Modelling the creep damage via irreversible entropy increase: Experiment and simulation

To evaluate the structural integrity of sodium-cooled fast reactor components under sustained creep loading, high-temperature creep tests were conducted at 525 °C on 316 H stainless steel base metal, welded joints, and associated regions (heat-affected zone and welded metal). And the stress effect on the creep properties of different regions in the welded joints were investigated in detail. During the long-term creep exposition, δ-ferrite gradually transformed into carbides and intermetallic phases, which diminished the resistance to the creep cavities formation at the interface of the δ-ferrite and austenite in the welded metal, making it the weakest region in the welded joint. To capture the creep deformation and damage evolution behaviors of 316 H steel, a creep rate model in the framework of micromechanics and a novel creep damage model grounded in the concept of the accumulation of irreversible entropy increase were established. The results predicted by the constitutive model were closely aligned with the experimental results. Additionally, the proposed damage model provided a novel continuum damage mechanics-based method of long-term creep rupture life prediction under different creep damage mechanisms. The creep damage model developed in this work provides a novel framework for reliability assessment of high-temperature service infrastructures, particularly enabling reliable extrapolation of short-term laboratory data to long-term service life predictions.