Plastic Collapse Load Analysis Of Pressure Structures Containing Volumetric Defect Subjected To Creep Damage

High-temperature technologies are widely used in many industries, ranging from electricity generation, petrochemical, nuclear power plants to aeroengines. The high temperature components are usually designed for long term operation. Creep damages are inevitable in the service. Flaws such as inclusions, pits, thinning and other volumetric defects can occur during manufacturing process and in-service period. These defects may cause severe reduction of the limit load of structures. The existing assessment procedures do not take into account the effect of creep damage, which may lead to inaccurate assessment results. Therefore the dissertation is aimed to investigate the limit load of pressurized components with volumetric defect subjected to high temperature creep damage. An elasto-plastic constitutive model with coupled creep damage effects is proposed. The limit load of high temperature structures with volumetric defects is systematically studied using finite element method. Safety assessment approach for structures considering creep damage effect is subsequently proposed based on the principle of "fitness for service". The main research work and conclusions are summarized as follows:(1) Base on the strain strengthening constitutive model, derived are the analytic solutions for limit load of thick and thin cylinders subjected to internal pressures. By using finite element method, the analytic solutions are verified and effects of dimensions and material parameters on the limit loads are also discussed. Through experimental observation, the mechanism that creep damage degrades the limit loads of2.25CrlMo steel structure has been studied. Creep damage is subsequently coupled into the Ramberg-Osgood equation. The modified model is able to reasonablely predict the elasto-plastic response of creep-damaged materials. The obtained results could be applied to the calculation of the limit load of structures with high-temperature creep damage through finite element analysis.(2) Based on the modified model, the failure behavior and limit load of high temperature pressurized components with volumetric defects are studied, when subjected to the internal pressure or bending moment using the elastic plastic finite element method. Emphasized are the effects of defect sizes, creep damage on the limit load of structures.(3) Considering the accumulation of creep damage effect, the safety assessment procedure is amended with the introduction of normalized creep time and defect size. The proposed safety assessment procedure is able to assess the load carrying capacity of the high-temperature structures with volumetric defects.