by Daniel Vasilikis
Graduate Diploma Thesis, February 2008
The present thesis investigates the structural stability of thin-walled steel cylinders surrounded by an elastic medium, subjected to uniform external pressure. A two dimensional model is developed, assuming no variation of load and deformation along the cylinder axis. The cylinder and the surrounding medium are simulated with nonlinear finite elements that account for both geometric and material nonlinearities. Cylinders of elastic material within a rigid boundary are considered first, and the numerical results are compared successfully with available closed-form analytical predictions. Subsequently, the external pressure response of confined thin-walled steel cylinders is examined, in terms of initial out-of-roundness of the cylinder, initial gap between the cylinder and the medium and the stiffness of the medium. Numerical results are presented in the form of pressure-deflection equilibrium paths, which show a rapid drop of pressure capacity after reaching the maximum pressure level. The distribution of plastic deformation, as well as the variation of cylinder-medium contact pressure around the cylinder cross-section are also depicted and discussed. Furthermore, the effects of uniform vertical preloading on the maximum pressure sustained by the cylinder are examined. Finally, the numerical results are compared with a simplified closed-form expression, proposed elsewhere, that could be used for design purposes.