Limit And Shakedown Analyses Of 3-D Defective Pressurized Pipelines

One of the most important subjects on plastic limit and shakedown analysis is to develop feasible and applicable numerical methods to evaluate the limit and shakedown load-carrying capacities of complex 3-D structures. Supported by one of the National Ninth Five-Year Key Scientific and Technology Projects, this dissertation studies systematically and thoroughly numerical methods for lower bound limit and shakedown analyses of 3-D structures and applies them to safety evaluation of defective pressure pipelines.A numerical method is developed for lower bound limit analysis of 3-D structures and a mathematical programming problem is formulated to determine the limit loads of structures. To overcome the difficulties of large dimension of the problem, the residual stress field is generated by performing an equilibrium iteration procedure of elasto-plastic finite element analysis. A revised SQP-algorithm is used to solve the nonlinear mathematical programming problem and an active-constraint set strategy is introduced, so that the number of constraints is reduced greatly. Through typical numerical examples, the effectiveness and superiority of the proposed numerical method is shown.A numerical method is proposed for static shakedown analysis of elastic-perfectly plastic 3-D structures. By introducing the conception of load vertex, the numerical difficulties caused by the variable of time parameter are overcome. Based on the numerical method developed in this thesis, the shakedown loads of typical 3-D structures are computed and the failure modes are analyzed using a post-processing program, which is greatly significant in engineering.In case of the limited kinematic strain hardening material model, a numerical method is proposed for static shakedown analysis of 3-D structures. A simplified method is developed to construct back-stress field. Some meaningful conclusions is arrived at through typical numerical examples.The proposed numerical procedures for 3-D structures are applied to determine the limit and shakedown loads of defective pressurized pipelines. Theeffect of various shapes and sizes of typical part-through slots on the limit and shakedown loads of straight pipelines and pipe elbows are investigated and evaluated comprehensively and systematically. A series of computational curves for limit and shakedown loads is obtained and the failure modes under different loads and defects are analyzed using the post-processing program. Some valuable conclusions are drawn. A simplified method for the limit analysis of defective complex piping systems is proposed and tested. The difficulties for safety evaluation of defective piping system structures are overcome. The numerical results provide theoretical foundations for the design and safety assessment of pressurized pipelines with defects.