Nonlinear stability analysis of thin-walled steel pipe confined in soft bilayer medium

This paper studies the structural stability of thin-walled cylindrical pipe surrounded by soft bilayer medium, subjected to uniform pressure. A two-dimensional (2D) finite element model (FEM) is developed by assuming a plane strain condition. Both the geometric and material nonlinearities are taken into account for the simulation. An elastic pipe confined in a uniform soft medium and bilayer medium is investigated, and the numerical results are compared with available closed-form analytical solutions. Then, the steel pipe with inelastic properties is investigated by tracing the pressure-displacement equilibrium paths of the crown position where the maximum radial displacement occurs. The pressure shows a significant drop after reaching the maximum level (critical buckling pressure). Furthermore, a geometric thickness parameter is introduced to define the border of the upper and bottom layer of the bilayer medium around the steel pipe. In addition, parametric studies show that the response of the steel pipe is significantly affected by the initial gap, the out-of-roundness imperfections, the deformable soft bilayer medium, and insensitive to the yield stress of inelastic pipe and the friction in the pipe-medium system. Finally, the present numerical results show close agreement with the experimental results for the inelastic confined pipe with an initial gap between the pipe and medium.