Modelling the manufacturing history, through life creep-fatigue damage and limiting defect sizes of a pipework joint using finite element based methods

摘要

The work reported in this paper describes a simulation of the manufacture, through-life operation and limiting defect size assessment of a pipework joint in a nuclear powerplant boiler. The objective of this work is to understand the critical factors that influence the integrity of the joint in-service and support accurate predictions of service life. This work differs from typical structural integrity assessments in that advanced modelling techniques have been used throughout the assessment process and include detailed simulations of the manufacturing process, a simulation of the entire in-service operating history including predictions of creep-fatigue damage and cracked body analysis to determine limiting defect sizes. Residual stresses resulting from the manufacturing process can be a key driver for creep and creep-fatigue damage. The calculation of creep-fatigue damage for assessment purposes is typically under-taken within the framework of an appropriate assessment code, such as EDF Energy's R5. The standard assessment approach usually requires calculation of stresses using elastic finite element analysis followed by hand calculations to calculate the damage. A combination of explicit and implicit finite element methods are employed to simulate a range of manufacturing processes which influence the in-service structural integrity of a branched pipework joint. Where available, test data have been compared to the results to assess the validity of the simulation. The simulation results then feed into a finite element based structural integrity assessment. The methods follow the principles outlined in the EDF Energy R5 assessment code but use the inelastic strains calculated directly from analysis. The methods are based around the general purpose finite element code Abaqus. The residual stresses generated during manufacture may adversely affect the critical defect sizes for the pipework joint. However, the complex geometry and loading complicate the assessment of the cracked body. Therefore, a finite element analysis representing the cracked body has been carried out on the pipework joint to evaluate the J-integrals at the locations of interest and hence calculate the critical defect sizes. The analysis considers the residual stresses determined from the finite element analysis of the manufacturing processes.