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- Carlsson, Jenny, 1984-
(författare)
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On Crack Dynamics in Brittle Heterogeneous Materials
- 2020
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Natural variation, sub-structural features, heterogeneity and porosity make fracture modelling of wood and many other heterogeneous and cellular materials challenging. In this thesis, fracture in such complicated materials is simulated using phase field methods for fracture. Phase field methods have shown promise in simulations of complex geometries as well as dynamics and require few additional parameters; only the material toughness and a length parameter, determining the width of a regularised crack, are needed.First, a dynamic phase field model is developed and validated against experiments performed on homogeneous brittle polymeric materials, wood fibre composites and polymeric materials with different hole patterns. Then, a high-resolution model of wood is developed and related to experiments, this time without considering fracture. Attention is finally focussed on high-resolution numerical analyses of fracture in wood and other cellular microstructures, considering both heterogeneity and relative density.The phase field model is found to reproduce crack paths, velocities and energy release rates well in homogeneous samples both with and without holes. In more complicated heterogeneous and porous materials, the model is also able to simulate crack paths, but the interpretation of the length scale is complicated by the inherent lengths of the micro-structural geometry. In sum, the thesis points to possibilities with the proposed method, as well as limitations in our current understanding of both quasistatic and dynamic fracture of heterogeneous and cellular materials. The findings of this thesis can contribute to an improved understanding of fracture in such materials.
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| 2. |
- Hassila, Carl Johan
(författare)
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Additive Manufacturing of Ni-Fe Superalloys : Exploring the Alloying Envelope and the Impact of Process on Mechanical Properties
- 2022
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Additive manufacturing of metals has received a lot of attention in the last decade as this family of manufacturing processes allows the manufacturing of complicated geometries which would be difficult to produce using conventional manufacturing techniques. Additive manufacturing of the Ni-Fe based superalloys 625 and 718 using the Powder Bed Fusion – Laser Beam (PBF-LB) process is facilitated by the fact that these alloys were developed as weldable alternatives to other high-strength, high-temperature Ni-based superalloys. However, given that these alloys were developed with casting and forging as the main manufacturing route, the alloying composition of these alloys may possibly be tuned to better suit the PBF-LB process. In this thesis, small changes to the alloy 625 and 718 alloy compositions were made, with the goal of either improving material properties or reducing the environmental footprint of the produced materials. For alloy 718, the influence of carbon content on the resulting microstructure and mechanical properties was investigated both in the as-built and heat-treated conditions using tensile and impact testing. A similar study, but also including corrosion experiments, was performed on an alloy 625 composition which had been tuned to allow it to be atomized using nitrogen instead of argon, a transition that results in environmental benefits as argon gas carries with it a larger environmental footprint compared to nitrogen gas. In addition to the above, as the process conditions in the PBF-LB process have a strong influence on the developing microstructure, their influence on rolling contact fatigue and residual stresses in printed alloys 625 and 718 were investigated. Rolling contact fatigue experiments were performed on alloy 625 and were complemented by a fractographic study which showed that the different grain structures achieved depending on the used process condition affected the pitting damage development. Meanwhile, the residual stress experiments were performed on PBF-LB processed alloy 625 and 718. The residual stresses in the materials were first calculated using experimental data attained from high energy synchrotron diffraction experiments. These results were then compared to the predicted stresses from a thermo-mechanical model. The thermomechanical model included a built-in mechanism-based material model which was shown to successfully simulate relaxation effects stemming from the cyclic heating of the material during the PBF-LB process. Lastly, a modelling approach using the thermo-mechanical model was developed which allowed the model to successfully predict the stresses also when using different scanning strategies.
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