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Peer Reviewed
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 Electrical Energy Systems
Plumier, Frédéric ULg; Paquay, Yannick ULg

Scientific conference (2015)

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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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Peer Reviewed
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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Peer Reviewed
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 (2015), 55(1), 73-91

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)

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See detailMultiscale modelling of the hydromechanical behaviour of argillaceous rocks
van den Eijnden, Abraham Pieter ULg

Doctoral thesis (2015)

Feasibility studies for deep geological radioactive waste disposal facilities have led to an increased interest in the geomechanical modelling of its host rock. In France, a potential host rock is the ... [more ▼]

Feasibility studies for deep geological radioactive waste disposal facilities have led to an increased interest in the geomechanical modelling of its host rock. In France, a potential host rock is the Callovo-Oxfordian claystone. The low permeability of this material is of key importance, as the principle of deep geological disposal strongly relies on the sealing capacity of the host formation. The permeability being coupled to the mechanical material state, hydromechanical coupled behaviour of the claystone becomes important when mechanical alterations are induced by gallery excavation in the so-called excavation damaged zone (EDZ). In materials with microstructure such as the Callovo-Oxfordian claystone [Robinet et al., 2012], the macroscopic behaviour has its origin in the interaction of its micromechanical constituents. In addition to the coupling between hydraulic and mechanical behaviour, a coupling between the micro (material microstructure) and macro scale will be made. By means of the development of a framework of computational homogenization for hydromechanical coupling, a doublescale modelling approach is formulated, for which the macroscale constitutive relations are derived from the microscale by homogenization. An existing model for the modelling of hydromechanical coupling based on the distinct de nition of grains and intergranular pore space [Frey, 2010] is adopted and modi ed to enable the application of rst order computational homogenization for obtaining macroscale stress and uid transport responses. This model is used to constitute a periodic representative elementary volume (REV) that allows the representation of the local macroscopic behaviour of the claystone. As a response to deformation loading, the behaviour of the REV represents the numerical equivalent of a constitutive relation at the macroscale. For the required consistent tangent operators, the framework of computational homogenization by static condensation [Kouznetsova et al., 2001] is extended to hydromechanical coupling. The theoretical developments of this extension are implemented in the nite element code Lagamine (Li ege) as an independent constitutive relation. For the modelling of localization of deformation, which in classical FE methods su ers from the well-known mesh dependency, the doublescale approach of hydromechanical coupling is combined with a local second gradient model [Collin et al., 2006] to control the internal length scale of localized deformation. By accepting the periodic boundary conditions as a regularization of the microscale deformation, the use of the multiscale model in combination with the local second gradient model can be used for modelling localization phenomena in HM-coupled settings with material softening. The modelling capacities of the approach are demonstrated by means of simulations of oedometer tests and biaxial compression tests. The approach is demonstrated to be a powerful way to model anisotropy in the mechanical as well as the hydraulic behaviour of the material both in the initial material state and as an e ect of hydromechanical alterations. For the application to the modelling of Callovo-Oxfordian claystone, microstructural REVs are calibrated to geometrical characteristics of the inclusion that form the microstructure under consideration and to macroscale experimental results of the mechanical behaviour. The calibrated constitutive relation is used in the simulation of gallery excavation processes. These computations give a proof of concept of the doublescale assessment of the hydromechanical behaviour of the excavation damaged zones around galleries in the context of nuclear waste disposal. [less ▲]

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Peer Reviewed
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 ▲]

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See detailMultiscale Simulations of Composites with Non-Local Damage-Enhanced Mean-Field Homogenization
Wu, Ling ULg; Noels, Ludovic ULg; Adam, Laurent et al

Conference (2012, July)

The mean-field homogenization (MFH) approach is an attractive framework for multiscale methods, as it provides predictions of the macroscopic behavior of particle or fiber reinforced composites at a ... [more ▼]

The mean-field homogenization (MFH) approach is an attractive framework for multiscale methods, as it provides predictions of the macroscopic behavior of particle or fiber reinforced composites at a reasonable computational cost. Efficient MFH methods have been available for a long time for linear elastic problems, using for example the Mori-Tanaka scheme [2], but they can also be extended in the non-linear regime after linearization of the constitutive behavior at the current strain state, as for the incremental approach, e.g. [1]. In this work, the application of ductile-damage theories to a multiscale analysis of continuous fiber reinforced composites is considered. Toward this end, the incremental MFH approach is extended to account for the damage behavior happening in the matrix material at the microscale and to derive the effective properties of particle or fiber reinforced composites. However, capturing the degradation, damage or failure of material happening at the microscopic scale could lead to loss of uniqueness in the solution as the governing partial differential equations may lose ellipticity at a given level of loading corresponding to the strain-softening onset. Thus, in order to avoid the strain/damage localization caused by matrix material softening, the gradient-enhanced formulation [3] is adopted to describe the material behavior of the matrix during the homogenization process, as we have recently proposed [4]. As illustration, the behavior of a fiber re-enforced elasto-plastic matrix is considered. The properties of the matrix correspond to an elasto-plastic material experiencing damage, with a non-local form of Lemaitre Chaboche model. The fibers are assumed linear elastic, see [4] for details. A loading-unloading cycle is applied in the direction transverse to the fibers. A maximal deformation of 10 % is reached before the unloading proceeds to zero-transverse deformation. The effective behavior predicted by the MFH models is compared to the solutions obtained by finite element computations performed on a unit periodic cell and on RVE where the micro-structure is fully meshed. The results for three fiber volume ratios are presented in Fig. 1. For the three fiber volume ratios, the homogenized property is dominated by the properties of the matrix, with an obvious elasto-plastic behavior exhibiting softening. For vI = 15% and 30%, rather good predictions are given by the MFH model, with, as expected, higher macroscopic stress and damage predicted by the MFH due to the incremental formulation. However for vI = 50%, the MFH model overestimates the macroscopic stress considerably. This error comes from the assumption of Mori - Tanaka based MFH. As it is shown to be an efficient multi-scale approach, the developed gradient enhanced MFH formulation presented can now be used to model the behavior of composite laminates experiencing damage. [less ▲]

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See detailMultiscale structure characterization of sawdust-waste water sludge extrudates dried in a pilot-scale fixed bed
Li, Jie ULg; Plougonven, Erwan ULg; Fraikin, Laurent ULg et al

in Biomass and Bioenergy (2015), 81(0), 98-107

Abstract Convective drying of waste water sludge and sawdust-sludge mixtures in a pilot-scale fixed bed was studied. Drying was performed in a cross-flow convective dryer using 500 g of wet material ... [more ▼]

Abstract Convective drying of waste water sludge and sawdust-sludge mixtures in a pilot-scale fixed bed was studied. Drying was performed in a cross-flow convective dryer using 500 g of wet material extruded through a disk with circular dies 12 mm in diameter. The structure of the bed mainly shows volume shrinkage and crack formation during drying. Several characterization techniques were used over a wide range of scales from nm up to mm. The overall bed structure was imaged with X-ray macrotomography, at a resolution of 0.36 mm per pixel. Single extrudates were scanned with X-ray microtomography, at a resolution of 41 μm per pixel. Pore structure of the dried samples were characterized by mercury porosimetry (7.5 nm < dp < 150 μm). Results show significant structural changes on all scales with increasing amounts of sawdust: shrinkage decreases, crack formation increases, and the pores become larger. This confirms the benefits of sawdust addition for sludge drying applications. [less ▲]

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See detailMultiscale texture characterization of wastewater sludges dried in a convective rig
Léonard, Angélique ULg; Blacher, Silvia ULg; Pirard, René ULg et al

in Drying Technology (2003), 21(8), 1507-1526

Secondary sludges from two different wastewater treatment plants are conditioned and dewatered in the same way before drying in a lab-scale convective rig at the same operating conditions. Several ... [more ▼]

Secondary sludges from two different wastewater treatment plants are conditioned and dewatered in the same way before drying in a lab-scale convective rig at the same operating conditions. Several techniques are used to characterize the texture of the dried materials over a wide range of scales from nm up to mm. Texture and porosity of the dried products are studied by SEM imaging, nitrogen absorption isotherms (0.5 < d(p) < 50 nm), mercury porosimetry (7.5 nm < d(p) < 150 mum) and X-ray microtomography (spatial resolution= 41 mum). The image analysis of cross-sections reconstructed by microtomography also allows following shrinkage and textural evolution during drying. [less ▲]

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See detailMultiscale variability of amphipod assemblages in Posidonia oceanica meadows
Sturaro, Nicolas ULg; Lepoint, Gilles ULg; Vermeulen, Simon et al

in Journal of Sea Research (2015), 95

The study of spatial patterns is of ecological importance in order to understand the causes of the distribution and abundance of organisms, and it also provides valuable basis for management and ... [more ▼]

The study of spatial patterns is of ecological importance in order to understand the causes of the distribution and abundance of organisms, and it also provides valuable basis for management and conservation. Amphipod crustaceans are key organisms in seagrass ecosystems. However, little attention has been paid to the spatial scales at which amphipod assemblages may vary. We examined variability patterns of amphipod populations inhabiting Posidonia oceanica meadows, over spatial scales spanning four orders of magnitude (1 to 1000 metres) and for two consecutive years. This study reports the scales that contributed most to spatial variation of amphipod assemblages and explores the potential processes of the observed patterns, with particular emphasis on habitat features. The number of species, the diversity and the density of some species, exhibited high variation across years. Most species showed the highest spatial variation in density and biomass at small scales (~1 and 10 m). Based on density data, the structure of amphipod assemblages did not differ at any scales investigated. The patchiness that occurred at small scales may have been only weakly related to habitat features. Instead, we postulated that behavioural processes of amphipods were likely good explanatory factors. Although, the small scale spatial variability can be an important feature of amphipod assemblages in P. oceanica meadows, many patterns probably remained undetected as they may occur at scales smaller than those investigated. [less ▲]

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