Paradoxical buckling behaviour of a thin cylindrical shell under axial compression

The widely accepted theory of buckling of thin cylindrical shells under axi al compressive loading emphasises the sensitivity of the buckling load to t he presence of initial imperfections. These imperfections are conventionall y taken to be minor geometric perturbations of a shell which is initially s tress-free. The original aim of the present study was to investigate the ef fect on the buckling load of imperfections in the form of local initial str ess, which are probably more typical of practice than purely geometric ones . Experiments were performed on a vertical "melinex" cylinder of diameter s imilar to 0.9 m and height similar to 0.7 m, with radius/thickness ratio si milar to 1800. The upper and lower edges of the cylinder were clamped to en d discs by means of circumferential belts - an arrangement that allowed sta tes of self-stress to be introduced to the shell readily by means of local "uplift" at the base. The upper disc was made sufficiently heavy to buckle the shell, and it was supported by a vertical central rod under screw contr ol. Many buckling tests were performed. Surprisingly, the buckling loads we re generally at the upper end of the range of fractions of the classical bu ckling load that have been found in many previous experimental studies. Eve n when the local uplift at the base caused a local "dimple" to be formed be fore the shell was loaded, the buckling load was relatively high. A surface -scanning apparatus allowed the geometric form of the shell to be monitored , and the progress of such a dimple to be followed; and it was found that a dimple generally grew in size and migrated in a stable fashion up the shel l as the load increased, until a point was reached when unstable buckling o ccurred. These unexpected and paradoxical features of the behaviour of the experimental shell may be attributed to the particular boundary conditions of the shell, which provide in effect statically determinate support condit ions. This study raises some new issues in the held of shell buckling, both for the understanding of buckling phenomena and for the rational design of shells by engineers against buckling. (C) 2000 Elsevier Science Ltd. All r ights reserved.