Understanding the influence of prior plastic loading on subsequent creep in 316H stainless steel via crystal plasticity finite element modelling

Structural components in the thermal power generation industry are regularly exposed to cyclic thermal stresses and as a result, develop complex loading histories which impact their lifetimes. The current assessment methodologies often introduce conservatism which can result in under-predicting the safe lifetimes of plant components. In order to better inform lifetime assessments, it is vitally important to understand both experimentally and through simulations, how the loading history of a material influences the mechanism by which subsequent macromechanical deformation takes place. Crystal plasticity finite element (CPFE) modelling is an effective tool that is able to capture the macroscopic response and meso-scale behaviour of crystalline materials, such as 316H stainless steel. The information gained from CPFE modelling provides a deeper understanding of the physical mechanisms and behaviours that lead to deformation, which will result in better informed lifetime assessments. In this thesis, the predictive capabilities of a crystal plasticity finite element model are scrutinised and novel experimental testing programs are developed.Specific areas of the CPFE model are subjected to a sensitivity analysis study to examine various sources of uncertainty; the input material parameters, the amount of scatter that is produced by altering the initial microstructure, and the influence of the boundary conditions. A further investigation into the predictive capabilities of the model is completed to analyse how thestress/strain response of a single grain is influenced by the crystallographic orientation of itself and the neighbouring grains. The influence of prior plastic deformation on subsequent creep and primary creep regeneration has been investigated experimentally and successfully simulated withthe CPFE model. The experimental results have highlighted the extent to which creep behaviour can be altered and demonstrate the need for accurate modelling of such complex loading cases such that these macromechanical responses can be represented in lifetime assessments