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See detailMultiscale Finite Element Modeling of Nonlinear Quasistatic Electromagnetic Problems
Niyonzima, Innocent ULg

Doctoral thesis (2014)

The effective use of composite materials in the technology industry requires the development of accurate models. Typical such materials in electrotechnical applications are lamination stacks and soft ... [more ▼]

The effective use of composite materials in the technology industry requires the development of accurate models. Typical such materials in electrotechnical applications are lamination stacks and soft magnetic composites, used in the so-called magnetoquasistatic (low frequency) regime. Current homogenization models (e.g. the classical homogenization method, mean field homogenization, ...) fail to handle all the difficulties raised by the modeling of these materials, particularly taking into account the complexity of their microstructure and their nonlinear/hysteretic behaviour. In this thesis we develop a multiscale computational method which allows to effectively solve multiscale magnetoquasistatic problems. The technique is inspired by the HMM (heterogeneous multiscale method), which involves the resolution of two types of problems: a macroscale problem that captures slow variations of the overall solution, and many mesoscale problems that allow to determine the constitutive laws at the macroscale and to construct accurate local fields. Macroscale and mesoscale weak, b-conform and h-conform formulations, are derived starting from the two-scale convergence and the periodic unfolding methods. We also use the asymptotic homogenization method for deriving the homogenized linear material laws and, in the end, we derive scale transitions for bridging the scales. Numerical tests carried out in the two-dimensional case allow to validate the models. In the case of b-conform formulations, it is shown that the macroscale solution approximates well the average of the reference solution and that the resolution of the mesoscale problems allows to reconstruct accurate local fields and to compute accurate Joule losses and this, for materials with (non)linear and hysteretic behavior. Similar findings were obtained for the h-conform formulations. In both cases, the deterioration of the accuracy for mesoscale problems located near the boundary of the computational domain could be treated by defining suit- able mesoscale problems near such boundaries. The extension of the model to three-dimensional problems, to multiphysical problems and the inclusion of the mesoscale domains with a stochastic distribution of phases are also some of the possible prospects for improving this work. [less ▲]

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See detailMultiscale finite element study of Ti-555
Habraken, Anne ULg; Gerday, Anne-Françoise ULg; Duchene, Laurent ULg

in Akhtar S. Khan; Babak Farrokh; Baig, M. (Eds.) et al Proceedings of the 14th International Symposium on Plasticity & Its Current Applications, Mechanics & Mechanisms of Finite Plastic Deformation (2008)

The material parameters of the 2 phases Ti-555 alloy have to be extracted in order to perform simulations on representative microscopic cells and guide the optimization of the alloy. The identification of ... [more ▼]

The material parameters of the 2 phases Ti-555 alloy have to be extracted in order to perform simulations on representative microscopic cells and guide the optimization of the alloy. The identification of the flow parameters of the BCC β-phase is discussed, using macroscopic and microscopic constitutive laws. The validation is performed, comparing experimental and numerical tensile tests obtained for different strain rates. [less ▲]

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See detailMultiscale image analysis of microcellular solids: application to hybrid silica xerogels
Gommes, Cédric ULg; Pirard, Jean-Paul ULg; Blacher, Silvia ULg

in Journal of Microscopy (2007), 226

A general methodology is proposed to characterize microcellular solids, the structure of which consists of a three-dimensional network of filamentary structures. The analysis is based on transmission ... [more ▼]

A general methodology is proposed to characterize microcellular solids, the structure of which consists of a three-dimensional network of filamentary structures. The analysis is based on transmission electron microscopy observation of the filaments individually and of their spatial arrangement. The micrographs are analyzed with greytone digital image analysis techniques, such as opening granulometry and correlation analysis. The methodology is applied to hybrid organic–inorganic low-density silica solids synthesized by the sol–gel method with an organically modified co-reactant. The quantitative impact of the coreactant on each structural level of the structure is assessed quantitatively. [less ▲]

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See detailA multiscale mean-field homogenization method for fiber-reinforced composites with gradient-enhanced damage models
Wu, Ling ULg; Noels, Ludovic ULg; Adam, L et al

in Computer Methods in Applied Mechanics & Engineering (2012), 233-236

In this work, a gradient-enhanced homogenization procedure is proposed for fiber reinforced materials. In this approach, the fiber is assumed to remain linear elastic while the matrix material is modeled ... [more ▼]

In this work, a gradient-enhanced homogenization procedure is proposed for fiber reinforced materials. In this approach, the fiber is assumed to remain linear elastic while the matrix material is modeled as elasto-plastic coupled with a damage law described by a non-local constitutive model. Toward this end, the mean-field homogenization is based on the knowledge of the macroscopic deformation tensors, internal variables and their gradients, which are applied to a micro-structural representative volume element (RVE). The macro-stress is then obtained from a homogenization procedure. The methodology holds for 2-phase composites with moderate fiber volume ratios, and for which, at the RVE size, the matrix can be considered as homogeneous isotropic and the ellipsoidal fibers can be considered as homogeneous transversely isotropic. Under these assumptions, the method is successfully applied to simulate the damage process occurring in unidirectional carbon-fiber reinforced epoxy composites submitted to different loading conditions. [less ▲]

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See detailMultiscale mesh generation on the sphere
Lambrechts, J.; Comblen, R.; Legat, V. et al

in Ocean Dynamics (2008), 58

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See detailA multiscale method for a robust detection of the default mode network.
Baquero, Katherine; Gómez, Francisco; Cifuentes, Christian et al

Poster (2013)

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See detailA multiscale model of sprouting angiogenesis during fracture healing.
Carlier, Aurélie ULg; Geris, Liesbet ULg; Van Oosterwyck, Hans

Conference (2012, September 18)

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See detailA multiscale model of the influence of oxygen during bone fracture healing.
Carlier, Aurélie ULg; Geris, Liesbet ULg; Van Oosterwyck, Hans

Poster (2013, April 03)

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See detailMultiscale modeling of back-stress evolution in equal-channel angular pressing: from one pass to multiple passes
Chen, E.; Duchene, Laurent ULg; Habraken, Anne ULg et al

in Journal of Materials Science (2010), 45(17), 4696-4704

Fine-grained materials produced by equalchannel angular pressing (ECAP) exhibit kinematic hardening due to the existence of a back-stress. This article presents a new dislocation-based model, which is ... [more ▼]

Fine-grained materials produced by equalchannel angular pressing (ECAP) exhibit kinematic hardening due to the existence of a back-stress. This article presents a new dislocation-based model, which is able to describe the tension/compression asymmetry of the ECAP processed commercial purity aluminum. By introducing strain relaxation, and relating the back-stress to the inhomogeneous dislocation density distribution in cell walls and in cell interiors, the model can accurately predict the evolution of the dislocation densities, the cell size, and the back-stress. Compared to the other back-stress models, it takes into account the microstructure evolution and gives a better prediction. [less ▲]

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See detailMultiscale modeling of equal channel angular extruded aluminium with strain gradient crystal plasticity and phenomenological models
Duchene, Laurent ULg; Geers, M. G. D.; Brekelmans, W. A. M. et al

in Rollett, A. (Ed.) Proceedings of the 15th International Conference on the Texture of Materials (ICOTOM-15) (2008)

The Equal Channel Angular Extrusion process is used to modify the microstructure of an AA1050 aluminum alloy in order to produce an ultra fine grained material. Due to the severe plastic deformation ... [more ▼]

The Equal Channel Angular Extrusion process is used to modify the microstructure of an AA1050 aluminum alloy in order to produce an ultra fine grained material. Due to the severe plastic deformation undergone by the material during the ECAE process, the subsequent behavior of the material is non-conventional and difficult to model with classical constitutive laws (e.g. ECAE aluminum presents a large initial back-stress which must be adequately incorporated in the model). In this study, the evolution of the back-stress during the ECAE process is analyzed. Two different numerical models were investigated in this respect. The first one is a single crystal strain gradient plasticity model based on dislocation densities. The second model is the Teodosiu and Hu’s hardening model, which is a microstructuraly based phenomenological model at the macroscale. The results provided by the two models are obviously distinct. Nevertheless, some common trends can be pointed out, among which the amplitude of the back-stress that is similar. In agreement with the cyclic deformation mode of the studied route C ECAE process, the evolution of the predicted back-stress is also cyclic in both models. [less ▲]

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See detailMultiscale modeling of in the influence of oxygen during bone fracture healing.
Carlier, Aurélie ULg; Van Gastel, Nick; Carmeliet, Geert et al

Poster (2012, September 17)

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See detailMultiscale modeling of sprouting angiogenesis
Carlier, Aurélie ULg; Geris, Liesbet ULg; Van Oosterwyck, Hans

Poster (2011, December 02)

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See detailMultiscale modeling of sprouting angiogenesis: tip cells are selected for the top.
Carlier, Aurélie ULg; Geris, Liesbet ULg; Van Oosterwyck, Hans

Poster (2012, September 05)

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See detailMultiscale modelling framework for the fracture of thin brittle polycrystalline films - Application to polysilicon
Mulay, Shantanu; Becker, Gauthier ULg; Vayrette, Renaud et al

in Computational Mechanics (2014)

Micro-electro-mechanical systems (MEMS) made of polycrystalline silicon are widely used in several engineering fields. The fracture properties of polycrystalline silicon directly affect their reliability ... [more ▼]

Micro-electro-mechanical systems (MEMS) made of polycrystalline silicon are widely used in several engineering fields. The fracture properties of polycrystalline silicon directly affect their reliability. The effect of the orientation of grains on the fracture behaviour of polycrystalline silicon is investigated out of the several factors. This is achieved, firstly, by identifying the statistical variation of the fracture strength and critical strain energy release rate, at the nanoscopic scale, over a thin freestanding polycrystalline silicon film, having mesoscopic scale dimensions. The fracture stress and strain at the mesoscopic level are found to be closely matching with uniaxial tension experimental results. Secondly, the polycrystalline silicon film is considered at the continuum MEMS scale, and its fracture behaviour is studied by incorporating the nanoscopic scale effect of grain orientation. The entire modelling and simulation of the thin film is achieved by combining the discontinuous Galerkin method and extrinsic cohesive law describing the fracture process. [less ▲]

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See detailMultiscale modelling of angiogenesis during normal and impaired bone regeneration
Carlier, Aurélie ULg

Doctoral thesis (2014)

Bone regeneration is, like many other healing events, a complex, well-orchestrated process involving a myriad of different cell types and regulated by countless biochemical, physical and mechanical ... [more ▼]

Bone regeneration is, like many other healing events, a complex, well-orchestrated process involving a myriad of different cell types and regulated by countless biochemical, physical and mechanical factors. But unlike other adult biological tissues, the majority of bone fractures can heal without the production of scar tissue, eventually recovering the original bone shape, size and strength. Despite bone’s remarkable healing capacity and the continuing research efforts, the impaired healing of complex orthopaedic cases is still not fully understood. This PhD work hypothesises that computational modelling can make a substantial contribution to the bone regeneration field by proposing and testing the underlying mechanisms of action as well as by designing and optimising experimental strategies in silico. In the first part of this work, an existing bioregulatory model of fracture healing is extended with an intracellular module of Dll4-Notch1 signalling in order to capture the ingrowth of new blood vessels through sprouting angiogenesis. The predictions of the new MOSAIC model are compared to experimental results and an extensive sensitivity analysis is performed on the newly introduced parameters. The potential of the MOSAIC model to investigate the influence of the molecular mechanisms on angiogenesis and consequently the bone formation process is illustrated. In the second part of this work, the MOSAIC model is further improved with a rigorous implementation of the influence of oxygen on the behaviour of skeletal cells. A comprehensive literature study is performed in order to ensure the correspondence of the oxygen ranges of the cell-specific oxygen-dependent processes with the state-of-the-art experimental knowledge. The oxygen model is corroborated with previously published experimental results. The robustness of the oxygen model with respect to the newly introduced oxygen thresholds is demonstrated by a sensitivity analysis. Some limitations and shortcomings of the oxygen model are identified together with suggestions for future work. In the last part of this work, the added value of the oxygen model is shown by applying it to three cases of impaired bone healing: the occurrence of nonunions in critical size defects, bone graft healing in a compromised environment and the impaired healing of bone fractures in NF1 patients. Not only is the oxygen model used to determine the underlying mechanisms of action, potential treatment strategies for the respective challenging orthopaedic conditions are also designed and optimised in silico. In conclusion, this PhD thesis demonstrates the potential of an integrative in vivo-in silico approach to advance our current understanding of bone regeneration as well as to design effective treatments of complex bone fractures. [less ▲]

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See detailMultiscale modelling of back-stress during equal-channel angular pressing
Chen, E.; Duchene, Laurent ULg; Habraken, Anne ULg et al

in Reviews of advanced materials science (2010), 25(1), 23-31

Equal-channel angular pressing (ECAP) is a well known process to produce ultrafinegrained materials. The mechanical properties of these materials, including a compressiontension asymmetry and a transient ... [more ▼]

Equal-channel angular pressing (ECAP) is a well known process to produce ultrafinegrained materials. The mechanical properties of these materials, including a compressiontension asymmetry and a transient hardening saturation in the beginning of the flow curve, largely depend on the evolution of the microstructure during ECAP. Consequently, the backstress induced by the dislocation microstructure exhibits kinematic hardening at the macroscopic scale. In this paper, commercial purity aluminium AA1050 is processed by ECAP route C. Tensile and compression specimens are machined from the post-ECAP samples. The back-stress level is estimated from the different yielding strengths of tensile tests and compression tests. Then two different models, a macroscopic phenomenological Teodosiu-type model and a microscopic dislocation-based multi-layer model, are used to predict the back-stress values. A set of parameters for Teodosiu's model is identified from simple shear tests, Bauschinger tests and orthogonal orthogonal tests. The dislocation-based multi-layer model is based on the Estrin-Tóth dislocation model and Sauzay's intragranular back-stress model. The predicted and experimental back-stresses due to ECAP are compared and critically evaluated. [less ▲]

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See detailMultiscale modelling of cohesive-frictional materials: from textured to random materials
Massart, T.J.; Sonon, B.; Mercatoris, Benoît ULg et al

Conference (2012)

Detailed reference viewed: 16 (1 ULg)
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See detailMultiscale modelling of the influence of VEGF on sprouting angiogenesis.
Carlier, Aurélie ULg; Geris, Liesbet ULg; Van Oosterwyck, Hans

Poster (2012, July 06)

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See detailMultiscale Quasistatic Homogenization for Laminated Ferromagnetic Cores
Niyonzima, Innocent ULg; Vazquez Sabariego, Ruth ULg; Dular, Patrick ULg et al

in Proceedings of the 7th European Conference on Numerical Methods in Electromagnetism (NUMELEC 2012) (2012, July 03)

In this paper, we investigate the modeling of ferromagnetic multiscale materials. We propose a computational homogenization method based on the heterogeneous multiscale method (HMM) with inclusion of a ... [more ▼]

In this paper, we investigate the modeling of ferromagnetic multiscale materials. We propose a computational homogenization method based on the heterogeneous multiscale method (HMM) with inclusion of a hysteresis model. The HMM involves: 1) a macroscale problem that captures the slow variations of the overall solution; 2) many microscale problems that allow to determine the constitutive law at the macroscale. At the microscale, a novel energy consistent hystere- sis model is incorporated. As application example, a laminated iron core is considered. [less ▲]

Detailed reference viewed: 92 (25 ULg)