Effect of Local Imperfections on the Collapse of Tubes Under Bending and Internal Pressure

Previous work by the authors investigated the inelastic response and stability of pipes bent in the presence of internal pressure [1,2]. It was shown that internal pressure tends to stabilize the pipe by reducing initial geometric imperfections and reducing the induced ovalization. Consequently pressurized pipe can sustain significantly higher bending strains before collapse than pipe bent in the absence of pressure. Pipelines have girth welds and other local imperfections such as dents. The present phase of this work uses experiment and analysis to investigate the effect of local dents on the collapse capacity of pressurized pipes under pure bending. A series of experiments was conducted on stainless steel 321 seamless tubes with diameters of 1.5 inches and D/t of 52. Small imperfections in the form of transverse dents were introduced to the specimens using a custom technique that limits the axial and circumferential spans of the dents. The dented tubes were loaded by pure bending at a fixed internal pressure (approximately one half the yield pressure) to collapse. Tubes with dent depths ranging from very small to about 1.7 times the pipe wall thickness were tested. It was found that such local imperfections tend to reduce the bending strain capacity of the pipe quite significantly. Smaller depth dents tend to cause relatively larger reduction in the bending strain at collapse whereas at larger depths the bending strain at collapse tends to level off. The inelastic response and the eventual localized collapse are being simulated using FE models. The material is represented as an anisotropic elastic-plastic solid using the flow theory of plasticity. The modeling includes simulation of the denting process followed by pressurization and bending. It will be shown that all aspects of the observed behavior including the sensitivity of collapse strain to the local imperfection are reproduced well by the models.