Inelastic Buckling of Geometrically Imperfect Tubes under External Hydrostatic Pressure

Abstract

The paper reports on the buckling of twelve thin-walled geometrically imperfect tubes, which were tested to destruction under uniform external hydrostatic pressure. The paper also reports on other similar tests to destruction, carried out on a large number of geometrically imperfect tubes. Theoretical studies were also carried out with well-known analytical solutions, together with a numerical solution using a finite element computer package, namely ANSYS. Whereas the theoretical analyses agreed with each other, they did not agree with the experimental data for the shorter tubes; this was because the shorter tubes collapsed by inelastic instability due to initial geometrical imperfections of the tubes. Exact analysis of slightly geometrically imperfect tubes, with random distribution has so far defied reliable theoretical solutions. However, the paper presents a design chart, which can cater for these geometrical imperfections. The design chart may also be suitable for large vessels such as submarines, off-shore drilling rigs, silos, etc. Circular cylinders under external pressure, often appear in the form of submarine pressure hulls, torpedoes, off-shore drilling rigs, silos, tunnels, immersed tubes, rockets, medical equipment, food cans, etc. Such vessels are good for resisting internal or external pressure, however under uniform external pressure they can collapse at a fraction of the pressure that will cause failure under internal pressure. Failure of these vessels under uniform external pressure is called non-symmetric bifurcation buckling or shell instability [1,2,3]. To improve the resistance of these vessels to the effects of uniform external pressure, the vessels are usually stiffened by ring stiffeners spaced at near equal distances apart. If, however, the ring stiffeners are not strong enough, the entire flank of the vessel can collapse bodily by a mode called general instability. Another mode of failure is known as axisymmetric deformation, where the cylinder implodes axisymmetrically, so that its cross-section keeps its circular form while collapsing. In this study, we will be concerned with elastic and inelastic shell instability; as such vessels can collapse at pressures of a fraction of that which cause the vessels to fail under internal