Plasticity-Instability Coupling in the Ultimate Behavior of Thick and Medium Thick Shells

The imperfection sensitivity of cylindrical shells under external pressure is studied, focusing attention on tubes of considerable thickness, such as those to be employed in very deep water pipe laying or in some innovative nuclear power plant proposals. In such tubes collapse is dominated by yielding, but the decreasing nature of the post-collapse evolution makes the effects of interaction with instability significant enough to demand consideration. On the other hand, available theories assessing the imperfection sensitivity of thin shells do not apply in this context and empirical design formulas used for the medium-thick tubes typical of present oil industry applications are extremely conservative. The interaction phenomenon becomes inherently different as the thickness increases and its correct description appears a must in view of a rational assessment of the load bearing capacity. This paper is intended as a first step in this direction.