Computational modeling of trans- and intergranular fracture in polycrystals
Doctoral thesis, 2024
Nickel-based superalloys are commonly used in demanding environments, where high temperatures are combined with considerable mechanical loads, such as turbine disks in jet engines and gas turbines. When exposed to a combination of severe loading conditions, including an oxygen-rich environment, high temperatures, and sustained tensile loading, the fracture mode of nickel-based superalloys can change from the default ductile transgranular fracture to environmentally assisted intergranular fracture.
In this thesis, a fully chemo-mechanically coupled modeling framework is presented for intergranular fracture. The framework is built on a cohesive zone law, that is degraded based on the local oxygen concentration. Oxygen transport is at the same time accelerated by traction gradients. The model is complemented by a moving oxygen boundary condition, which follows the crack tip upon crack propagation. It is demonstrated that the framework can qualitatively predict experimental results such as the reduction of ultimate tensile strength and the dependence of the average crack growth rates on the environmental oxygen content, load level, and dwell time in cyclic loading.
In order to capture the formation of transgranular cracks, a thermodynamical framework for ductile phase-field fracture is developed. It is based on a large deformation crystal plasticity model and incorporates size dependence via gradient-extended hardening. A micromorphic damage irreversibility approach is adopted to ensure thermodynamic and variational consistency of the proposed model. A damage dependent micro-flexible boundary condition for gradient-extended hardening is developed, which allows grain boundaries to retain their resistance to slip transmission during hardening. Numerical simulations show that the fracture model can describe transgranular crack growth in two- and three-dimensional polycrystals, while considering microstructural effects such as crystal orientation, grain geometry and void coalescence.
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In this thesis, a fully chemo-mechanically coupled modeling framework is presented for intergranular fracture. The framework is built on a cohesive zone law, that is degraded based on the local oxygen concentration. Oxygen transport is at the same time accelerated by traction gradients. The model is complemented by a moving oxygen boundary condition, which follows the crack tip upon crack propagation. It is demonstrated that the framework can qualitatively predict experimental results such as the reduction of ultimate tensile strength and the dependence of the average crack growth rates on the environmental oxygen content, load level, and dwell time in cyclic loading.
In order to capture the formation of transgranular cracks, a thermodynamical framework for ductile phase-field fracture is developed. It is based on a large deformation crystal plasticity model and incorporates size dependence via gradient-extended hardening. A micromorphic damage irreversibility approach is adopted to ensure thermodynamic and variational consistency of the proposed model. A damage dependent micro-flexible boundary condition for gradient-extended hardening is developed, which allows grain boundaries to retain their resistance to slip transmission during hardening. Numerical simulations show that the fracture model can describe transgranular crack growth in two- and three-dimensional polycrystals, while considering microstructural effects such as crystal orientation, grain geometry and void coalescence.
Environmentally assisted fatigue
Moving boundary condition
Gradient crystal plasticity
Nickel-based superalloy
Phase-field modeling
Transgranular fracture
Intergranular fracture
Micro-flexible boundary condition
Author
Kim Louisa Auth
Chalmers, Industrial and Materials Science, Material and Computational Mechanics
Included papers
Auth, K.L., Brouzoulis, J., Ekh, M. Phase-field modeling of ductile fracture across grain boundaries in polycrystals.
Manuscript
A thermodynamic framework for ductile phase-field fracture and gradient-enhanced crystal plasticity
European Journal of Mechanics, A/Solids,;Vol. 108(2024)
Journal article
Modeling of environmentally assisted intergranular crack propagation in polycrystals
International Journal for Numerical Methods in Engineering,;Vol. 124(2023)p. 5183-5199
Journal article
A fully coupled chemo-mechanical cohesive zone model for oxygen embrittlement of nickel-based superalloys
Journal of the Mechanics and Physics of Solids,;Vol. 164(2022)
Journal article
Popular science description
English
Understanding the fracture mechanics of nickel-based superalloys
Nickel-based superalloys are a class of materials known for their exceptional strength and resistance against temperatures reaching several hundred degrees Celsius. These materials are for this reason common in the aerospace industry, where they are used in critical components like jet engines.
Materials like nickel-based superalloys consist of crystal structures and they can either be made from a single crystal or from many tiny crystals, in which case they are called polycrystals. The tiny crystals are called grains and the positions where the different grains meet are called grain boundaries. When subjected to severe conditions — high temperatures, oxygen-rich environments like air, and prolonged tension — these materials can break under significantly lower loads than they normally would. This happens due to a phenomenon known as environmentally assisted fatigue. This phenomenon has been found to be the root cause of several examples of jet engine failures in passenger air planes.
To prevent such failures and to design better components in the future, it is important to understand the physical mechanisms behind the fracture behavior in these materials and to have computer models that can predict failure of superalloys. The research in this thesis focuses on developing and building such computer models for describing the fracture behavior in the grain structures. Specifically, the models investigate why and how cracks grows through some of the crystals or along their grain boundaries.
The computer models can help to understand the causes of material failure on the grain scale, where it is difficult and expensive to conduct experiments. The gained understanding can help engineers to avoid these causes of failure in the design of new engineering components. Further, predictive fracture models are useful tools for improving turbine efficiency and life spans, to reduce emissions, fuel consumptions and environmental impact.
Nickel-based superalloys are a class of materials known for their exceptional strength and resistance against temperatures reaching several hundred degrees Celsius. These materials are for this reason common in the aerospace industry, where they are used in critical components like jet engines.
Materials like nickel-based superalloys consist of crystal structures and they can either be made from a single crystal or from many tiny crystals, in which case they are called polycrystals. The tiny crystals are called grains and the positions where the different grains meet are called grain boundaries. When subjected to severe conditions — high temperatures, oxygen-rich environments like air, and prolonged tension — these materials can break under significantly lower loads than they normally would. This happens due to a phenomenon known as environmentally assisted fatigue. This phenomenon has been found to be the root cause of several examples of jet engine failures in passenger air planes.
To prevent such failures and to design better components in the future, it is important to understand the physical mechanisms behind the fracture behavior in these materials and to have computer models that can predict failure of superalloys. The research in this thesis focuses on developing and building such computer models for describing the fracture behavior in the grain structures. Specifically, the models investigate why and how cracks grows through some of the crystals or along their grain boundaries.
The computer models can help to understand the causes of material failure on the grain scale, where it is difficult and expensive to conduct experiments. The gained understanding can help engineers to avoid these causes of failure in the design of new engineering components. Further, predictive fracture models are useful tools for improving turbine efficiency and life spans, to reduce emissions, fuel consumptions and environmental impact.
Research Project(s)
Computational modeling of crystal plasticity and intergranular decohesion coupled to stress-assisted oxidation in high-temperature polycrystalline alloys
Swedish Research Council (VR) (2018-04318), 2019-01-01 -- 2023-12-31.
Categorizing
Subject Categories (SSIF 2011)
Applied Mechanics
Metallurgy and Metallic Materials
Infrastructure
C3SE (Chalmers Centre for Computational Science and Engineering)
Identifiers
ISBN
978-91-8103-121-8
Other
Series
Doktorsavhandlingar vid Chalmers tekniska högskola. Ny serie: 5579
Publisher
Chalmers
Public defence
2024-11-29 09:00 -- 12:00
Virtual Development Lab, Chalmers Tvärgata 4C, Chalmers.
Opponent: Professor Andrew McBride, James Watt School of Engineering, University of Glasgow, United Kingdom