| 1. |
- Fischer, Tim, et al.
(författare)
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Relating stress/strain heterogeneity to lath martensite strength by experiments and dislocation density-based crystal plasticity
- 2024
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Ingår i: International journal of plasticity. - : Elsevier BV. - 0749-6419 .- 1879-2154. ; 174
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Tidskriftsartikel (refereegranskat)abstract
- To enhance the fundamental understanding for micromechanical lath martensite deformation, the microstructure as well as macro- and microscopic tensile properties of as -quenched 15-5 PH stainless steel are systematically analysed depending on the austenitisation temperature. Based on electron backscatter diffraction (EBSD) and backscattered electron (BSE) analysis, it is noted that the martensite morphology alters from a less defined to a more clearly defined parallel arrangement of the block and lath structure with increasing temperature. For an indepth quantification of the hierarchical boundary strengthening contributions in relation to local stress/strain heterogeneity, separate high-fidelity virtual microstructures are realised for the different scales (prior austenite grains, packets and blocks). This is consistent with the materials transformation process. The virtual microstructures are simulated employing the crystal plasticity finite element method (CPFEM) adapted for handling high dislocation density and encompassing all relevant strengthening mechanisms by boundaries, dislocations and solute atoms. While accurately capturing the measured size -dependent stress-strain behaviour, the simulations reveal in line with the experiments (Hall-Petch) that blocks are the most effective dislocation motion barrier, causing increased strain hardening and stress/strain heterogeneity. Furthermore, since strain localisation is predicted strongest in the distinct block structure, the experimentally observed early plastic material yielding is thought to be favoured here.
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| 2. |
- Fischer, Tim, et al.
(författare)
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Sensitivity of local cyclic deformation in lath martensite to flow rule and slip system in crystal plasticity
- 2023
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Ingår i: Computational materials science. - : Elsevier BV. - 0927-0256 .- 1879-0801. ; 222, s. 112106-
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Tidskriftsartikel (refereegranskat)abstract
- The prediction of the cyclic deformation behaviour in lath martensite-based high-strength steels requires constitutive models that reflect the local stress and strain fields as accurately as possible. At the same time, the constitutive models should act as efficiently as possible in order to achieve the required high number of cycles in a finite time. Only few research works have studied the sensitivity of the local cyclic deformation in lath martensite to the power law-based flow rule (Hutchinson or Chaboche-Cailletaud) and the active body-centred cubic (bcc) slip systems ({110}(111) and {112}(111)) in the crystal plasticity finite element method (CPFEM). This paper, therefore, aims to provide some guidance in the selection of suitable flow rule and slip systems. Based on full-field micromechanical modelling of a medium-carbon steel under symmetric strain-controlled cyclic loading, it can be shown that the two most commonly used flow rules according to Hutchinson and Chaboche-Cailletaud are equally capable of predicting the local stress and strain distributions within the hierarchical martensitic microstructure. However, using the Hutchinson flow rule increases the computational performance for the quasi-rate-independent problem considered here. The local distributions found differ strongly from those in the parent austenitic microstructure. If plastic deformation is assumed not only on the slip systems {110}(111), as often done, but also on the {112}(111) type, a redistribution of the bimodal distributed local stresses occurs at a significantly lower stress level. The unimodal distributed local strains are less affected by this. In addition, it is found that slightly different critical resolved shear stress (CRSS) values for both slip system types influence the local stress and strain distributions less severely than the additional plastic slip activation in the material.
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| 3. |
- 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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| 4. |
- Subasic, Mustafa, et al.
(författare)
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Mechanical Characterization of Fatigue and Cyclic Plasticity of 304L Stainless Steel at Elevated Temperature
- 2023
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Ingår i: Experimental mechanics. - : Springer Nature. - 0014-4851 .- 1741-2765. ; 63:8, s. 1391-1407
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Tidskriftsartikel (refereegranskat)abstract
- Background: The mechanical characterization of the cyclic elastoplastic response of structural materials at elevated temperatures is crucial for understanding and predicting the fatigue life of components in nuclear reactors. Objective: In this study, a comprehensive mechanical characterization of 304L stainless steel has been performed including metallography, tensile tests, fatigue tests, fatigue crack growth tests and cyclic stress-strain tests. Methods: Isothermal tests were conducted at both room temperature and 300 °C for both the rolling direction and the transverse direction of the hot rolled steel. Mechanical properties were extracted from the uniaxial experiments by fitting relevant material models to the data. The cyclic plasticity behavior has been modelled with a radial return-mapping algorithm that utilizes the Voce nonlinear isotropic hardening model in combination with the Armstrong-Frederick nonlinear kinematic hardening model. The plasticity models are available in commercial FE software and accurately capture the stabilized hysteresis loops, including a substantial Bauschinger effect. Results: The material exhibits near isotropic properties, but its mechanical performance is generally reduced at high temperatures. Specifically, in the rolling direction, the Young’s modulus is reduced by 16 % at 300 °C, the yield strength at 0.2 % plastic strain is lower by 23 %, and the ultimate tensile strength is lower by 30 % compared to room temperature. Fatigue life is also decreased, leading to an accelerated fatigue crack growth rate compared to room temperature. A von Mises radial return mapping algorithm proves to be effective in accurately modelling the cyclic plasticity of the material. The algorithm has also been used to establish a clear correlation between energy dissipation per cycle and cycles to failure, leading to the proposal of an energy-based fatigue life prediction model. Conclusions: The material exhibits reduced mechanical performance at elevated temperatures, with decreased monotonic strength, compared to room temperature. Fatigue life is also compromised, resulting in accelerated fatigue crack growth. The material’s hardening behavior differs at room temperature and elevated temperature, with lower peak stress values observed at higher temperatures. The radial return mapping algorithm can be used to determine the dissipated energy per cycle which together with fatigue testing has been used to propose a low cycle fatigue life prediction model at both temperatures.
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| 5. |
- Fischer, Tim, et al.
(författare)
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Creep-fatigue properties of austenitic cast iron D5S with tension and compression dwell : A dislocation density-based crystal plasticity study
- 2022
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Ingår i: Materials Science & Engineering. - : Elsevier BV. - 0921-5093 .- 1873-4936. ; 860, s. 144212-
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Tidskriftsartikel (refereegranskat)abstract
- To predict and better understand the creep-fatigue behaviour of austenitic cast iron D5S under tension and compression dwell at 800 degrees C, a physics-based crystal plasticity model that describes the complex rate-and temperature-dependent deformation of the material as a function of the dislocation density is implemented. In addition to the tension and compression dwell direction, the effect of three different dwell times (30, 180 and 600 s) on the creep-fatigue properties is investigated. The dislocation density-based crystal plasticity simulations are compared to experimental tests from a prior work. While relaxation tests and low-cycle fatigue (LCF) tests without dwell assist in systematically identifying the material parameters, creep-fatigue (CF) data is used to validate the predictions. The virtual testing is performed on a large-scale representation of the actual test specimen with a polycrystalline structure. To analyse the fatigue damage mechanism, small-scale predictions are also conducted using a micromechanical unit cell approach. Here, a single graphite nodule frequently found in the material is embedded into the austenitic matrix. In the present work, a close agreement is achieved between the predicted CF behaviour and the experimental results. Consistent with the experimental findings, the simulation results show that the addition of compression dwell leads to an uplift of the overall tensile stress level, which significantly reduces the fatigue life of the material. The unit cell studies demonstrate that during this uplift, a strong localisation of stresses and strains arises at the graphite/matrix interface, triggering the nucleation and growth of cavities and/or debonding.
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| 6. |
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| 7. |
- Skriver Hansen, Andreas, 1983, et al.
(författare)
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Synthesizing knowledge and evidence to inform the management of coastal seas’ biodiversity and NCPs – a case study from Sweden
- 2022
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Ingår i: Presentation given at ECSA 59, 5-8 September 2022, Kursaal, San Sebastian, Spain. - Essen : University of Duisburg Essen, Faculty of Biology, Aquatic Ecology.
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Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- While being among the most productive ecosystems on earth, coastal ecosystems are heavily impacted by anthropogenic causes on land and at sea. Fisheries, nutrient discharges from land uses and global climate change, for example, adversely affect coastal seas’ biodiversity, functioning and related nature’s contributions to people (NCPs). Data and knowledge of particular cause-effect relationships in these complex systems is often insufficient or missing. Therefore, a coupled model framework was developed that allows the combination of evidence from environmental monitoring data, literature, expert knowledge and stakeholder interviews and thereby enables the exploration of causal relations between human-induced environmental change, biodiversity and NCPs. The framework was applied to the Kosterhavet National Park in Sweden. First, a Bayesian Belief Network (BBN) was built to link anthropogenic land and sea uses and climate change via abiotic and biotic cause-and-effect relationships to two ecological key species in the national park: eelgrass (Zostera marina) and northern shrimp (Pandalus borealis). Subsequently, social-cultural and social-economic cause-and-consequence chains were developed, to derive the potential implications of changes in eelgrass coverage and shrimp stock on human well-being. It was shown that the sustainable management of the two target species in Kosterhavet National Park requires local management in the park (e.g. recreational use and fishery) and regional management across coastal water bodies (e.g. nutrient management). Significant declines in eelgrass and northern shrimp were found to impact the people’s place attachment, tradition, knowledge and identity more than their regional economy. The coupled model framework is currently implemented as an online tool to facilitate its application and support decision making.
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| 8. |
- Van Tran, Khanh, et al.
(författare)
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Torsion of a rectangular bar : Complex phase distribution in 304L steel revealed by neutron tomography
- 2022
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Ingår i: Materials & design. - : Elsevier BV. - 0264-1275 .- 1873-4197. ; 222
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Tidskriftsartikel (refereegranskat)abstract
- Metastable austenitic stainless steel (304L) samples with a rectangular cross-section were plastically deformed in torsion during which they experienced multiaxial stresses that led to a complex martensitic phase distribution owing to the transformation induced plasticity effect. A three-dimensional character-ization of the phase distributions in these cm-sized samples was carried out by wavelength-selective neutron tomography. It was found that quantitatively correct results are obtained as long as the samples do not exhibit any considerable preferential grain orientation. Optical microscopy, electron backscatter diffraction, and finite element modeling were used to verify and explain the results obtained by neutron tomography. Altogether, neutron tomography was shown to extend the range of microstructure charac-terization methods towards the meso-and macroscale.
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| 9. |
- 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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| 10. |
- Brandberg, August, 1990-
(författare)
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Micro-mechanical characterization and modeling of paper and paperboard
- 2021
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Fiber networks made of cellulose fibers from trees are used as information carriers (paper) and as packaging (paperboard). This thesis investigates the mechanical performance of paper and paperboard via micro-mechanical modeling and presents new methods for the mechanical characterization of the micro scale, necessary in such models. In Paper A the effect of the fiber-fiber bond geometry on the sheet stiffness is investigated. In thick, low density sheets, the fiber lumen remains open resulting in a more compliant bonded segment. By finite element simulations, we demonstrate the effect of the lumen configuration on the stiffness of the bonded segment. Most important for the stiffness of the segment is the average state of the fiber lumen which has a marked effect on the macroscopic response of fiber networks when the network is sparse. Compression strength is central in many industrial applications. In Paper B we recreated the short span compression test in a simulation setting. The networks considered are three-dimensional and have a grammage of 80--400 gm^-2. By modeling compression strength at the level of individual fibers and bonds, we show that widespread fiber level buckling is unlikely to appear at the loads at which the macroscopic sheet fails. In Paper C we develop a micro-mechanical model to study the creation of curl in paper sheets subjected to a moisture gradient through the thickness of a sheet. A moisture gradient is created during the printing process if the ink is water based, which may lead to out-of-plane deformations (curl). The effect of transverse fiber shrinkage is captured using a multiscale model where the fiber-fiber bond is modeled with volume elements. We show how the swelling anisotropy of individual fibers contributes to the curl of the sheet in such settings. In Paper D we present how to uniquely and compactly describe the distribution of fiber shapes (length, width, wall thickness, curl) used in network simulations. Using a canonical vine structure, fiber shapes measured using an optical image analyzer are used to construct a multivariate distribution function. New fiber geometries can then be generated by sampling from this distribution. Having access to such a complete description with both the distribution of fiber properties and the dependence between properties is shown to be superior to previously presented methods using micro-mechanical simulations of thermo-mechanical (TMP) long fiber sheets. In Paper E we compare sheet testing, micro-mechanical tensile testing, and nanoindentation as methods to extract the elastic material properties of individual pulp fibers. Nanoindentations are performed parallel to and orthogonal to the axis of the fiber after it has gone through all steps of papermaking, and indentation moduli are extracted. By relating the indentation modulus to the components of the anisotropic stiffness tensor, the longitudinal and transverse elastic modulus can be determined via an iterative error minimization scheme. We show that nanoindentation is an alternative to traditional methods with the advantage of yielding the transverse modulus and enabling measurement of the fiber properties after papermaking.
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