For the last 10 years, silicone-based admixtures have been successfully used for bulk waterproofing treatment of cementitious materials. However, a reduction in mechanical properties of treated materials is rather observed and becomes a major problem for the in-situ application. A new concept of a knowledge-based integral water repellent has been designed in such a way that the negative effect on mechanical properties is significantly reduced. The technology comprises the delayed release of the hydrophobic agent (silicone resin) which is achieved by encapsulation of the resin in SiO2 shell. A multidisciplinary research was conducted in order to propose a model of the delayed release and the silica shell reaction mechanism in cement paste. Therefore, a study on the microcapsules reaction in calcium hydroxide solution was conducted by means of FTIR, DSC-TGA, surface tension measurements and chemical analysis by ICP-OES. It was shown that microcapsules flocculate in presence of Ca2+, what appears to be the main factor that contribute to the delayed release of the resin.The influence of the microcapsules on Ordinary Portland (OPC) and Blast furnace slag (BFS) cement hydration process was compared with the emulsion of silane monomer and silicone resin. It was shown that the emulsion delays the setting and influences the early age hydration by prolonging the dormant period and decreasing the hydration heat. Cement microstructure and hydration products development was observed by SEM/ESEM. Quantitative analysis of hydration products was assessed by Rietveld analysis of XRD diffractograms. Emulsion induced a significant delay in the cement paste setting by changing the amount and morphology of ettringite and portlandite at very early age. Differently, microcapsules didn’t show any effect on these properties. Experimental work on relevant mortars is done in order to prove the concept of a delayed release as a solution towards the decrease in mechanical properties. The influence of the new additive on setting, volume change, dynamic E-modulus and the compressive strength was analysed. New analytical techniques (AutoShrink, Ultrasonic Pulse Velocity and ConSensor) in combination with traditional ones (Penetration resistance test and compressive strength on cubes) were used. Microcapsules have successfully lowered the negative impact of silanes on the compressive strength and dynamic Young’s modulus. Moreover, it was showed that the microcapsules slightly influence autogenous deformation by increasing the shrinkage of mortars.Microencapsulation of the silicone resin proved to be a promising solution for the bulk hydrophobic treatment of cementitious materials with no-influence on cement hydration. / Doctorat en Sciences de l'ingénieur et technologie / info:eu-repo/semantics/nonPublished

La fissuration d’éléments minces en béton dès le jeune âge correspond à une réalité observée sur des ouvrages en construction. Cette fissuration concerne des bétons courants de bâtiments, pour lesquels les matériaux de qualité optimale ne sont pas toujours disponibles, en particulier au niveau des granulats. Le but de cette thèse est de définir une approche expérimentale d’étude de ces phénomènes et de proposer des interprétations pour les sensibilités relatives à la fissuration en fonction de la nature des granulats et de leurs taux de saturation initiaux. La sensibilité des moyens de mesures existants conçus pour des bétons spéciaux (BAP et BHP) a d’abord dû être<p>vérifiée. Un nouveau dispositif expérimental a été développé dans le but de mieux décrire l’évolution de la résistance et de la capacité de<p>déformation en traction du béton jeune. Les indicateurs globaux (macroscopiques) apparaissent plus sensibles que les indicateurs de la microstructure pour rendre compte des différences de comportement observées. L’évolution du module élastique, du retrait plastique et endogène corrélées à l’évolution de la capacité de déformation et de la résistance en traction permettent de mieux caractériser le risque<p>potentiel de fissuration par retrait empêché. La fin de prise correspond à une phase critique. L’influence de la saturation des granulats est<p>indirecte, et résulte des variations du rapport Eau d’ajout/Ciment, à rapport Eau efficace/Ciment constant. La nature des granulats intervient au niveau des évolutions relatives de la résistance en traction et du module élastique. La méthodologie peut être appliquée à l’étude d’autres types de bétons et d’autres paramètres de formulation. / Doctorat en Sciences de l'ingénieur / info:eu-repo/semantics/nonPublished

The macroscopic behavior of complex heterogeneous materials is strongly governed by the interactions between their elementary constituents within their microstructure. Beside experimental efforts characterizing the behaviors of such materials, there is growing interest, in view of the increasing computational power available, in building models representing their microstructural systems integrating the elementary behaviors of their constituents and their geometrical organization. While a large number of contributions on this aspect focus on the investigation of advanced physics in material parameter studies using rather simple geometries to represent the spatial organization of heterogeneities, few are dedicated to the exploration of the role of microstructural geometries by means of morphological parameter studies.<p>The critical ingredients of this second type of investigation are (I) the generation of sets of representative volume elements ( RVE ) describing the geometry of microstructures with a satisfying control on the morphology relevant to the material of interest and (II) the discretization of governing equations of a model representing the investigated physics on those RVEs domains. One possible reason for the under-representation of morphologically detailed RVEs in the related literature may be related to several issues associated with the geometrical complexity of the microstructures of considered materials in both of these steps. Based on this hypothesis, this work is aimed at bringing contributions to advanced techniques for the generation and discretization of microstructures of complex heterogeneous materials, focusing on geometrical issues. In particular, a special emphasis is put on the consistent geometrical representation of RVEs across generation and discretization methodologies and the accommodation of a quantitative control on specific morphological features characterizing the microstructures of the covered materials.<p>While several promising recent techniques are dedicated to the discretization of arbitrary complex geometries in numerical models, the literature on RVEs generation methodologies does not provide fully satisfying solutions for most of the cases. The general strategy in this work consisted in selecting a promising state-of-the-art discretization method and in designing improved RVE generation techniques with the concern of guaranteeing their seamless collaboration. The chosen discretization technique is a specific variation of the generalized / extended finite element method that accommodates the representation of arbitrary input geometries represented by level set functions. The RVE generation techniques were designed accordingly, using level set functions to define and manipulate the RVEs geometries. <p>The RVE methodologies developed are mostly morphologically motivated, incorporating governing parameters allowing the reproduction and the quantitative control of specific morphological features of the considered materials. These developments make an intensive use of distance fields and level set functions to handle the geometrical complexity of microstructures. Valuable improvements were brought to the RVE generation methodologies for several materials, namely granular and particle-based materials, coated and cemented geomaterials, polycrystalline materials, cellular materials and textile-based materials. RVEs produced using those developments have allowed extensive testing of the investigated discretization method, using complex microstructures in proof-of-concept studies involving the main ingredients of RVE-based morphological parameter studies of complex heterogeneous materials. In particular, the illustrated approach offers the possibility to address three crucial aspects of those kinds of studies: (I) to easily conduct simulations on a large number of RVEs covering a significant range of morphological variations for a material, (II) to use advanced constituent material behaviors and (III) to discretize large 3D RVEs. Based on those illustrations and the experience gained from their realization, the main strengths and limitations of the considered discretization methods were clearly identified. / Doctorat en Sciences de l'ingénieur / info:eu-repo/semantics/nonPublished

The goal of this thesis is the development of theoretical methods targeting the implementation of topology optimization in structural engineering applications. In civil engineering applications, structures are typically assemblies of many standardized components, such as bars, where the largest gains in efficiency can be made during the preliminary design of the overall structure. The work is aimed mainly at truss-like structures in civil engineering applications, however several of the developments are general enough to encompass continuum structures and other areas of engineering research too. The research aims to address the following challenges:<p>- Discrete variable optimization, generally necessary for truss problems in civil engineering, tends to be computationally very expensive,<p>- the gap between industrial applications in civil engineering and optimization research is quite large, meaning that the developed methods are currently not fully embraced in practice, and<p>- industrial applications demand robust and reliable solutions to the real-world problems faced by the civil engineering profession.<p><p>In order to face these challenges, the research is divided into several research papers, included as chapters in the thesis.<p>Discrete binary variables in structural topology optimization often lead to very large computational cost and sometimes even failure of algorithm convergence. A novel method was developed for improving the performance of topology optimization problems in truss-like structures with discrete design variables, using so-called Kinematic Stability Repair (KSR). Two typical examples of topology optimization problems with binary variables are bracing systems and steel grid shell structures. These important industrial applications of topology optimization are investigated in the thesis. A novel method is developed for topology optimization of grid shells whose global shape has been determined by form-finding. Furthermore a novel technique for façade bracing optimization is developed. In this application a multiobjective approach was used to give the designers freedom to make changes, as the design advanced at various stages of the design process. The application of the two methods to practical<p>engineering problems, inspired a theoretical development which has wide-reaching implications for discrete optimization: the pitfalls of symmetry reduction. A seemingly self-evident method of cardinality reduction makes use of geometric symmetry reduction in structures in order to reduce the problem size. It is shown in the research that this assumption is not valid for discrete variable problems. Despite intuition to the contrary, for symmetric problems, asymmetric solutions may be more optimal than their symmetric counterparts. In reality many uncertainties exist on geometry, loading and material properties in structural systems. This has an effect on the performance (robustness) of the non-ideal, realized structure. To address this, a general robust topology optimization framework for both continuum and truss-like structures, developing a novel analysis technique for truss structures under material uncertainties, is introduced. Next, this framework is extended to discrete variable, multiobjective optimization problems of truss structures, taking uncertainties on the material stiffness and the loading into account. Two papers corresponding to the two chapters were submitted to the journal Computers and Structures and Structural and Multidisciplinary Optimization. Finally, a concluding chapter summarizes the main findings of the research. A number of appendices are included at the end of the manuscript, clarifying several pertinent issues. / Doctorat en Sciences de l'ingénieur / info:eu-repo/semantics/nonPublished

This doctoral research work focuses on the simulation of progressive collapse of reinforced concrete structures. It aims at contributing to the ‘alternate load path’ design approach suggested by the General Services Administration (GSA) and the Department of Defense (DoD) of the United States, by providing a detailed yet flexible numerical modelling tool. <p><p>The finite element formulation adopted here is based on a multilevel approach where the response at the structural level is naturally deduced from the behaviour of the constituents (concrete and steel) at the material level. One-dimensional nonlinear constitutive laws are used to model the material response of concrete and steel. These constitutive equations are introduced in a layered beam approach, where the cross-sections of the structural members are discretised through a finite number of layers. This modelling strategy allows deriving physically motivated relationships between generalised stresses and strains at the sectional level. Additionally, a gradual sectional strength degradation can be obtained as a consequence of the progressive failure of the constitutive layers. This means that complex nonlinear sectional responses exhibiting softening can be obtained even for simplified one dimensional constitutive laws for the constituents.<p><p>This numerical formulation is used in dynamic progressive collapse simulations to study the structural response of a multi-storey planar frame subject to a sudden column loss. The versatility of the proposed methodology allows assessing the influence of the main material and design parameters in the structural failure. Furthermore, the effect of particular modelling options of the progressive collapse simulation technique, such as the column removal time or the strategy adopted for the structural verification, can be evaluated.<p><p>The potential strain rate effects on the structural response of reinforced concrete frames are also investigated. To this end, a strain rate dependent material formulation is developed, where the rate effects are introduced in both the concrete and steel constitutive response. These effects are incorporated at the structural level through the multilayered beam approach. In order to assess the degree of rate dependence in progressive collapse, the results of rate dependent simulations are presented and compared to those obtained via the rate independent approach. The influence of certain parameters on the rate dependent structural failure is also studied.<p><p>The differences obtained in terms of progressive failure degree for the considered parametric variations and modelling options are analysed and discussed. The parameters observed to have a major influence on the structural response in a progressive collapse scenario are the ductility of the steel bars, the degree of symmetry and/or continuity of the reinforcement and the column removal time. The results also depend on the strategy considered (GSA vs DoD). The strain rate effects are confirmed to play a significant role in the failure pattern. Based on these observations, general recommendations for the design of progressive collapse resisting structures are finally derived.<p><p><p><p><p>L’effondrement progressif est un sujet de recherche qui a connu un grand développement suite aux événements désastreux qui se sont produits au cours des dernières décennies. Ce phénomène est déclenché par la défaillance soudaine d’un nombre réduit d’éléments porteurs de la structure, qui provoque une propagation en cascade de l’endommagement d’élément en élément jusqu’à affecter une partie importante, voire la totalité de l’ouvrage. Le résultat est donc disproportionné par rapport à la cause. La plupart des codes de construction ont inclus des prescriptions pour le dimensionnement des structures face aux actions accidentelles. Malheureusement, ces procédures se limitent à fournir des ‘règles de bonne pratique’, ou proposent des calculs simplifiés se caractérisant par un manque de détail pour permettre leur mise en oeuvre.<p><p>Cette thèse de doctorat intitulée Simulation de l’Effondrement Progressif des Structures en Béton Armé: Influence des Paramètres Materiaux et de Dimensionnement et Investigation des Effets de Vitesse a pour but de contribuer à la simulation numérique de l’effondrement progressif des structures en béton armé. Une formulation aux éléments finis basée sur une approche multi-échelles a été développée, où la réponse à l’échelle structurale est déduite à partir de la réponse au niveau matériel des constituants (le béton et l’acier). Les sections des éléments structuraux sont divisées en un nombre fini de couches pour lesquelles des lois constitutives unidimensionnelles sont postulées. Cet outil permet une dégradation graduelle de la résistance des sections en béton armé suite à la rupture progressive des couches. Des comportements complexes au niveau des points de Gauss peuvent être ainsi obtenus, et cela même à partir de lois unidimensionnelles pour les constituants.<p><p>Cette formulation est utilisée pour la simulation de l’effondrement progressif d’ossatures 2D, avec prise en compte des effets dynamiques. La versatilité de la présente stratégie numérique permet d’analyser l’influence de différents paramètres matériaux et de dimensionnement, ainsi que d’autres paramètres de modélisation, sur la réponse structurale face à la disparition soudaine d’une colonne.<p><p>Les effets de la vitesse de déformation sur le comportement des matériaux constituants est aussi un sujet d’attention dans ce travail de recherche. Des lois constitutives prenant en compte ces effets sont postulées et incorporées au niveau structural grâce à l’approche multi-couches. Le but est d’étudier l’influence des effets de la vitesse de chargement sur la réponse structurale face à la disparition d’un élément porteur. Les resultats obtenus à l’aide de cette approche avec effets de vitesse sont comparés à ceux obtenus avec des lois indépendantes de la vitesse.<p><p>Les différences dans la réponse à la disparition d’une colonne sont analysées pour les variations paramétriques étudiées. Les paramètres ayant une influence importante sont notamment: la ductilité des matériaux constituants et la disposition et/ou la symétrie des armatures. Les effets de vitesse sont également significatifs. Sur base de ces résultats, des recommandations sont proposées pour le dimensionnement et/ou l’analyse des structures face à l’effondrement progressif.<p> / Doctorat en Sciences de l'ingénieur / info:eu-repo/semantics/nonPublished

<p align='justify'>This thesis is devoted to the micromechanical study of the size-dependent strengthening in Transformation Induced Plasticity (TRIP) steels. Such grades of advanced high-strength steels are compelling for the automotive industry, due to their improved mechanical properties. Among others, they combine a good strength versus ductility balance. In this context, many research works have been carried out to study these grades of steels. In particular, from a numerical point of view, earlier studies within the framework of classical plasticity do not properly reproduce the strengthening levels characterizing TRIP steels and obtained experimentally.</p> <p><p align='justify'>In this study, the strain gradient plasticity theory presented by Fleck and Hutchinson (2001) is chosen to account for the strengthening effect resulting from the phase transformation. A two-dimensional embedded cell model of a simplified microstructure composed of small cylindrical metastable austenitic inclusions, partially undergoing the phase transformation, within a ferritic matrix is used.</p><p><p align='justify'>First, the single-parameter version of the strain gradient plasticity theory under small strain assumption is used for the simulations. The impact of the higher order boundary conditions is assessed. It is shown that, when the plastic flow is unconstrained at the elasto-plastic boundaries, the transformation strain has no significant impact on the overall strengthening. The strengthening is essentially coming from the composite effect with a marked inclusion size effect resulting from the appearance during deformation of new boundaries (at the interface between parent and product phases) constraining the plastic flow.</p><p><p align='justify'>Second, the multi-parameter version of the strain gradient plasticity theory, incorporating separately the rotational and extensional gradients in the formulation, is employed under small strain assumption. The effect of the plastic strain gradients resulting from the transformation strain is better captured. In particular, the results show a significant influence of the shear component of the transformation strain. An implicit confinement effect is revealed at the elasto-plastic boundaries which is partly responsible for the transformation strain effect. Size effects on the overall strengthening are also revealed, due to a combined size dependent effect of the transformation strain and of the evolving composite structure.</p><p align='justify'>Third, the extension of the strain gradient plasticity theory to a finite strain description is applied. A significant effect of the transformation strain is obtained with the multi-parameter version of the theory as well as an optimal austenite grain size improving the damage resistance of the martensite, in agreement with the typical grain size of the current TRIP-assisted steels (Jacques et al. 2007).</p> / Doctorat en Sciences de l'ingénieur / info:eu-repo/semantics/nonPublished

<p align="justify">In a context of restoration of historical masonry structures, it is crucial to properly estimate the residual strength and the potential structural failure modes in order to assess the safety of buildings. Due to its mesostructure and the quasi-brittle nature of its constituents, masonry presents preferential damage orientations, strongly localised failure modes and damage-induced anisotropy, which are complex to incorporate in structural computations. Furthermore, masonry structures are generally subjected to complex loading processes including both in-plane and out-of-plane loads which considerably influence the potential failure mechanisms. As a consequence, both the membrane and the flexural behaviours of masonry walls have to be taken into account for a proper estimation of the structural stability.</p><p><p align="justify">Macrosopic models used in structural computations are based on phenomenological laws including a set of parameters which characterises the average behaviour of the material. These parameters need to be identified through experimental tests, which can become costly due to the complexity of the behaviour particularly when cracks appear. The existing macroscopic models are consequently restricted to particular assumptions. Other models based on a detailed mesoscopic description are used to estimate the strength of masonry and its behaviour with failure. This is motivated by the fact that the behaviour of each constituent is a priori easier to identify than the global structural response. These mesoscopic models can however rapidly become unaffordable in terms of computational cost for the case of large-scale three-dimensional structures.</p><p><p align="justify">In order to keep the accuracy of the mesoscopic modelling with a more affordable computational effort for large-scale structures, a multi-scale framework using computational homogenisation is developed to extract the macroscopic constitutive material response from computations performed on a sample of the mesostructure, thereby allowing to bridge the gap between macroscopic and mesoscopic representations. Coarse graining methodologies for the failure of quasi-brittle heterogeneous materials have started to emerge for in-plane problems but remain largely unexplored for shell descriptions. The purpose of this study is to propose a new periodic homogenisation-based multi-scale approach for quasi-brittle thin shell failure.</p><p><p align="justify">For the numerical treatment of damage localisation at the structural scale, an embedded strong discontinuity approach is used to represent the collective behaviour of fine-scale cracks using average cohesive zones including mixed cracking modes and presenting evolving orientation related to fine-scale damage evolutions.</p><p><p align="justify">A first originality of this research work is the definition and analysis of a criterion based on the homogenisation of a fine-scale modelling to detect localisation in a shell description and determine its evolving orientation. Secondly, an enhanced continuous-discontinuous scale transition incorporating strong embedded discontinuities driven by the damaging mesostructure is proposed for the case of in-plane loaded structures. Finally, this continuous-discontinuous homogenisation scheme is extended to a shell description in order to model the localised behaviour of out-of-plane loaded structures. These multi-scale approaches for failure are applied on typical masonry wall tests and verified against three-dimensional full fine-scale computations in which all the bricks and the joints are discretised.</p> / Doctorat en Sciences de l'ingénieur / info:eu-repo/semantics/nonPublished

Chaque année, d'innombrables collisions se produisent entre des avions en vol et des oiseaux. L'impact aviaire, menace redoutée par les pilotes, concerne tant l'aviation civile que son pendant militaire. Les statistiques démontrent que, même si fort heureusement le nombre d'accidents graves reste limité, les incidents sont de plus en plus nombreux.<p>Parmi les acteurs qui luttent contre ce danger, les constructeurs d'avions jouent un rôle prépondérant. Contraints par des réglementations internationales, ils s'attachent à produire des éléments de structure qui résistent à l'impact d'oiseaux.<p>Dans la mise au point de leur produits, les avionneurs démontrent cette résistance à l'aide d'essais d'impact :on accélère un simulant d'oiseau jusqu'à la vitesse voulue -- de l'ordre de la vitesse de croisière nominale de l'avion -- et on le projette sur un aileron ou un morceau de fuselage. <p><p>La présente thèse doctorale, co-dirigée par les professeurs Philippe Bouillard de l'ULB et Marc Pirlot de l'ERM, contribue doublement à l'amélioration de ces techniques d'essais dynamiques.<p><p><p>D'une part, elle réalise la mise au point et la validation d'un lanceur pyrotechnique à double étage pour l'accélération du simulant d'oiseau. Un canon de calibre 20 mm est combiné avec un accélérateur de calibre 160 mm. La combustion d'un mélange de poudre propulsive contenu dans une douille adaptée génère les gaz à haute température et à haute pression nécessaires à l'accélération d'un simulant d'oiseau dûment confiné dans un conteneur de protection. Un dispositif de séparation arrête le conteneur afin que seul le simulant d'oiseau percute l'élément d'avion à l'essai. La solution pyrotechnique à double étage mise au point est validée par de nombreux tirs instrumentés en vitesse, en accélération et en pression ;elle se révèle conforme aux exigences de sécurité et de reproductibilité. Le lanceur pyrotechnique présente par rapport aux solutions pneumatiques, utilisées à notre connaissance dans tous les autres centres d'essais, des avantages indéniables de compacité ainsi que de rapidité et de souplesse de mise en oeuvre.<p><p><p>D'autre part, la migration des alliages métalliques vers les matériaux composites est amorcée depuis plusieurs années déjà dans le monde de la construction aéronautique. Pour optimiser les structures, une connaissance des caractéristiques de ces matériaux est indispensable. Les modes de rupture font partie des caractéristiques encore mal connues. La mesure du déplacement hors-plan lors du tir sur panneaux plans est une des manières de quantifier le comportement du matériau sous l'action d'un impact. Cette mesure s'opère généralement de manière statique, après le tir. Une méthode de mesure dynamique a été mise au point, basée sur l'emploi de techniques de stéréoscopie par corrélation numérique d'images. Cette technique a été validée au moyen d'une méthode métrologique indépendante d'extensométrie laser. <p><p><p>ABSTRACT<p><p>Countless collisions occur each year between airplanes and birds. Bird strike is a concern to both civilian and militay aircraft. Statistics show that, although the number of serious accidents fortunately remains low, the number of incidents keeps increasing.<p>Amongst the actors tackling this issue, aircraft manufacturers play an important role. In compliance with international regulations, they have to produce structural elements that withstand bird impact. During the development of their products, aircraft manufacturers have to demonstrate this resistance through bird impact trials :a bird surrogate is accelerated to the required velocity - often close to the nominal cruise speed of the aircraft - and launched onto a flap or a piece of fuselage.<p><p>This PhD thesis has been co-supervised by Professor Philippe Bouillard (ULB) and Professor Marc Pirlot (ERM-KMS). Its contribution to the improvement of the aforementioned dynamic trials is twofold.<p><p><p>One one hand, a two-stage pyrotechnical launcher for bird surrogates has been developed and assessed. A 20 mm caliber gun is connected to a 160 mm diameter launcher. The combustion of a propellant mixture in a cartridge case generates high pressure, high temperature gases which accelerate a bird surrogate protected by a cylindrical container. A stripper refrains the container from hitting the target pane. <p>The pyrotechnical solution has been assessed through an important number of firings where pressure, velocity and acceleration have been measured. The solution has proven compliance with both the safety requirements and the repeatability specifications. Its advantages compared to the pneumatic solutions used, as far as we know, in all other test centres, include compactedness as well as flexibility and high firing rate.<p><p><p>On the other hand, migration towards composite materials has been initiated years ago in the area of aeronautical constructions.<p>To optimize structures, a thorough knowledge of these new materials is required. Failure modes belong to the still badly known features of carbon reinforced plastics. Measuring the out-of-plane deformation when firing on a flat pane is one way of quantifying the material's behaviour under impact. This measurement is most frequently made in a static way, after completion of the firing. A dynamic measuring method has been developed, based upon stereoscopic digital image correlation techniques. This technique has been validated by means of an independent laser extensometer measuring method. <p> / Doctorat en Sciences de l'ingénieur / info:eu-repo/semantics/nonPublished

<p align='justify'>L’adaptation des surfaces pour des fonctions prédéterminées par le choix des matériaux métalliques ou des couches minces ayant des propriétés mécaniques avancées peut potentiellement permettre de réaliser des nouvelles applications à petites échelles. Concevoir de telles applications utilisant des nouveaux matériaux nécessite en premier lieu la connaissance des propriétés mécaniques des matériaux ciblés à l’échelle microscopique et nanoscopique. Une méthode souvent appliquée pour caractériser les matériaux à petites échelles est la nanoindentation, qui peut être vue comme une mesure de dureté à l’échelle nanoscopique.</p><p><p align='justify'>Ce travail présente une contribution relative à l'interprétation des résultats de la nanoindentation, qui fait intervenir un grand nombre de phénomènes physiques couplés à l'aide de simulations numériques. A cette fin une approche interdisciplinaire, adaptée aux phénomènes apparaissant à petites échelles, et située à l’intersection entre la physique, la mécanique et la science des matériaux a été utilisée. Des modèles numériques de la nanoindentation ont été conçus à l'échelle atomique (modèle discret) et à l'échelle des milieux continus (méthode des éléments finis), pour étudier le comportement du nickel pur. Ce matériau a été choisi pour ses propriétés mécaniques avancées, sa résistance à l'usure et sa bio-compatibilité, qui peuvent permettre des applications futures intéressantes à l'échelle nanoscopique, particulièrement dans le domaine biomédical. Des méthodes avancées de mécanique du solide ont été utilisées pour prendre en compte les grandes déformations locales du matériau (par la formulation corotationelle), et pour décrire les conditions de contact qui évoluent au cours de l'analyse dans le modèle à l'échelle des milieux continus (traitement des conditions de contact unilatérales et tangentielles par une forme de Lagrangien augmenté).</p><p><p align='justify'>L’application des modèles numériques a permis de contribuer à l’identification des phénomènes qui gouvernent la nanoindentation du nickel pur. Le comportement viscoplastique du nickel pur pendant nanoindentation a été identifié dans une étude expérimentale-numérique couplée, et l'effet cumulatif de la rugosité et du frottement sur la dispersion des résultats de la nanoindentation a été montré par une étude numérique (dont les résultats sont en accord avec des tendances expérimentales).</p> <p><p align='justify'>Par ailleurs, l’utilisation de l’outil numérique pour une autre application à petites échelles, la manipulation des objets par contact, a contribué à la compréhension de la variation de l’adhésion électrostatique pendant micromanipulation. La déformation plastique des aspérités de surface sur le bras de manipulateur (en nickel pur) a été identifiée comme une source potentielle d’augmentation importante de l'adhésion pendant la micromanipulation, qui peut potentiellement causer des problèmes de relâche et de précision de positionnement, observés expérimentalement.</p><p><p align='justify'>Les résultats présentés dans cette thèse montrent que des simulations numériques basées sur la physique du problème traité peuvent expliquer des tendances expérimentales et contribuer à la compréhension et l'interprétation d'essais couramment utilisé pour la caractérisation aux petites échelles. Le travail réalisé dans cette thèse s’inscrit dans un projet de recherche appelé "mini-micro-nano" (mµn), financé par la Communauté Française de Belgique dans le cadre de "l'Action de Recherche Concertée", convention 04/09-310.</p> / Doctorat en Sciences de l'ingénieur / info:eu-repo/semantics/nonPublished

In the global framework of improving vibro-acoustic numerical prediction quality together with the need to decrease the number of prototyping stages, this manuscript focuses on achieving greater accuracy for acoustic numerical simulations by making use of a parametric updating technique, which enables tuning the model parameters inside physically meaningful boundaries. The improved model is used for the next prototyping stages, allowing more accurate results within reduced simulation times. The updating technique is based on recent works dealing with the constitutive relation error method (CRE) applied to acoustics. The updating process focuses on improving the acoustic damping matrix related to the absorbing properties of the materials covering the borders of the acoustic domain. / Doctorat en sciences appliquées / info:eu-repo/semantics/nonPublished