A Validated Model for Deformation and Fracture in 316H Stainless Steel Pipes in Nuclear Power Plants: Incorporating the Microstructure, Stress Triaxiality and Damage Indicators

Accurately modelling creep deformation and damage in 316H stainless steel under high temperatures and complex loading is vital for the long-term structural integrity of nuclear power plants components, such as pipes and pressure vessels. This is particularly important in the UK's advanced gas-cooled reactors, where 316H is widely used. Reliable models can inform life assessment procedures, such as R5, enhancing safety and extending component lifespan under creep conditions. Microstructural features such as grains, grain boundaries and carbides, often overlooked in assessment codes, are addressed in this study. A combined modelling and experimental approach is presented to understand the creep behaviour of 316H stainless steel at 550°C, focusing on stresses that induce glide-controlled dislocation creep. A crystal plasticity finite element model is developed to capture primary and secondary creep deformation stages, incorporating state variables for each slip system and thermal recovery, alongside specific power law terms for plasticity and creep deformation. The model is validated through experiments on notched samples, with creep damage observed via optical microscopy and grain structures characterised using Electron Backscatter Diffraction. A submodelling strategy is proposed to embed crystal plasticity domains within larger volumes, enabling detailed local deformation analysis while maintaining realistic boundary conditions.