On the collapse of dented cylinders under external pressure

In this paper the reduction in the collapse pressure of long cylinders which have local dents is evaluated through a combination of experiment and analysis. A number of stainless steel tubes, with diameter-to-thickness ratios of approximately 33, 24 and 19, were indented to various degrees with spherical indenters of two diameters. The geometry of each dent was recorded using an imperfection scanning system and the cylinders were subsequently collapsed under external pressure. Denting reduces the local collapse resistance of the cylinder. For larger dents the collapse pressure was found to approach the propagation pressure of the tube. Collapse was found to be relatively insensitive to the detailed geometry of a dent but to be critically dependent on the maximum ovalization of its most deformed cross section (Delta(Od)). The collapse pressures of tubes with dents produced by indenters of different diameters could be well correlated through this measure of the dent geometry. The denting and collapse processes were simulated numerically using appropriately nonlinear elastoplastic shell analyses. Both steps of such simulations were shown to be in good agreement with experimental results for a broad variation of the parameters of the problem. The key role of the geometric parameter Delta(Od) was exploited in order to generate a Universal Collapse Resistance Curve for dented cylinders. It was possible to show that the post-limit load response (P - Delta) of a cylinder, with a small but axially uniform initial ovality, provides a very good lower bound to the collapse pressures of the dented cylinders plotted against the dent parameter Delta(Od). The significance of this curve is that it can be used to estimate the reduction in the collapse pressure of a cylinder with any dent geometry from only one relatively simple measurement of the geometry of the dented section.