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A weakest link model for multiple mechanism brittlefracture - Model development and application
KTH, School of Engineering Sciences (SCI), Engineering Mechanics, Vehicle Engineering and Solid Mechanics, Solid Mechanics.ORCID iD: 0000-0002-8408-8489
KTH, School of Engineering Sciences (SCI), Engineering Mechanics, Vehicle Engineering and Solid Mechanics, Solid Mechanics.ORCID iD: 0000-0002-9509-2811
KTH, School of Engineering Sciences (SCI), Engineering Mechanics, Vehicle Engineering and Solid Mechanics, Solid Mechanics.ORCID iD: 0000-0003-1498-5691
KTH, School of Engineering Sciences (SCI), Engineering Mechanics, Vehicle Engineering and Solid Mechanics, Solid Mechanics.ORCID iD: 0000-0003-2470-7679
2020 (English)Report (Other academic)
Abstract [en]

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.

Place, publisher, year, edition, pages
2020. , p. 46
Series
TRITA-SCI-RAP ; 2020:005
National Category
Applied Mechanics
Research subject
Solid Mechanics
Identifiers
URN: urn:nbn:se:kth:diva-279001OAI: oai:DiVA.org:kth-279001DiVA, id: diva2:1457158
Note

QC 20200812

Available from: 2020-08-10 Created: 2020-08-10 Last updated: 2024-03-18Bibliographically approved
In thesis
1. Modeling of structural integrity of aged low alloy steels using non-local mechanics
Open this publication in new window or tab >>Modeling of structural integrity of aged low alloy steels using non-local mechanics
2020 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

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.

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2020. p. 55
Series
TRITA-SCI-FOU ; 2020:024
National Category
Applied Mechanics
Research subject
Solid Mechanics
Identifiers
urn:nbn:se:kth:diva-279003 (URN)978-91-7873-603-4 (ISBN)
Public defence
2020-09-18, Live-streaming via Zoom: https://kth-se.zoom.us/j/68393256338 If you lack computer or computerskills, please contact Per-Lennart Larsson at plla@kth.se for information, Stockholm, 10:00 (English)
Opponent
Supervisors
Note

QC 20200817

Available from: 2020-08-17 Created: 2020-08-10 Last updated: 2022-06-26Bibliographically approved

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Boåsen, MagnusDahlberg, Carl F. O.Efsing, PålFaleskog, Jonas

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