Ageing of low alloy steel in nuclear applications commonly takes the form as a hardening and an embrittlement of the material. This is due to the evolution of the microstructure during irradiation and at purely thermal conditions, as a combination or separate. Irradiation introduces evenly distributed solute clusters, while thermal ageing has been shown to yield a more inhomogeneous distribution. These clusters affect the dislocation motion within the material and results in a hardening and in more severe cases of ageing, also a decreased work hardening slope due to plastic strain localization into bands/channels. Embrittlement corresponds to decreased fracture toughness due to microstructural changes resulting from ageing. The thesis presents a possible framework for modeling of ageing effects in low alloy steels.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 this undergoes during plastic deformation. Several evolution laws for the length scale were developed and implemented in a FEM-code considering 2D plane strain. This was used to solve a test problem of pure bending in order to investigate the effects of the length scale evolution. As all length scale evolution laws considered in this study results in a decreasing length scale; this leads to a loss of non-locality which causes an overall softening at cases where the strain gradient is dominating the solution. The results are in tentative agreement with phenomena of strain localization that is occurring in highly irradiated materials.In Paper II, the scalar stress measure for cleavage fracture is developed and generalized, here called the effective normal stress measure. This is used in a non-local weakest link model which is applied to two datasets from the literature in order to study the effects of the effective normal stress measure, as well as new 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.

Åldring av låglegerade stål i kärntekniska användningsområden framträder typiskt som ett hårdnande och en försprödning av materialet. Detta på grund av utvecklingen av mikrostrukturen under bestrålning och under rent termiska förhållanden. Bestrålning introducerar jämt fördelade kluster av legeringsämnen. Termisk åldring har däremot visats ge upphov till en mer ojämn fördelning. Klustren hämmar dislokationsrörelsen i materialet och ger därigenom upphov till en ökning av materialets sträckgräns, vid en mer påtaglig åldring det även leda till ett sänkt arbetshårdnande på grund av lokalisering av plastisk töjning i s.k. kanaler/band. Försprödning är en sänkning av materialets brottseghet som en följd av de mikrostrukturella förändringar som sker vid åldring. Arbetet som presenteras i den här avhandlingen har gjorts i syfte till att ta fram ett möjligt ramverk för modellering av låglegerade stål.I Artikel I, används en töjningsgradientbaserad plasticitetsteori för att kunna fånga längdskalebeteenden. Längdskalan i teorin antas vara relaterad till dislokationernas medelfria väg och den förändring den genomgår vid plastisk deformation. Flera utvecklingslagar för längdskalan har analyserats och implementerats i en finita element kod för 2D plan deformation. Denna implementering har använts för att lösa ett testproblem bestående av ren böjning med syfte att undersöka effekterna av utvecklingen hos längdskalan. Alla de utvecklingslagar som presenteras i artikeln ger en minskande längdskala, vilket leder till vad som valt att kallas förlust av icke-lokalitet. Fenomenet leder till ett övergripande mjuknande vid fall där den plastiska töjningsgradienten har stor inverkan på lösningen. Resultaten är i preliminär överenstämmelse med de typer av lokalisering av plastisk töjning som observerats i starkt bestrålade material.I Artikel II utvecklas ett generaliserat spänningsmått i syfte att beskriva klyvbrott, här benämnt effektivt normalspänningsmått. Detta har använts i samband med en icke-lokal svagaste länk modell, som har applicerats på två experimentella studier från den öppna litteraturen i syfte att studera effekterna av det effektiva normalspänningsmåttet. Utöver detta presenteras även nya experiment på ytspruckna provstavar under fyrpunktsböj. I artikeln visas att modellen återskapar sannolikheten för brott för alla undersökta experimentuppställningar, d.v.s. modellen visas vara väl duglig för att överföra brottseghet mellan geometrier.

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.