Creep-Fatigue damage analysis of modified 9Cr-1Mo steel based on a Voronoi crystalline model

Power plants operating at elevated temperatures mainly undergo creep-dominant creep-fatigue interaction behavior. In order to safely operate power plants for long service durations, it is very important to evaluate the creep-dominant creep-fatigue damage interactions. Therefore, in this paper, the creep-fatigue interaction lifespan was evaluated through finite element analyses. Firstly, in order to consider the prior austenite grain structure of modified 9Cr-1Mo steel, a finite element model was generated using the Voronoi tessellation method. Secondly, to reflect each nonlinear creep and fatigue damage accumulation in the grain structure, Dyson and modified Wahab damage models were employed. Pure creep and pure fatigue simulations were conducted which verified that the Voronoi tessellation model and each damage model accurately accounts for the real creep or fatigue damaging mechanisms. As a consequence, finite element analyses for creep-fatigue interaction were conducted several times for various stress conditions and new criteria for the creep-fatigue life evaluation were proposed with a concern about stress sensitivity. The results were between the conventional criteria suggested by RCCMRx and ASME-NH code.