Les matériaux polymères sont largement utilisés pour des applications structurelles dans le secteur automobile et leurs comportements complexes nécessitent des modèles précis pour la simulation éléments finis. Les polymères possèdent un comportement dépendant du temps et de la vitesse. La dépendance à la vitesse peut être observée par un accroissement de la rigidité et de la limite élastique en fonction de la vitesse de déformation. Le long temps nécessaire pour retrouver des contraintes nulles après sollicitation du matériau met en évidence la dépendance du temps sur le comportement. De plus, particulièrement pour les polymères chargés, le phénomène de cavitation se traduisant par la création et la croissance de micro-cavités et de microfissures conduit à un changement de volume durant la déformation. Dans ce travail, un modèle de comportement est développé pour un polymère semi-cristallin chargé de talc utilisé dans l’industrie automobile. Un modèle constitutif viscoélastique-viscoplastique non-associatif avec endommagement non-local est proposé dans le but de simuler les phénomènes observés expérimentalement. Dans le modèle développé, une surface de charge non symétrique est utilisée pour prendre en compte la pression hydrostatique. La viscoplasticité non-associative couplée avec l’endommagement conduit aux déformations viscoplastiques non-isochoriques caractérisées expérimentalement. Les paramètres du modèle proviennent d’essais expérimentaux réalisés sous différentes conditions et `a différentes vitesses de déformation. Pour ces essais, plusieurs techniques de mesure, telles que la corrélation d’images et l’extensommetrie optique sont utilisées pour les mesures de champs de déplacements. La bonne corrélation entre les données expérimentales et les simulations numériques mettent en évidence la précision du modèle développé afin de modéliser le comportement des matériaux polymères semi-cristallins. / Polymer materials are widely used for structural applications in the automotive sector and their behaviours are complex and require accurate models for finite element simulations. Polymer materials exhibit rate and time dependent behaviours. The rate dependency can be observed by an increase of the stiffness and the yield stress at increasing strain rate. The long time to recover the zero stress after solicitation of the material highlight the time dependent behaviour. Furthermore, particularly for filled polymers, the cavitation phenomenon cause the creation and growth of micro-voids and microcracks called damage and leads to volume change during the deformation. In this work, a behaviourmodel for mineral filled semi-crystalline polymer used in automotive industry is developed. A constitutive viscoelastic-viscoplastic non-associated model coupled with nonlocal damage is proposed in order to simulate the phenomena observed experimentally. In the constitutive model, a non symmetric yield surface is used to take the hydrostatic pressure into account. The non associated viscoplasticity coupled with damage leads to the non-isochoric viscoplastic deformation characterised experimentally. The material parameters arise from experimental tests carried out under various loadings and strain rates. For these experimental tests, different measurement techniques like Digital Image Correlation and optical extensometry are used for the displacements and the strain field measurements. The good agreement between the experimental data and the numerical simulations highlights the accuracy of the developed model for polymer modelling.

Les phases de conception et de validation de pièces mécaniques nécessitent des outils de calculs rapides et fiables, permettant de faire des choix technologiques en un temps court. Dans ce cadre, il n'est pas possible de calculer la réponse exacte pour l'ensemble des configurations envisageables. Les métamodèles sont alors couramment utilisés mais nécessitent un grand nombre de réponses, notamment dans le cas où celles-ci sont non-linéaires. Une solution est alors d'exploiter plusieurs sources de données de qualité diverses pour générer un métamodèle multi-fidélité plus rapide à calculer pour une précision équivalente. Ces données multi-fidélité peuvent être extraites de modèles réduits.Les travaux présentés proposent une méthode de génération de métamodèles multi-fidélité pour l'optimisation de structures mécaniques par la mise en place d'une stratégie d'enrichissement adaptatif des informations sur la réponse de la structure, par utilisation de données issues d'un solveur LATIN-PGD permettant de générer des données de qualités adaptées, et d'accélérer le calcul par la réutilisation des données précédemment calculées. Un grand nombre de données basse-fidélité sont calculées avant un enrichissement intelligent par des données haute-fidélité.Ce manuscrit présente les contributions aux métamodèles multi-fidélité et deux approches de la méthode LATIN-PGD avec la mise en place d'une stratégie multi-paramétrique pour le réemploi des données précédemment calculées. Une implémentation parallèle des méthodes a permis de tester la méthode sur trois cas-tests, pour des gains pouvant aller jusqu'à 37x. / Engineering simulation provides the best design products by allowing many design options to be quickly explored and tested, but fast-time-to-results requirement remains a critical factor to meet aggressive time-to-market requirements. In this context, using high-fidelity direct resolution solver is not suitable for (virtual) charts generation for engineering design and optimization.Metamodels are commonly considered to explore design options without computing every possibility, but if the behavior is nonlinear, a large amount of data is still required. A possibility is to use further data sources to generate a multi-fidelity surrogate model by using model reduction. Model reduction techniques constitute one of the tools to bypass the limited calculation budget by seeking a solution to a problem on a reduced order basis (ROB).The purpose of the present work is an online method for generating a multi-fidelity metamodel nourished by calculating the quantity of interest from the basis generated on-the-fly with the LATIN-PGD framework for elasto-viscoplastic problems. Low-fidelity fields are obtained by stopping the solver before convergence, and high-fidelity information is obtained with converged solution. In addition, the solver ability to reuse information from previously calculated PGD basis is exploited.This manuscript presents the contributions to multi-fidelity metamodels and the LATIN-PGD method with the implementation of a multi-parametric strategy. This coupling strategy was tested on three test cases for calculation time savings of more than 37x.

Viscoplasticity models allow for the prediction of the inelastic behavior of materials, taking into account the rate-dependence. In order to model the response under non-isothermal conditions experienced by many components, such as those in turbomachinery, however, it is necessary to incorporate temperature-dependence. Additionally, for materials subjected to thermal shock, temperature rate-dependence is also important. The purpose of this research is to develop a method of determining Chaboche viscoplasticity parameters that allows for consistent behavior with changing temperature. A quartet of candidate materials, 304 stainless steel, IN617, DS GTD-111, and Ti6242S, were chosen for their applications in turbomachinery, such as gas turbines, nuclear, and aerospace applications. The focus of this research is geared towards establishing the temperature-dependence of the constants used in the model in order to obtain more accurate modeling of non-isothermal fatigue loadings than those achieved through linear interpolation of constants at several temperatures. The goal is to be able to more accurately predict the deformation behavior of components subjected to cyclic temperature and mechanical loadings which will ultimately allow for more accurate life prediction. The effects of orientation in directionally solidified (DS) materials is also examined in order to gain insight as to the expected behavior of parameters with changing orientation.

Material flow in friction stir welding is largely uncharacterized due to the difficulty in material flow measurement and visualization in metals. This study investigates plasticine for use as an analog for modeling material flow in friction stir welding (FSW) of metals. Qualitative comparisons between welded plasticine and metal sections exhibit many similarities. The transient temperature response of the plasticine also shows the same qualitative behavior as welds conducted in metal. To quantify its similarity to metal, the plasticine is further analyzed through compression tests to characterize its strain, strain-rate, and temperature sensitivities. A detailed analysis is presented which defines the criteria for rigorous mechanical and thermal similarity between metals and analog materials. The mechanical response of the plasticine is quantitatively similar to many aluminum and steel alloys. In addition to the mechanical properties of the plasticine, thermal properties are measured and thermal similarity is investigated. Generally, complete thermal similarity cannot be achieved in FSW. However, given the similarities between other critical parameters, and observed qualitatively similarity, it is possible to satisfy similarity approximately, such that information can be obtained from the physical model and extrapolated to metals. Using plasticine, material flow behavior in FSW is investigated under various operating conditions. The physical model permits visualization and characterization of material flow around a suspended welding tool. Depending on operating conditions, several material flow regimes are observed, including simple extrusion with substantial tool/material slip, defect formation, a region of rotating material adjacent to the tool, and vertical deformation. Material flow and frictional heating in FSW are also investigated using a three-dimensional numerical model. Two mechanical boundary conditions are investigated, including 1) a sticking constant velocity, and 2) a slipping variable shear stress model. The constant velocity model generally over-predicts the extent of material flow in the weld region. The variable shear model predicts simple extrusion of material around the tool, and substantial tool/material slip. Additionally, the variable shear model exhibits a region of diminishing shear stress, velocity, and pressure at the back advancing side of the pin, suggesting formation of an internal void. The limited deformation, low velocities, and indication of void formation agree well with flow visualization studies using plasticine under identical operating parameters.

No description available.

The coastal population is growing, putting extra stress on coastal sediments and protection features, such as beach dunes. Moreover, global warming will increase the frequency of storms, and coastal dunes and other defense infrastructure will be subjected to increased erosion and scouring, endangering the people they are meant to protect. Understanding soil dynamics and fluid interaction is crucial to predict the effects of sand erosion. In particular, the study of wind erosion of sands in coastal dunes is essential due to the protective role these earthen structures have during storm events. One of the challenges about predicting wind erosion in coastal dunes is its extended spatial scale and the associated economic and logistics costs of sampling and characterizing the sediments. Because of this, in-situ testing for sediment characterization is essential. In particular, the usage of free-fall penetrometers (FFP) is appealing due to their portability and robustness. The sediment properties obtained with this type of testing can later be used to assess wind erosion susceptibility by determining, for example, the wind velocity to initiate the erosion process. FFP testing involves dropping an instrumented probe that impacts the soil and measures the kinematics or kinetics during the penetration process. For example, deceleration measurements are used to compute an equivalent quasi-static failure, which is not in line with the dynamic process characteristic of FFP testing. This preassumed failure mechanism is used to back-calculate the sand's geomechanical properties. However, soil behavior is highly complex under rapid loading, and incorporating this behavior into FFP sediment characterization models is challenging. Advanced numerical modeling can improve the understanding of the physics behind FFP testing. This thesis presents various advancements in numerical modeling and erosion models to bridge FFP in-situ testing with predicting the initiation of wind erosion of sands. First, improvements oriented to the Material Point Method (MPM) for modeling in-situ FFP testing are proposed. The numerical results show that the simulation of FFP deployment in sands is affected by strain localization and highlight the importance of considering constitutive models sensitive to different loading rates. Because of the importance of rate effects in soil behavior, the second aspect of this thesis proposes a novel consistency framework. Two constitutive models are adapted to study strain-rate sensitive non-cohesive materials: i) a strain-softening Mohr-Coulomb, and ii) a NorSand model. In addition to increased strength, the proposed framework captures increased dilatation, an early peak deviatoric stress, and relaxation. Finally, a novel sand erosion model is derived using a continuum approximation and limit equilibrium analysis. The erosion law considers geotechnical parameters, the effects of slope, and moisture suction, in a combined manner. The proposed model is theoretically consistent with existing expressions in the literature. It covers a wide range of environmental and geometrical conditions and helps to reconcile the results from FFP testing with the prediction of the initiation of wind erosion. The model was validated in a wind tunnel and is demonstrated to be a viable alternative for predicting sand erosion initiation. This thesis opens up new research prospects, such as improving the soil characterization models or the direct prediction of sand erosion using rapid, reliable, and efficient in-situ testing methods. / Doctor of Philosophy / With global warming and climate change, it is expected that the frequency and intensity of storms will increase. This increment will put extra stress on coastal sediments such as beach sand and coastal dunes, making them prone to erosion. Coastal dunes lose their ability to withstand storms as they erode, potentially making coastal flooding more frequent. In light of this, all stakeholders involved in the protection against coastal disasters must have the tools to predict, prepare for, and mitigate for situations like the ones stated above. An essential aspect of the prediction component is dependent on a successful sediment characterization, for example, determining how much wind the sand can withstand before it erodes. Free-fall penetrometers (FFP) are devices designed to conduct the characterization mentioned above. However, the procedures used to perform this characterization are mainly based on empirical or semi-empirical expressions. Computer models, capable of simulating the physics behind FFP testing, can bring more insight into the process of interaction between FFP devices, sands, and water and can be the basis to improve the characterization methods. The latter results can be utilized for instance to predict wind erosion, including several properties of the sand, such as its mineralogy and shape. This study contributes to developing the computer simulations of FFP deployment and the wind erosion prediction models. Eventually, these developments can help engineers and coastal managers to anticipate and prepare for more frequent coastal hazards.

Thermomechanical fatigue life prediction is important in the design of Ni-base superalloy components in gas turbine engines and requires a stress-strain analysis for accurate results. Crystal viscoplasticity models are an ideal tool for this stress-strain analysis of Ni-base superalloys as they can capture not only the anomalous yielding behavior, but also the non-Schmid effect, the strain rate dependence, and the temperature dependence of typically large grained directionally-solidified and single crystal alloys. However, the model is difficult to calibrate even for isothermal conditions because of the interdependencies between parameters meant to capture different but similar phenomena at different length scales, many tied to a particular slip system. The need for the capacity to predict the material response over a large temperature range, which is critical for the simulation of hot section gas turbine components, causes the determination of parameters to be even more difficult since some parameters are highly temperature dependent. Rapid parameter determination techniques are therefore needed for temperature-dependent parameterizations so that the effort needed to calibrate the model is reduced to a reasonable level. Specific parameter determination protocols are established for a crystal viscoplasticity model implemented in ABAQUS through a user material subroutine. Parameters are grouped to reduce interdependencies and a hierarchical path through the groups and the parameters within each group is established. This dual level hierarchy creates a logical path for parameter determination which further reduces the interdependencies between parameters, allowing for rapid parameter determination. Next, experiments and protocols are established to rapidly provide data for calibration of the temperature-dependencies of the viscoplasticity. The amount of data needed to calibrate the crystal viscoplasticity model over a wide temperature range is excessively large due to the number of parameters that it contains which causes the amount of time spent in the experimentation phase of parameter determination to be excessively large. To avoid this lengthy experimentation phase each experiment is designed to contain as much relevant data as possible. This is accomplished through the inclusion of multiple strain rates in each experiment with strain ranges sufficiently large to clearly capture the inelastic response. The experimental and parameter determination protocols were exercised by calibrating the model to the directionally-solidified Ni-bas superalloy DS-CM247LC. The resulting calibration describes the material's behavior in multiple loading orientations and over a wide temperature range of 20 °C to 1050 °C. Several parametric studies illustrate the utility of the calibrated model.

A constitutive model has been developed to model the shock response of single crystal aluminum from peak pressures ranging from 2-110 GPa. This model couples a description of higher-order thermoelasticity with a dislocation-based viscoplastic formulation, both of which are formulated for single crystals. The constitutive model has been implemented using two numerical methods: a plane wave method that tracks the propagating wave front; and an extended one-dimensional, finite-difference method that can be used to model spatio-temporal evolution of wave propagation in anisotropic materials. The constitutive model, as well as these numerical methods, are used to simulate shock wave propagation in single crystals, polycrystals, and pre-textured polycrystals. Model predictions are compared with extensive existing experimental data and are then used to quantify the influence of the initial material state on the subsequent shock response. A coarse-grained model is then proposed to capture orientation-dependent deformation heterogeneity, and is shown to replicate salient features predicted by direct finite-difference simulation of polycrystals in the weak shock regime. The work in this thesis establishes a general framework that can be used to quantify the influence of initial material state on subsequent shock behavior not only for aluminum single crystals, but for other face-centered cubic and lower symmetry crystalline metals as well.

Dans l'étude des éléments combustibles des réacteurs à eau pressurisée, cette thèse s'inscrit dans la compréhension et la modélisation du comportement viscoplastique du dioxyde d'uranium (UO2) à l'échelle du polycristal. Lors de fonctionnement de type incidentel du réacteur, le combustible subit une forte élévation de la température avec un gradient thermique de la pastille engendrant des déformations viscoplastiques contrôlées par des mouvements de dislocations. D'abord, un modèle de plasticité cristalline a été développé de manière à décrire l’anisotropie viscoplastique du matériau en fonction de la température et de la vitesse de sollicitation. Des simulations par éléments finis (EF) sur monocristaux ont permis d’identifier que les trois modes de glissement généralement observés dans l'UO2 sont importants pour décrire le comportement anisotrope du matériau. Dans un second temps, les coefficients de la matrice d'interactions entre dislocations ont été déterminés spécifiquement pour l’UO2 afin d’améliorer la modélisation des polycristaux. En effet, en calculant par EF les dislocations géométriquement nécessaires, qui sont responsables d’une forte augmentation de la densité de dislocations stockées dans les polycristaux, les interactions entre dislocations permettent de simuler l’effet dé taille de grain et l’écrouissage des pastilles. Finalement, le modèle, adapté pour les polycristaux, a été validé par comparaison avec les essais expérimentaux sur pastille et par comparaison du comportement intra-granulaire simulé avec des mesures EBSD. Grâce à cette dernière comparaison, il est possible de remonter indirectement aux hétérogénéités de déformation dans les grains / This thesis is part of the study of fuel elements of pressurized water reactors and, more specifically, focus on the understanding and modelling of the viscoplastic behavior of uranium dioxide (UO$_2$) at polycrystalline scale. During the incidental operation of the reactor, the fuel undergoes a strong increase of temperature and thermal gradient between the center and the periphery of the pellet leading to viscoplastic strains due to dislocation movement mechanisms. First, a crystal plasticity model was developed in order to describe the viscoplastic anisotropy of the material considering the temperature and the loading rate. Finite element (FE) simulations on single crystals enabled to highlight that the three slip modes generally observed in UO$_2$ are crucial to describe the anisotropic behavior of the material. Secondly, coefficients of the interaction matrix have been identified specifically for UO$_2$ in order to improve the polycrystal modelling. Indeed, by calculating geometrically necessary dislocations (GNDs), which are responsible of the great increase of the stored dislocation density in polycrystals, the interactions between dislocations enable to simulate de grain size sensitivity and hardening of the fuel pellet. Finally, the model adapted for polycrystals, have been validated by comparing FE simulations with pellet compression tests and by comparing the simulated intra-granular behavior with EBSD measurements. Thanks to the latter comparison, it is possible to indirectly compare the strain heterogeneities in the grains

The focus of this work is on the modeling and simulation of shock wave propagation in reactive metal powder mixtures. Reactive metal systems are non-explosive, solid-state materials that release chemical energy when subjected to sufficiently strong stimuli. Shock loading experiments have demonstrated that ultra-fast chemical reactions can be achieved in certain micron-sized metal powder mixtures. However, the mechanisms of rapid mixing that drive these chemical reactions are currently unclear. The goal of this research is to gain an understanding of the shock-induced deformation that enables these ultra-fast reactions. The problem is approached using direct numerical simulation. In this work, a finite element (FE) model is developed to simulate shock wave propagation in discrete particle mixtures. This provides explicit particle-level resolution of the thermal and mechanical fields that develop in the shock wave. The Ni/Al powder system has been selected for study. To facilitate mesoscale FE simulation, a new dislocation-based constitutive model has been developed to address the viscoplastic deformation of fcc metals at very high strain rates. Six distinct initial configurations of the Ni/Al powder system have been simulated to quantify the effects of powder configuration (e.g., particle size, phase morphology, and constituent volume fractions) on deformation in the shock wave. Results relevant to the degree of shock-induced mixing in the Ni/Al powders are presented, including specific analysis of the thermodynamic state and microstructure of the Ni/Al interfaces that develop during wave propagation. Finally, it is shown that velocity fluctuations at the Ni/Al interfaces (which arise due to material heterogeneity) may serve to fragment the particles down to the nanoscale, and thus provide an explanation of ultra-fast chemical reactions in these material systems.