On the collapse of dented tubes under combined bending and internal pressure

The paper uses experiment and analysis to investigate the effect of local dents on the collapse curvature of pressurized pipes under pure bending. A series of experiments is conducted on stainless steel 321 seamless tubes with diameters of 1.5 in. and D/t of 52. Small transverse dents are introduced to the specimens using a custom technique that limits their axial and circumferential spans. The dented tubes are internally pressurized to approximately one half the yield pressure and then bent to collapse in a pure bending testing machine. Tubes with dent depths ranging from very small to about 1.7 times the tube 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 to the collapse curvature, or bending strain, whereas for deeper ones the collapse curvature tends to level off at about 50% of the value of the intact case. The tube denting, pressurization, and bending to collapse have been simulated using solid finite element models. In addition to proper modeling the elastic-plastic behavior of the material, the model includes measured yield anisotropies, residual stresses, and wall eccentricity. The model reproduced quite well all aspects of the observed behavior including denting, bending and the ensuing collapse of the dented section, resulting in an inward kink with outward protrusions on either side. All predicted collapse curvatures were slightly lower than measured values. The cause of this discrepancy was traced to several small misalignments of the experimental dents, each of which tends to delay collapse somewhat. (C) 2011 Elsevier Ltd. All rights reserved.