Modeling of structural integrity of aged low alloy steels using non-local mechanics

Ageing of low alloy steels affects the structural integrity assessment as it most commonly causes embrittlement and a hardening of the material. This is due to theevolution of the microstructure during operation in the specific application. In nuclear applications, the most common causes of ageing of low alloy steels areirradiation and thermal ageing. Embrittlement in this type of materials is generally divided into hardening and non-hardening embrittlement. The formation of clusters or precipitates of solute atoms typically cause the former, and the weakening of grain boundaries generally cause the latter. This thesis is devoted to thedevelopment of models that can be used to describe the material properties of aged low alloy steels in terms of plastic properties and fracture toughness, and to thestudy of the effects of thermal ageing on the mechanical properties of a low alloy steel. In Paper I, a strain gradient plasticity framework is applied in order to capture length scale effects. The constitutive length scale is assumed to be related to the dislocation mean free path and the changes it undergoes during plastic deformation. Several evolution laws for the length scale were developed and implemented in a FEM-code. This was used to solve a test problem in order to probe the effects of the length scale evolution. All length scale evolution laws considered in this study results in a decreasing length scale, which causes an overall softening in cases where the strain gradient dominates the solution. The results are in tentative agreement with phenomena of strain localization that occurs in highly irradiated materials. In Paper II, a scalar stress measure for cleavage fracture is developed and generalized, here called the effective normal stress measure. This is used in a nonlocal weakest link model which is applied to two datasets from literature in order to study the effects of the effective normal stress measure, as well as to experiments considering four-point bending of specimens containing a semi-elliptical surface crack. The model is shown to reproduce the failure probability of all considered datasets, i.e. well capable of transferring toughness information between different geometries. In Paper III, a thermally aged weld from the Ringhals nuclear power plant is studied experimentally and compared to a reference material using fracture toughness testing. The main objective of the study was to investigate the effect of thermal ageing on the cleavage or brittle fracture toughness, with a specific focus on the effect of crack tip constraint. The testing showed that thermal ageing had enabled brittle fracture initiation from grain boundaries, resulting in a bimodal toughness distribution due to multiple mechanisms for brittle fracture initiation. In Paper IV, the non-local weakest link model in Paper II is further developed to account for multiple mechanism brittle fracture. The model is developed for brittle fracture initiation from grain boundaries and second phase particles. The grain boundary mechanism is inferred from simulations of polycrystalline aggregates using crystal plasticity. When applied to the experimental results of Paper III, the model is able to describe the fracture toughness distribution with a remarkable accuracy.QC 20200817