| 1. |
- Halilovic, Armin E., et al.
(författare)
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A conceptual modeling approach for investigating multiple failure mechanisms in the environmentally driven ductile-to-brittle transition region
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Recension (övrigt vetenskapligt/konstnärligt)abstract
- A continuum modeling approach that considers two separate failure mechanisms of steels subjected to hydrogen embrittlement is proposed based on experimental observations. The brittle failure is modeled using a cohesive zone approach, where both the cohesive strength and the fracture energy are degraded when exposed to hydrogen. The ductile failure is modeled using the Gurson model that includes a strain driven nucleation of void. Here, the nucleation model also incorporates hydrogen degradation where an increase in hydrogen is assumed to increase the volume of nucleated voids. This modeling approach is divided into two parts where the first step is to utilize a conceptual degradation of both failure modes and calibrate modeling parameters, and the second part incorporates a coupled diffusion-mechanical approach.
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| 2. |
- Halilović, Armin E.
(författare)
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A Fracture Mechanics Approach to Study Hydrogen Embrittlement in High Strength Martensitic Steels
- 2023
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- High strength steels that are subjected to hydrogen experience embrittlement where the mechanical properties are reduced, and premature failure of components may occur. Although the phenomenon has been recognized for over 150 years, it is not clear what drives embrittlement. The goal of this thesis has been to investigate hydrogen embrittlement in high strength martensitic steels by fracture toughness testing. Since a well-recognized standard to test a materials susceptibility to hydrogen embrittlement is missing, the first step has been to develop an experimental-numerical method that produces reproducible results that can be transferred from laboratory to in-service components, which is presented in Paper I. Here it is seen that the environmentally driven ductile-to-brittle transition region in elastic-plastic fracture toughness depends on the hydrogen exposure time. The presented numerical evaluation approach removes the need to perform unloadings, and the results correlates well with standards. The proposed method is then applied to two different application areas presented in Paper II and Paper IV. In Paper II the proposed experimental method is utilized to develop a framework that can be used to study hydrogen kinetics ahead of a crack frontduring in-situ conditions for delayed hydrogen cracking using neutron imaging. In Paper IV, the experimental method is applied to specimens with different crack tip constraints to mitigate the gap between laboratory experiments and in-service components. It is seen that the environmentally driven ductile-to-brittle transition region is obtained for specimens with different constraints, and that both the plastic strains as well as the hydrostatic stress play a critical role in hydrogen embrittlement. The results from Paper I are used as the basis for the numerical framework presented in Paper III. Here, a conceptual modeling approach is adopted that incorporates two separate failure mechanisms observed in the experiments performed in Paper I. It is seen that both a ductile and brittle failure mechanism must be employed to capture the full range of crack extension resistance curves. Furthermore, to capture the slope of the degraded J-R curves, it is necessary to employ a degradation of fracture energy, the cohesive strength as well as the strain driven nucleation.
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| 3. |
- Halilovic, Armin E., et al.
(författare)
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An experimental fracture mechanics study of the combined effect of hydrogen embrittlement and loss of constraint
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Recension (övrigt vetenskapligt/konstnärligt)abstract
- This work presents a systematic investigation of the combined effect of hydrogen embrittlement and loss of constraint. The fracture mechanics experiments are performed on an advanced martensitic high strength steel using a single-edge-notch bend specimen, with different crack over height ratio, subjected to electrochemical in-situ hydrogen charging at various loading rates. It is found that the environmentally driven ductile-to-brittle transition region in fracture toughness is obtained for both the high and low constraint specimen configurations. This region is characterized by a change from transgranular dimple rupture to an intergranular mode of fracture. The transition region for the low constraint specimen is shifted towards longer hydrogen exposure times, which is an effect of the reduced hydrostatic stress ahead of the crack front compared to the high constraint specimen. The low constraint specimen exhibits significant plastic straining, which is reflected in a significant decrease in the fracture toughness due to hydrogen transgranular assisted dimple rupture.
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| 4. |
- Lindblom, David, et al.
(författare)
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In-situ neutron imaging of delayed hydrogen cracking in highstrength steel - experiments and modeling
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Recension (övrigt vetenskapligt/konstnärligt)abstract
- Hydrogen delayed fracture, also known as hydrogen-induced cracking, is a type of brittle fracture that occurs due to the slow diffusion and accumulation of hydrogen atoms, leading todecreased ductility and eventual cracking under constant load. This paper presents an in-situobservation, using neutron imaging, of delayed crack propagation caused by hydrogen embrittlement in a high strength martensitic steel specimen. The experiments involved mechanicalloading of a single-edge-notch bend specimen while submerged in an electrolyte solution (H2O+ 3.5% NaCl) under cathodic polarization to facilitate hydrogen ingress. Neutron transmission images were obtained in-situ and used to monitor intermittent crack propagation wasrecorded over a period of 12 hours. The stress state at each crack configuration was extracted from a three-dimensional elastic-plastic finite element simulation, which was tailoredto match the quantitative information acquired from the neutron radiographs of the fractureprocess. To gain insight into the evolution of hydrogen concentration with crack propagation,a modeling scheme for stress-assisted hydrogen diffusion was employed. These simulationsprovided qualitative information on the relation between intermittent crack propagation andthe subsequent supply of hydrogen to the crack tip. Finally, a failure locus was constructedbased on the calculated hydrogen concentration levels and the experimentally determinedcrack growth resistance.
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