Post-buckling Analysis For Infinitely Long Cylindrical Sandwich Shells Subjected To External Pressure

Under water pipelines may exist some initial imperfections due to the limitation of manufacturing and processing technologies. Under the action of water pressure, buckling may occur in the pipelines with initial imperfections and induce structural failure of the pipelines.When buckling happens, the circular cross section of the pipelines starts to deform as an elliptical circle. The ovalization continues till two opposite points on the inner surface of the pipeline contact with each other, forming a collapse mode of dog-bone shape. If the external pressure is greater than a critical pressure, called buckle propagation pressure, the collapse mode will spread along the pipeline. In recent years, the exploitation of oil has extended from the shallow area to the depths of the ocean. Due to the temperature descendant from the oil layer to the sea floor the oil may condensate and consolidate, leading to the inhabitation of the oil flow in the pipeline. Undoubtedly, this put forward a higher demand on the pipeline design. The underwater thermal insulating pipelines are one of the most commonly used means in deep sea oil exploitation. This thesis studies the post-buckling phenomenon of infinitely long, thermal insulating sandwich pipelines subjected to hydrostatic pressure. The main research contents are Governing equations for nonlinear local buckling of cylindrical sandwich shells subjected to hydrostatic external pressure are established. The mechanical behaviors in the sandwich shells are analyzed using the First Order Shear Deformation Theory. The equilibrium of moments and forces are set up by considering the rotations of the middle planes of the facesheets with respect to the neutral surface of the sandwich shell and the deformations of the neutral surface of the sandwich shell. Based on the theory for thin shells with nonlinear large deflection, the compatibility equation is inferred from nonlinear displacement-strain relationships. Introducing the rotation functions, the deflection function and the stress function, the governing equations for nonlinear buckling of long cylindrical sandwich shells are expressed in terms of these functions.Governing equations are solved using Ritz methods. According to the problem description,boundary conditions are determined to ensure the continuity of the displacements. A local collapse buckling mode is proposed based on the classical solution from plane strain analysis.The stress function as well as the rotation functions is solved from the governing equations using the assumed displacement function. The expression of the total potential energy is obtained and is minimized with respect to the undetermined parameters. The formulas to calculate the post buckling equilibrium path and the length of transition zone during buckle propagation along thepipelines.Parametric studies are performed. The effects of different radius-to-thickness ratio, different core-to-facesheet thickness ratio on the post buckling pressure and the length of transition zone are investigated. Also influences of material properties, including the shear modulus of the core and the Young modulus of the facesheets, are discussed. Finally the sandwich shell is simplified as a single-walled shell by setting the core thickness to be zero. The resulting pressures are compared with the classical solutions of a single-walled shell subjected to external pressure. Thus the accuracy of the analytical solutions are justified.Finite element simulations are conducted. Abaqus software is applied to simulate the postbuckling of cylindrical sandwich shells of finite length. Eigenvalue analyses are performed to obtain the buckling pressures of the sandwich shells and Riks method is used to analyze the postbuckling behaviors of the cylindrical sandwich shells with imperfection modes obtained from buckling analyses. The buckling pressures from eigenvalue analyses and the length of transition zone from post buckling analyses are compared with the analytical solutions. It shows they are in good agreement.