Research On High Temperature Structure Design Codes Based On Strain Classification

There has been increasing requirments for the safety of equipments serving at high temperature and high pressure due to creep phenomena. For the typical design codes, security is ensured by limitation of stress in the structure, but for long time creep condition it’s not enough because huge deformation in the structure. The deformation, however, will exhauste the ductility of structure on the one hand and casuse functional failure on the other hand. For this purpose, both ASME NH and RCC-MR put limits on inelastic membrane strain and inelastic membrane+bending strain, but these limits are under controversy because multiaxial condition is not considered, so as to welded condition.Membrane strain and membrane+bending strain can not be obtained directly from a structure, it can only be obtained after a procedure of linearization. In this paper, the procedure of strain linearization in ABAQUS is discussed and a Matlab program is written to help calculate the strain tensor. This procedure is proved to be effective and accurate. During the consequent research in this paper, the above procedure is used to calculate the classified strain.By using constitutive equation with damage variables, creep failure of a 550℃ 316 elbow and a 550 "C 316 welded pipe is simulated in ABAQUS. Comparasion is made between failure time defined by damage variable and that by starin design code. During the creep simulation of weled pipe, impacts of different HAZ creep property and weld angle on classified strain results are studied. Conclusion are made as follows:1. For the 316 pipe studied in this paper, strain limits from ASME NH and RCC-MR is proved to be conservative. Damage variables in these structures can be ensured much less than damage critical value, which means these strain limits can be used in structure with high multiaxial conditions.2. For both non-welded structure and welded structure, membrane strain is always the first to be achieve limits other than membrane+bending strain, which means membrane stain will be the primary strain limitation during design.3. For welded structure, classified strain is not sensitive to HAZ creep property and weled angle, which means strain limits in ASME NH is not very suitable for welded structure. But these limits are finally proved to be conservative because they are very small.