Influence of Imperfections on the Collapse of Thick Tubes Subjected to External Pressure

Abstract

Cylindrical shells pressurized from outside are required for several engineering applications, and a growing need of tubes with significant thickness has been recently experienced in the oil industry (very deep water pipelines) and in the frame of integrated primary system nuclear reactors (steam generators). Their collapse behaviour has been explored little if at all, both experimentally and numerically, as witnessed by the extremely conservative attitude that codes assume for very thick tubes. A numerical investigation has been performed in this context at the Politecnico di Milano, which was originally intended as a support for requesting a relaxation of ASME regulations. In fact, in 2007 Code Case N-759 [1] was approved, permitting significant thickness saving in the tube design. Nevertheless, the numerical investigation was pursued to assess the influence of different parameters, such as eccentricity, initial stresses and material hardening, on the collapse of tubes with diameter to thickness ratios D/t<20. Results are thought to be useful under at least two respects: first, they provide some understanding on the collapse behaviour in a thickness range so far unexplored; secondly, they give an indication on the assumptions on which computer codes ought to be based when numerical analyses are required.