| 1. |
- Auth, Kim Louisa, 1995, et al.
(författare)
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A fully coupled chemo-mechanical cohesive zone model for oxygen embrittlement of nickel-based superalloys
- 2022
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Ingår i: Journal of the Mechanics and Physics of Solids. - : Elsevier BV. - 0022-5096. ; 164
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Tidskriftsartikel (refereegranskat)abstract
- For nickel-based superalloys subjected to high temperatures and oxygen-rich environments, mechanical loading in combination with oxygen diffusion along grain boundaries leads to an acceleration of crack propagation. To account for these phenomena, a fully coupled thermodynamically consistent chemo-mechanical modeling framework for stress-assisted oxygen embrittlement of grain boundaries in polycrystals is proposed. We formulate an extended cohesive zone model where the grain boundary strength is reduced by the presence of oxygen and the oxygen diffusion is enhanced by tensile mechanical loading. We show that the model can qualitatively predict experimental results such as: reduction of ultimate tensile strength and accelerated crack growth due to dwell time combined with mechanical loading and saturation of crack growth rates for increasing environmental oxygen pressure levels. In addition, numerical simulation results of intergranular crack growth are shown for a 2D polycrystalline structure. An emphasis is put on the difference in cracking behavior after dwelling with or without mechanical loading.
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| 2. |
- Auth, Kim Louisa, 1995
(författare)
-
Computational modeling of oxygen-assisted fracture in nickel-based superalloys
- 2022
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Nickel-based superalloys are a commonly used material in applications where high strength is required at high temperatures. A typical such example is jet engines and, in the case of polycrystalline nickel-based superalloys, components like turbine disks. Under severe loading conditions, such as cyclic loading combined with sustained dwell times at high temperatures, polycrystalline nickel-based superalloys are known to experience accelerated fatigue crack growth in oxygen-rich environments compared to vacuum. Environmentally assisted crack initiation could for example be identified as cause for turbine disk fracture in some cases of mechanical turbine failure of passenger airplanes. It has been shown by experimental work in the past that oxygen is the main deteriorating species in the case of intergranular fracture in nickel-based superalloys. In this work we develop a fully coupled chemo-mechanical cohesive zone model accounting for the interaction between oxygen transport into the grain boundaries and their stress state. The model is presented in a thermodynamic framework. Additionally, a chemo-mechanical cohesive finite element carrying both the displacement and the concentration field is suggested. The finite element formulation is complemented by a moving boundary condition for the concentration field, accounting for the increase in environment-exposed surface as edge cracks grow into the structure. Aside from handling edge cracks, the moving boundary condition yields physically meaningful results even in the case of crack initiation at interior grain boundaries. Numerical experiments are conducted on bi- and polycrystals. It is shown that the modeling framework can qualitatively reproduce experimentally observed phenomena, like the reduction of tensile strength in oxygen rich environments, the acceleration of crack growth rates upon increasing environmental oxygen concentration and saturation thereof for high environmental oxygen levels. It is also demonstrated that edge cracks can be propagated past preexisting cracks inside the polycrystal while maintaining realistic oxygen boundary conditions.
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| 3. |
- Auth, Kim Louisa, 1995, et al.
(författare)
-
Modeling of environmentally assisted intergranular crack propagation in polycrystals
- 2023
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Ingår i: International Journal for Numerical Methods in Engineering. - 0029-5981 .- 1097-0207. ; 124:23, s. 5183-5199
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Tidskriftsartikel (refereegranskat)abstract
- Polycrystalline nickel-based superalloys experience accelerated intergranular crack growth when exposed to dwell times in oxygen-rich environments and a combination of high temperature and tensile mechanical loading. Increasing crack growth rates are observed for increasing amounts of environmental oxygen in a certain oxygen concentration range, while below and above that range crack growth rates remain approximately constant. A fully coupled chemo-mechanical modeling framework accounting for the degradation of grain boundaries by oxygen has been presented by the authors. In this work, we expand the framework by a moving boundary condition to capture a realistic oxygen flux in grain boundary cracks for both edge cracks connected to the environment and interior cracks. In numerical simulation results, the behavior of the moving boundary condition is shown for intergranular crack propagation through a polycrystal subjected to cyclic loading. Finally, the capabilities of the modeling framework to qualitatively predict the dependence of the average crack growth rate on the environmental oxygen content, load level, and dwell time are evaluated and it is shown that predictions qualitatively agree with experimental observations for intergranular fracture.
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