Plastic failure analysis of defective pipes with creep damage under multi-loading systems.

The objective of this study is to investigate the effect of local wall thinning on the plastic limit load of modified 9Cr-1Mo steel pipe with existing creep damage based on the plastic limit load concept under high temperature. The creep damage is obtained from the Liu–Murakami creep model. A modified Ramberg–Osgood model is derived by using the relationship among the hardness, the creep damage, the yield strength and the ultimate tensile strength at high temperature to characterize the material deterioration during the creep process, where the non-uniform distribution of the material deterioration caused by the stress levels is considered. Orthogonal analysis for pipes with different type of defect under multi-loading systems are performed by using finite element method. The failure modes for pipes at limit state are revealed and the effect of local wall thinning on the plastic limit load of pipe with existing creep damage is presented. The regression formulae of the rupture time and the plastic limit load ratios for the pipes with different defect ratios and load combination ratios are established based on the numerical results. The limit loads for pipe with specified local wall thinning ratios after a period of time at high temperature can be determined conveniently through the regression formulae. [ABSTRACT FROM AUTHOR]