Collapse of Thick Tubes Pressurized from Outside: an Accurate Predictive Formula

Several engineering applications require cylindrical shells subjected to external pressure, and an increasing need for tubes of significant thickness has been experienced in recent years. So far, little study has been devoted to very stocky tubes, and a great amount of uncertainty exists on some important aspects, such as the consequences of imperfections on their failure pressure. This can only be computed by performing numerical analyses that consider both material (plasticity) and geometric (large displacement) nonlinearities. Such a procedure is feasible, but its use for design purposes is cumbersome, and handy alternatives are worth searching. In this paper, a comparatively simple formula is proposed, based on an interpretation of the relative role that plasticity and instability play in the thick tube range. The formula depends on a crucial coefficient, which can be defined so as to provide safe but reasonably accurate approximations to the collapse pressures computed numerically for tubes made of different metals. The proposal may be useful for preliminary design purposes and can be considered as a first contribution toward a precise assessment of the collapse behavior of tubes in a thickness range so far overlooked.