steel plate is first formed into a U shape, then into an O shape, welded at the seam, and mechanically expanded before being shipped on site. The UOE forming process deforms the pipe material plastically and induces residual strains in the pipe. Such pipes are commonly buried on side and then are pressurized under the high head of the fluids they convey which induce hoop stresses as high as 80% of the pipe yield strength. When buried pipelines cross the regions of discontinuous permafrost, they undergo differential frost heaving, inducing significant bending deformations, which potentially induce local buckling in the pipe wall. To control local buckling, design standards impose threshold limits on buckling strains. Such threshold values are primarily based on costly full-scale experimental results. Past nonlinear finite element analysis attempts aiming at determining the threshold buckling strains have neglected the presence of residual stresses induced by the UOE forming and were thus found to grossly overestimate the buckling strains compared to those based experiments. Within the above context, the present study focuses on developing a numerical technique to predict the residual stresses induced during UOE forming, and incorporating the induced residual stresses in 3D nonlinear FEA modeling to more reliably predict buckling strain limits. Comparisons with conventional analysis techniques that omit residual stresses reveal the importance of incorporating residual stresses induced in forming when quantifying buckling strains.