| 1. |
- Boåsen, Magnus, et al.
(författare)
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A generalized probabilistic model for cleavage fracture with a length scale - Influence of stress state and application to surface cracked experiments
- 2019
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Ingår i: Engineering Fracture Mechanics. - : PERGAMON-ELSEVIER SCIENCE LTD. - 0013-7944 .- 1873-7315. ; 214, s. 590-608
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Rapport (övrigt vetenskapligt/konstnärligt)abstract
- A probabilistic model for the cumulative probability of failure by cleavage fracture with a material related length scale is further developed in this study. A new generalized effective stress measure is proposed, based on a normal stress decomposition of the stress tensor, capable of describing a state of normal stress in the range from the mean stress to the maximum principal stress. The effective stress measure associated with a material point is evaluated from the stress tensor averaged over the material related length scale. The model is shown to be well capable to predict both the fracture toughness at loss of both in-plane and out-of-plane constraint by model application to two different datasets from the open literature. The model is also shown to be well capable of predicting the probability of failure of discriminating experiments on specimens containing semi-elliptic surface cracks. A comparison where the master curve methodology is used to predict the probability of failure of the experiments is also included.
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| 2. |
- Boåsen, Magnus, et al.
(författare)
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A weakest link model for multiple mechanism brittle fracture — Model development and application
- 2021
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Ingår i: Journal of the mechanics and physics of solids. - : Elsevier BV. - 0022-5096 .- 1873-4782. ; 147
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Tidskriftsartikel (refereegranskat)abstract
- A multiple mechanism weakest link model for intergranular and transgranular brittle fracture is developed on the basis of experimental observations of a thermally aged low alloy steel. The model development is carried out in tandem with micro mechanical analysis of grain boundary cracking using crystal plasticity modeling of polycrystalline aggregates with the purpose to inform the weakest link model. The fracture modeling presented in this paper is carried out by using a non-local porous plastic Gurson model where the void volume fraction evolution is regularized over two separate length scales. The ductile crack growth preceding the final brittle fracture is well predicted using this type of modeling. When applied to the brittle fracture tests, the weakest link model predicts the fracture toughness distribution remarkably well, both in terms of the constraint and the size effect. Included in the study is also the analysis of a reference material.
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| 3. |
- Boåsen, Magnus, et al.
(författare)
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A weakest link model for multiple mechanism brittlefracture - Model development and application
- 2020
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Rapport (övrigt vetenskapligt/konstnärligt)abstract
- A multiple mechanism weakest link model for intergranular and transgranularbrittle fracture is developed on the basis of experimental observations in a thermallyaged low alloy steel. The model development is carried out in tandemwith micro mechanical analysis of grain boundary cracking using crystal plasticitymodeling of polycrystalline aggregates with the purpose to inform theweakest link model. The fracture modeling presented in this paper is carriedout by using a non-local porous plastic Gurson model where the void volumefraction evolution is regularized over two separate length scales. The ductilecrack growth preceding the nal brittle fracture is well predicted using this typeof modeling. When applied to the brittle fracture tests, the weakest link modelpredicts the fracture toughness distribution remarkably well, both in terms ofthe constraint and the size eect. Included in the study is also the analysis of areference material.
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| 4. |
- Boåsen, Magnus, et al.
(författare)
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Analysis of thermal embrittlement of a low alloy steel weldment using fracture toughness and microstructural investigations
- 2020
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Rapport (övrigt vetenskapligt/konstnärligt)abstract
- A thermally aged low alloy steel is investigated in terms of its fracture toughness and microstructural evolution and compared to a reference. The main purpose of the study is to investigate the effects of thermal embrittlement on the brittle fracture toughness, and its effects on the influence of loss of crack tip constraint. Ageing appears to enable brittle fracture initiation from grain boundaries besides initiation from second phase particles, making the fracture toughness distribution bimodal as a result. The consequence is that the constraint effect is significantly reduced when grain boundary initiation dominates the toughness distribution, as compared to the reference material where the constraint effect is significant. The microstructure is investigated at the nano scale using atom probe tomography where nanometer sized Cu-rich clusters are found primarily situated on dislocation lines.
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| 5. |
- Boåsen, Magnus, et al.
(författare)
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Analysis of thermal embrittlement of a low alloy steel weldment using fracture toughness and microstructural investigations
- 2022
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Ingår i: Engineering Fracture Mechanics. - : Elsevier BV. - 0013-7944 .- 1873-7315. ; 262
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Tidskriftsartikel (refereegranskat)abstract
- A thermally aged low alloy steel weld metal is investigated in terms of its fracture toughness and microstructural evolution and compared to a reference. The main purpose of the study is to investigate the effects of embrittlement due to thermal ageing on the brittle fracture toughness, and its effects on the influence of loss of crack tip constraint. The comparison of the investigated materials has been made at temperatures that give the same median fracture toughness of the high constraint specimens, ensuring comparability of the low constraint specimens. Ageing appears to enable brittle fracture initiation from grain boundaries besides initiation from second phase particles, making the fracture toughness distribution bimodal. Consequently, this appears to reduce the facture toughness of the low constraint specimens of the aged material as compared to the reference material. The microstructure is investigated at the nano scale using atom probe tomography where nanometer sized Ni-Mn-rich clusters, precipitated during ageing, are found primarily situated on dislocation lines.
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| 6. |
- Boåsen, Magnus, 1991-
(författare)
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Modeling framework for ageing of low alloy steel
- 2019
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- 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.
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| 7. |
- Boåsen, Magnus, 1991-
(författare)
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Modeling of structural integrity of aged low alloy steels using non-local mechanics
- 2020
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- 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.
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| 8. |
- Boåsen, Magnus, et al.
(författare)
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On flux effects in a low alloy steel from a Swedish reactor pressure vessel
- 2017
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Ingår i: Journal of Nuclear Materials. - : Elsevier. - 0022-3115 .- 1873-4820. ; 484, s. 110-119
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Tidskriftsartikel (refereegranskat)abstract
- This study aims to investigate the presence of Unstable Matrix Defects in irradiated pressure vessel steel from weldments of the Swedish PWR Ringhals 4 (R4). Hardness tests have been performed on low flux (surveillance material) and high flux (Halden reactor) irradiated material samples in combination with heat treatments at temperatures of 330, 360 and 390 °C in order to reveal eventual recovery of any hardening features induced by irradiation. The experiments carried out in this study could not reveal any hardness recovery related to Unstable Matrix Defects at relevant temperatures. However, a difference in hardness recovery was found between the low and the high flux samples at heat treatments at higher temperatures than expected for the annihilation of Unstable Matrix Defects–the observed recovery is here attributed to differences of the solute clusters formed by the high and low flux irradiations.
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| 9. |
- Dahlberg, Carl F. O., 1980-, et al.
(författare)
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Evolution of the length scale in strain gradient plasticity
- 2019
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Ingår i: International journal of plasticity. - : PERGAMON-ELSEVIER SCIENCE LTD. - 0749-6419 .- 1879-2154. ; 112, s. 220-241
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Tidskriftsartikel (refereegranskat)abstract
- An equivalence is assumed between a microstructural length scale related to dislocation density and the constitutive length scale parameter in phenomenological strain gradient plasticity. An evolution law is formed on an incremental basis for the constitutive length scale parameter. Specific evolution equations are established through interpretations of the relation between changes in dislocation densities and increments in plastic strain and strain gradient. The length scale evolution has been implemented in a 2D-plane strain finite element method (FEM) code, which has been used to study a beam in pure bending. The main effect of the length scale evolution on the response of the beam is a decreased strain hardening, which in cases of small beam thicknesses even leads to a strain softening behavior. An intense plastic strain gradient may develop close to the neutral axis and can be interpreted as a pile-up of dislocations. The effects of the length scale evolution on the mechanical fields are compared with respect to the choice of length evolution equation.
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| 10. |
- Halilovic, Armin, et al.
(författare)
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An experimental-numerical screening method for assessing environmentally assisted degradation in high strength steels
- 2021
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Ingår i: Engineering Fracture Mechanics. - : Elsevier BV. - 0013-7944 .- 1873-7315. ; 245
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Tidskriftsartikel (refereegranskat)abstract
- In this work, an experimental-numerical screening method for studying the elastic-plastic properties in high strength steel subjected to environmentally assisted degradation due to hydrogen is proposed. The experiments were performed on single-edge-notch bend specimens loaded with a monotonic constant displacement rate, and the specimens were electrochemically hydrogen pre-charged and/or in-situ. A systematic investigation was conducted of the influence of current density, pre-charging time and loading rate on the fracture mechanical properties. It was found that the loading rate had the greatest effect on the J-R curves, and that the environmental ductile-to-brittle transition region was obtained in a less than a day of experimental time. In this transition region it was found from the fractography that the dominating mode of failure changed from dimple to dominating intergranular fracture.
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| 11. |
- Hultgren, Gustav, et al.
(författare)
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Fracture toughness assessment of surface cracks in slender ultra-high-strength steel plates
- 2023
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Ingår i: Engineering Fracture Mechanics. - : Elsevier Ltd. - 0013-7944 .- 1873-7315. ; 289
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Tidskriftsartikel (refereegranskat)abstract
- Safe design against unstable fractures in load-bearing structures is crucial at sub-zero temperatures where the brittle fracture toughness can be unfavourable, especially for high-stress designs incorporating ultra-high-strength steels. The brittle fracture toughness of surface cracks in structural steel with a minimum yield strength of 1300 MPa is, for this reason, tested in the present study at sub-zero temperatures. The realistic flaws are compared with single-edge notched specimens (SEN(B)) from thicker plates with the same chemical composition, using a representative fracture toughness for a three-dimensional crack front according to the Master Curve method. A novel approach determines the latter without considering the local temperature and constraint variation through empirical relations. The experimental result shows a difference in the reference temperature between the two specimen types, which likely is the natural variation of the manufactured materials and/or a loss of constraint due to the difference in the scaled specimen deformation level.
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| 12. |
- Lindgren, Kristina, 1989, et al.
(författare)
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Cluster formation in in-service thermally aged pressurizer welds
- 2018
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Ingår i: Journal of Nuclear Materials. - : Elsevier BV. - 0022-3115 .- 1873-4820. ; 504, s. 23-28
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Tidskriftsartikel (refereegranskat)abstract
- Thermal aging of reactor pressure vessel steel welds at elevated temperatures may affect the ductile-to-brittle transition temperature. In this study, unique weld material from a pressurizer, with a composition similar to that of the reactor pressure vessel, that has been in operation for 28 years at 345 °C is examined. Despite the relatively low temperature, the weld becomes hardened during operation. This is attributed to nanometre sized Cu-rich clusters, mainly located at Mo- and C-enriched dislocation lines and on boundaries. The welds have been characterized using atom probe tomography, and the characteristics of the precipitates/clusters is related to the hardness increase, giving the best agreement for the Russell-Brown model.
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| 13. |
- Lindgren, Kristina, 1989, et al.
(författare)
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Evolution of precipitation in reactor pressure vessel steel welds under neutron irradiation
- 2017
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Ingår i: Journal of Nuclear Materials. - : Elsevier. - 0022-3115 .- 1873-4820. ; 488, s. 222-230
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Tidskriftsartikel (refereegranskat)abstract
- Reactor pressure vessel steel welds are affected by irradiation during operation. The irradiation results in nanometre cluster formation, which in turn affects the mechanical properties of the material, e.g. the ductile-to-brittle transition temperature is shifted to higher levels. In this study, cluster formation is characterised in high Ni (1.58%) low Cu (0.04%) steel welds identical to Ringhals R4 welds, using atom probe tomography in both surveillance material and in material irradiated at accelerated dose rates. Clusters containing mainly Ni and Mn, but also some Si and Cu were observed in all of the irradiated materials. Their evolution did not change drastically during irradiation; the clusters grew and new clusters were nucleated. Hence, both the cluster number density and the average size increased with irradiation time. Some flux effects were observed when comparing the high flux material and the surveillance material. The surveillance material has a lower cluster number density, but larger clusters. The resulting impact on the mechanical properties of these two effects cancel out, resulting in a measured hardness that seems to be on the same trend as the high flux material. The dispersed barrier hardening model with an obstacle strength factor of 0.15 was found to reproduce the increase in hardness. In the investigated high flux materials, the clusters' Cu content was higher.
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| 14. |
- Lindgren, Kristina, 1989, et al.
(författare)
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Post-irradiation annealing of high flux irradiated and surveillance material reactor pressure vessel weld metal
- 2022
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Ingår i: Journal of Nuclear Materials. - : Elsevier BV. - 0022-3115 .- 1873-4820. ; 562
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Tidskriftsartikel (refereegranskat)abstract
- In this study, high flux irradiated and surveillance high Ni and Mn and low Cu welds identical to those of the belt-line region of Ringhals R4 were subjected to annealing at temperatures between 390 and 455 °C for 24–30 h, in order to study the dissolution of irradiation induced clusters and possible matrix defects using hardness testing and atom probe tomography. It was found that the cluster characteristics did not change during annealing at 390 °C, meaning that the size, number density and composition of the clusters, which mainly consist of Ni and Mn, did not change. Thus, the observed decrease in hardness during annealing of the high flux irradiated material is believed to be due to dissolution of matrix defects that were stable at the operating temperature. Cluster dissolution was observed after annealing at 410 °C in the high flux irradiated material, leaving around 10% of the original clusters. These clusters contained more Cu and less Ni and Mn than before annealing. The cluster dissolution at temperatures above 400 °C correlated with the decrease in hardness. The larger clusters of the surveillance material required a higher temperature or longer time to be dissolved compared to the clusters of the high flux material.
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| 15. |
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| 16. |
- Tomstad, Asle Joachim, et al.
(författare)
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On the influence of stress state on ductile fracture of two 6000-series aluminium alloys with different particle content
- 2023
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Ingår i: International Journal of Solids and Structures. - : Elsevier BV. - 0020-7683 .- 1879-2146. ; 269, s. 112149-
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Tidskriftsartikel (refereegranskat)abstract
- Tension-torsion tests were conducted on two 6000-series aluminium alloys with different area fraction of con-stituent particles. The two alloys, denoted alloy A and B, have previously been characterized and found to have similar matrix material, albeit the three times higher area fraction of constituent particles in alloy B than in alloy A. Single notch tube specimens of the two alloys were subjected to fifteen proportional load paths by varying the ratio of axial force and twisting moment, probing stress states from torsion to plane-strain tension. The overall failure strain in the notch was estimated analytically based on the experimental data, whereas finite element simulations were used to determine the stress and strain fields within the notch region and to estimate the local failure strain. The experiments showed that the increased particle content led to a reduction in the local failure strain of alloy B compared with alloy A that varied from 16% to 60%, depending on the stress state, with an average reduction of 39%. While the overall trend was an increasing failure strain with decreasing stress triaxiality, significant influence of the Lode parameter was observed, and thus the increase was not monotonic. Applying a porous plasticity model, localization analyses were conducted to examine the underlying mechanisms for the complex variation of the failure strain with stress state.
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