Comparative Study on Creep-Fatigue Damage Assessment Between R5 Procedures and a Full Inelastic Analysis

This research aims to investigate the differences in structural integrity assessment results between R5 Volume 2/3 procedures and a detailed inelastic analysis. The membrane sidewall/roof bimetallic stub tube subjected to creep-fatigue loading is selected as an example. Based on a linear elastic analysis, the maximum equivalent stress is found to be at the weldment located at the top of the stub tube. The R5 Volume 2/3 procedures enhance the start of a dwell stress and inelastic strain ranges by applying a weld strain enhancement factor and a stress concentration factor, and the assessment predicts a total creep-fatigue damage of 54% for a 45 year life. As a comparison, a detailed inelastic analysis has been conducted for the same bi-metallic stub tube using an incremental, step-by-step analysis. Temperature dependent material properties are used to simulate stress-strain response and a user subroutine is employed for creep deformation. Assessment results for the detailed inelastic analysis confirm the weldment is similarly affected as indicated by the elastic analysis results but the comparable total predicted creep-fatigue damage is less than 35%. Cold eye review of both assessment results concluded that the significant deviation is attributed to the additional factors applied for the assessment of weldments, including weldment geometry, flaw effects and weld micro-cracking, as provided in the R5 Volume 2/3 procedures.