Publications

2024

• Low frequency dynamics and coherent flow structures on a thin airfoil at stall
  
  • Kharsansky Atallah Ivan
  , 2024. In the last years, particular emphasis was put on understanding the mechanisms behind stall on wings working at transitional Reynolds numbers (Re≈10^5) as is the case for unmanned aired vehicles, e.g., drones. In this regime, there is a melting pot of phenomena, among which we can find multistability and low-frequency oscillations. Despite the vast literature on these subjects, there is still a gap in the understanding of the mechanisms behind and the interaction between them. The objective of this work is to bridge this gap and to provide a clearer picture of the dynamics involved. To this end, we have conducted synchronised pressure-forced and particle image velocimetry measurements in a NACA0012 on a wind tunnel. Self-sustained bistable dynamics are observed over a broad range of Re and at angles slightly higher than the angle of stall. The system jumps intermittently between two states of high and low lift. We show that this process is random and memoryless. The mean residence time follows a super-exponential evolution typical of subcritical bifurcations. Flow topology and dynamics in each state are analyzed at different Re. Above a critical Re, the high lift state features a small laminar separation bubble, low fluctuations and no sign of low-frequency unsteadiness. Below this value, a longer bubble is present, accompanied by a low-frequency broadband peak with a Strouhal number one order of magnitude lower than the one for wake vortex shedding. On the other hand, the low lift state exhibits similar features for all Re tested, featuring intermittency between long bubbles and massive separation. Broadband low-frequency unsteadiness is also observed, with a Strouhal number close to the one found for the high lift state. Employing an extension of the Multiscale proper orthogonal decomposition, we associate this low-frequency energy with a shrinking-expansion of the bubble. Two stall scenarios are proposed depending on the Re regime. A first one where the transitions are triggered by the bubble breathing and another driven by background noise. Finally, a stochastic model based on the lift coefficient is developed. The model sucessfully reproduces the large-scale dynamics and statistics of the bistability. It also predicts the existence of an unstable branch linked by two saddle-node bifurcations with consequences on the stability of the high and low lift states. (10.70675/8fd76227zb1c9z4d53zb017zf38ece464c54)
  DOI : 10.70675/8fd76227zb1c9z4d53zb017zf38ece464c54
• Reduced order modelling of fully coupled electro-mechanical systems through invariant manifolds with applications to microstructures
  
  • Frangi Attilio
  • Colombo Alessio
  • Vizzaccaro Alessandra
  • Touzé Cyril
  International Journal for Numerical Methods in Engineering, Wiley, 2024, 126 (3), pp.e7641. This article presents the first application of the direct parametrisation method for invariant manifolds to a fully coupled multiphysics problem involving the nonlinear vibrations of deformable structures subjected to an electrostatic field. The formulation proposed is intended for model order reduction of electrostatically actuated resonating Micro‐Electro‐Mechanical Systems (MEMS). The continuous problem is first rewritten in a manner that can be directly handled by the parametrisation method, which relies upon automated asymptotic expansions. A new mixed fully Lagrangian formulation is thus proposed, which contains only explicit polynomial nonlinearities, which is then discretised in the framework of finite element procedures. Validation is performed on the classical parallel plate configuration, where different formulations using either the general framework or an approximation of the electrostatic field due to the geometric configuration selected are compared. Reduced‐order models along these formulations are also compared to full‐order simulations operated with a time integration approach. Numerical results show a remarkable performance both in terms of accuracy and the wealth of nonlinear effects that can be accounted for. In particular, the transition from hardening to softening behaviour of the primary resonance while increasing the constant voltage component of the electric actuation is recovered. Secondary resonances leading to superharmonic and parametric resonances are also investigated with the reduced‐order model. (10.1002/nme.7641)
  DOI : 10.1002/nme.7641
• Reduced contact model applied to the explicit time integration of a nuclear component undergoing large rotations and frictional impacts
  
  • Serre Matthieu
  , 2024. This work addresses the need for precise dynamic simulations to predict the non-linear vibrations of the thermal sleeve in a reactor tank. Accurate prediction of wear requires simulations that provide highly localized impact data. While the modeling of flexible bodies undergoing rigid body motions is well-established, incorporating frictional impacts remains challenging. Traditional methods using impulse-based techniques do not meet the precision requirements, leading to the adoption of explicit time integration schemes with small time steps and the penalty method for shock treatment.The conducted work involved a detailed examination of time integration for Lagrange’s equation, beginning with large rotations and rigid body dynamics using Newmark-derived integrators. These were tested through Hamiltonian case studies. The simultaneous integration of elastic vibrations and large rotations is then studied using two different formalisms, both based on modal decomposition. The first approach, Linear Theory of Elastodynamics (LTE), treats elastic and rigid body motions independently, with coupling occurring only during shocks. The second formalism includes time-dependent coupling between reference motion and elastic vibrations, modeled using the original CDIN algorithm. A qualitative comparison of the two approaches shows the LTE to be significantly more efficient, and the corresponding energy drift acceptable for the thermal sleeve’s geometry.The next phase focuses on modeling the conical contact between the thermal sleeve and the adapter using the penalty method.A parametric study based on static finite element computations produced an analytical formulation for the contact forces. The final part of the investigation involved simulating the in-service behavior of the thermal sleeve, recalibrating the model to match experimental data and computing wear energy on the contact surfaces. (10.70675/dd2740ccz0ba8z44c3z9c66z16ebb4432fb0)
  DOI : 10.70675/dd2740ccz0ba8z44c3z9c66z16ebb4432fb0
• Robotized measurements : automated acoustic characterization of structures
  
  • Pascal Caroline
  , 2024. This thesis aims at developing a tool for automated acoustic characterization using robotized measurements. Leveraging advancements in robotics, mechanics, and applied mathematics, the work focuses on employing a robotic arm equipped with sensors to estimate the sound field emitted by unknown structures. Since the accuracy of the measurements depends on the precise positioning of the sensor, discrepancies between the nominal and actual models of the robotic arm must be corrected. To address this, a comprehensive geometric calibration procedure is proposed and experimentally validated through two case studies involving three robotic arms and two types of sensors. Notably, significant improvements in accuracy are achieved by incorporating task-specific and sensor-infused features. The theoretical and numerical foundations of Sound Field Estimation (SFE), which aims at reconstructing the acoustic field radiated by an unknown source based on a limited set of measurements, are then explored. Among the reviewed methods, the Boundary Elements Method (BEM) stands out for its efficiency and flexibility despite its computational cost. Numerical simulations allow for the validation of the method's performances and the analysis of its robustness in the presence of external disturbances, highlighting the importance of robot calibration to ensure reliable results. Finally, to support the experimental implementation of our tool, a framework based on the ROS middleware is introduced to manage interfaces between the robot, sensors, and their environment. Prior to acoustic measurements, a robotized geometric characterization process is presented to determine the shape and position of the studied object. The robotized setup developed to automate acoustic measurements is then presented, with particular focus on the effects of the robot on the measurements. Under conditions where these effects are minimized, two autonomous measurement campaigns led to a successful reconstruction of the sound field radiated by an unknown loudspeaker, demonstrating the potential of the proposed tool. (10.70675/d68c712ez446ez4a4bza568zabaf3dc8aac2)
  DOI : 10.70675/d68c712ez446ez4a4bza568zabaf3dc8aac2
• Vibrations sous chargement turbulent en géométrie cylindrique avec admissions impactantes : Application à un espace annulaire de cuve de réacteur à eau pressurisée
  
  • Kocher Maud
  , 2024. Les cuves de réacteurs à eau pressurisée subissent des vibrations, dues notamment à l’écoulement dans l'espace séparant la cuve de l’enveloppe de cœur : leur compréhension et leur maîtrise constituent un enjeu tant scientifique que de sûreté.La description conventionnelle de ces vibrations repose sur un spectre de pression fluctuante de référence, des longueurs de corrélation longitudinales et circonférentielles, et l’élaboration d’un chargement modal sur la base de l’intégrale de joint acceptance. Elle suppose une pression turbulente homogène sur la surface du cylindre interne, hypothèse a priori non adaptée à la configuration industrielle, avec la présence de trois ou quatre branches d'admission.Via une démarche couplée expérimentale – numérique, l’objectif est de quantifier les conséquences de cette simplification et d’améliorer la représentation du chargement, en s’appuyant sur un cas de référence élémentaire, constitué de deux cylindres co-axiaux soumis à deux jets impactants perpendiculaires à l’axe des cylindres.Sur une maquette en eau modulable conçue à cet effet, après la caractérisation, au moyen de capteurs de pression fluctuante en paroi, du chargement turbulent avec cylindre interne fixe, les vibrations obtenues en disposant le cylindre interne sur des languettes flexibles sont analysées : la forte inhomogénéité des vitesses de convection (entre 0 et 4 m/s), ainsi que des spectres de pression fluctuante (plusieurs ordres de grandeur selon la fréquence) et de leurs longueurs de corrélation (évolutions différentes selon la fréquence et variations du simple au double) numériquement prédite, est effectivement observée, selon un accord calculs-essais particulièrement remarquable.L’ensemble des résultats d’essais et de simulations LES constitue ainsi un cas-test de référence bien documenté de vibrations sous écoulement turbulent en espace annulaire avec admissions impactantes. Il servira de base pour des modélisations numériques et un moyen d’essai représentant des configurations plus complexes. (10.70675/9e1ef11ez2cacz4db9z95cdzd4f13b0914ba)
  DOI : 10.70675/9e1ef11ez2cacz4db9z95cdzd4f13b0914ba
• Analyse comparative des modèles champs de phase pour la simulation de la propagation de fissure en milieux anisotropes
  
  • Loiseau Flavien
  • Lazarus Veronique
  , 2024. L'étude de la fissuration dans les milieux anisotropes est de plus en plus importante en mécanique des structures. Notamment, les procédés de fabrication additive, appréciés pour leur capacité de réparabilité, sont en plein essor. Afin de garantir que leur emploi est sécurisé, il est nécessaire de comprendre et de modéliser leur dégradation mécanique. Cependant, la modélisation de la fissuration dans ces matériaux reste partielle. Bien que certains modèles puissent représenter qualitativement la fissuration, peu d'études quantitatives sont disponibles à ce jour. Cette étude propose une analyse comparative de différents modèles de simulation par champs de phase pour la propagation des fissures en milieux anisotropes. L'objectif est de développer des simulations prédictives précises et efficaces pour ces phénomènes. Une première analyse qualitative des modèles existants est réalisée pour identifier leurs similarités et différences, en termes de modélisation mécanique et d'implémentation numérique. Ensuite, différents modèles sont calibrés sur des données expérimentales existantes obtenues sur des éprouvettes fabriquées par dépôt de fil fondu. Les prédictions des différents modèles sont alors comparées aux résultats expérimentaux. Les résultats attendus de cette étude incluent une meilleure compréhension des performances relatives des différents modèles de simulation par champs de phase, ainsi que des recommandations pour leur utilisation dans des contextes pratiques. Cette recherche pourrait ainsi contribuer à améliorer la conception et la durabilité des structures produites par fabrication additive.
• Turbulence-induced vibration in annular flow of a rigid cylinder mounted on a cantilever beam
  
  • Lagrange Romain
  • Plado Costante Loucas
  • Kocher Maud
  Journal of Fluids and Structures, Elsevier, 2024, 131, pp.104213. <div><p>This study investigates the fluid-structure interaction of two coaxial cylinders separated by a Newtonian fluid under turbulent axial flow. The theoretical framework treats the inner cylinder as a rigid body mounted on a flexible blade modeled as a Rayleigh beam. The goals of this study are to determine the free vibration modes and frequencies, identify the fluid-elastic instability threshold, and establish an analytical expression for the mean-square displacement of the structure. The approach integrates various fluid forces and torques, such as Archimedean thrust, fluid-elastic forces for a quiescent fluid, fluid-elastic forces due to flow, and the effects of fluid turbulence. The new approach reveals that vibration modes, frequencies, instability thresholds, and meansquare displacement each depend on a different set of dimensionless parameters: 8, 11, and 12, respectively. These parameters include the cylinder aspect ratio and fluid gap radius ratio. By incorporating models from the literature for viscous friction coefficients, turbulent pressure power spectral density, and coherence function, the study demonstrates stability conditions and the scaling of mean-square displacement with Reynolds number squared. The study, presented in a fully dimensionless formulation, aims to assist engineers in constructing small-scale experiments representative of pressure vessel vibrations. To facilitate this, a Python code for system stability determination and mean-square displacement calculation is provided.</p></div> (10.1016/j.jfluidstructs.2024.104213)
  DOI : 10.1016/j.jfluidstructs.2024.104213
• Austenite-martensite interfacial patterns and energy dissipation of phase transformation in Ni-Mn-Ga single crystal
  
  • Zhang Chengguan
  • Chen Xue
  • Gao Xingke
  • Brisset François
  • Hubert Olivier
  • He Yongjun
  Acta Materialia, Elsevier, 2024, 281, pp.120418. The martensitic phase transformation occurs in Shape Memory Alloys (SMA) via the nucleation/propagation of the Austenite-Martensite interface (A-M interface), which is a transition region (domain) between the two coexisting phases. For providing compatibility, the transition region consists of various martensitic twin structures (laminates), forming different interfacial patterns, such as parallel laminates and branching laminates. Due to the energy accumulation in the interfacial structures (e.g., elastic mismatch and twin-boundary surface energy), the interfacial patterns should be relevant to the driving force (energy dissipation) and the associated kinetics of the phase transformation. In this paper, we adopt a special thermal loading, small-temperature-gradient "heatingcooling-reheating", to control the A-M interface's forward and reverse propagation to generate different interfacial patterns in a Ni-Mn-Ga single crystal SMA with the observation on the twin structures (by optical microscope and SEM) and the InfraRed measurement on the temperature hysteresis of the interface propagation (for characterizing the thermal driving force). Simple energetic analysis indicates that the thermal driving force (energy dissipation) is directly related to the stored energy in the interfacial structure, particularly the large mismatch elastic energy near the habit plane. This study not only provides the details of the various interfacial patterns and their dependence on the 2 / 34 loading path, but also indicates the driving force and the associated mechanism about the pattern evolution to understand the phase transformation process. (10.1016/j.actamat.2024.120418)
  DOI : 10.1016/j.actamat.2024.120418
• Phase field fracture: Towards a generic method to follow the equilibrium path of the structure
  
  • Loiseau Flavien
  • Lazarus Veronique
  , 2024.
• Seismic bearing capacity of shallow foundations on soil reinforced by rigid inclusions
  
  • Shen Y
  • Pérez-Herreros J
  • Cuira F
  • Burlon S
  • Semblat J-F
  , 2024. The kinematic exterior approach, in the framework of the yield design theory, is widely utilised for assessing foundation bearing capacity. This study aims at extending this approach to shallow foundations on reinforced soil by rigid inclusions. The interaction diagram in the (V, H, M) space is obtained by combining the interaction curves derived from different failure mechanisms. The resisting forces provided by the rigid inclusions are evaluated using a multicriterion approach and are considered at the intersection of the inclusions with the failure surface. Furthermore, the soil inertia due to the horizontal seismic excitation is also considered in the analysis. The results reveal that the bearing capacity under seismic load is unaffected in the zone of interest for foundations with a sufficient safety factor under static loading.
• Development of a New Laboratory Earthquake Setup Featuring a Paraffin oil-based Gel as Analogue Material
  
  • Aoude Abdallah
  • Stefanou Ioannis
  • Semblat Jean-François
  • Rubino Vito
  , 2024. In this study, we present the development of a new experimental setup composed of an analogue fault surrounded by paraffin oil-based gel, which allows us to simulate earthquake-like events in the laboratory. The apparatus is designed to test the possibility of mitigating earthquake-like instabilities using control theory. We present the physical properties of the paraffin oil-based gel as functions of strain rate, strain, and temperature. Our results show a linear relation between the stress and strain up to 30% shear strain, along with a low viscosity at high strain rate. Furthermore, we engineered the frictional properties of the analogue fault using 3D-printed patches placed along its surface. Finally, an experimental earthquake simulation, using the setup, demonstrates a sudden slip event within the gel, propagating at a speed between cs and 1.41 cs, where cs represents the shear wave velocity of the gel, which is consistent with theoretical and previous experimental results. (10.11159/icceia24.140)
  DOI : 10.11159/icceia24.140
• Far-field sound field estimation using robotized measurements and the boundary elements method
  
  • Pascal Caroline
  • Marchand Pierre
  • Chapoutot Alexandre
  • Doaré Olivier
  , 2024, 270 (11), pp.816-827. Sound Field Estimation (SFE) is a numerical technique widely used to identify and reconstruct the acoustic fields radiated by unknown structures. In particular, SFE proves to be useful when data is only available close to the source, but information in the whole space is required. However, the practical implementation of this method is still hindered by two major drawbacks: the lack of efficient implementation of existing numerical methodologies, and the time-consuming and tedious roll-out of acoustic measurements. This paper aims to provide a solution to both issues. First, the measurements step is fully automated by using a robotic arm, able to accurately gather geometric and acoustic data without any human assistance. In this matter, a particular attention has been paid to the impact of the robot on the acoustic pressure measurements. The sound field prediction is then tackled using the Boundary Element Method (BEM), and implemented using the FreeFEM++ BEM library. Numerically simulated measurements have allowed us to assess the method accuracy, and the overall solution has been successfully tested using actual robotized measurements of an unknown loudspeaker (10.3397/IN_2024_2661)
  DOI : 10.3397/IN_2024_2661
• Perceptual evaluation of sound synthesis of wind turbine noise
  
  • Meunier Sabine
  • El Sawaf Ossen
  • Marquis-Favre Catherine
  • Couzon Gauthier
  • Strauss Nicolas
  • Cotté Benjamin
  • Mascarenhas David
  • Chatron Jacques
  , 2024, pp.2426-2433. This study brings together several laboratories with the aim of assessing the health effects of "audible" noise (&gt; 20 Hz) and infrasound (&lt;20 Hz) emitted by wind turbines. To study loudness and annoyance due to this noise, perceptual tests are planned at the LMA, where a restitution cabin has been developed, specifically designed to diffuse very low frequencies and infrasound. As recording wind turbine noise is only possible at low wind speeds for a good quality sound reproduction, it would be interesting to be able to use sound synthesis of wind turbine noise. From sounds recorded in a wind farm for different meteorological conditions, the corresponding sounds have been synthesized. A physical model synthesis was performed, based on an extended-source aeroacoustic model taking into account propagation over flat ground. Dissimilarity tests including recorded and synthesized sounds enabled a 2D perceptual space to be built. Synthesized and the corresponding recorded sounds are closed together in the perceptual space, but some differences can be perceived, mainly due to difference in amplitude of fluctuation and spectral balance. The analysis of the perceptual space opens up interesting prospects for improving the sound synthesis and its use for future perceptual tests. (10.3397/IN_2024_3181)
  DOI : 10.3397/IN_2024_3181
• Computation of Green's functions for the acoustic scattering by an elastic structure excited by a turbulent flow in water
  
  • Pacaut Louise
  • Serre Gilles
  • Mercier Jean-François
  • Chaillat Stéphanie
  • Cotté Benjamin
  , 2024, 270 (5), pp.5995-6006. To model the hydrodynamic noise produced by an elastic ship hull or propeller excited by a turbulent boundary layer, we need an efficient method to compute the acoustic scattering by an elastic body surrounded by a fluid. In 3D, Boundary Element Methods (BEM) are used to reduce the computational costs, for both the fluid and the elastic body. A natural way to compute the boundary integral representation (BIR) of the sound pressure is to use formulations based on the free space acoustic and elastic Green's functions. However, since the turbulent flow along the elastic body is known only statistically, the use of these Green's functions would be too expensive. A remedy is to compute a Green's function adapted to the physical problem, thus satisfying the transmission conditions of the fluid-structure problem. This so-called "tailored Green's function" is determined by solving a coupled acoustic-elastic problem with the BEM, and leads to a simplified BIR of the sound pressure compatible with a stochastic source term. We first validate the computation of the tailored Green's function over a classic spherical geometry. Then we compare the scattering of multiple quadrupoles by elastic or rigid NACA0012 profiles. (10.3397/IN_2024_3671)
  DOI : 10.3397/IN_2024_3671
• Toughening effects of out-of-crack-path architected zones
  
  • Triclot Julie
  • Corre Thomas
  • Lazarus Véronique
  • Gravouil Anthony
  International Journal of Fracture, Springer Verlag, 2024, 248, pp.237-255. The increasing use of architected materials has broadened the possibilities of mechanical behaviour. In this article, we aim to explore these new possibilities in terms of in-service behaviour, especially in terms of crack propagation by performing an in-depth study in the framework of Linear Elastic Fracture Mechanics (LEFM). The specific configuration studied here is the case where the architected zones are symmetrically positioned adjacently to the crack path and no propagation occurs within the zone. This problem is addressed both numerically and experimentally. Numerically, an path-following algorithm is used to simulate the crack propagation. Different toughening aspects of the addition of architected zones are identified. First, a temporary increase in crack propagation resistance is shown. It comes from a temporary increase of stored elastic energy in the architected zones, thus acting as mechanical springs. Second, a snap-back instability appears, linked to the release of the previously stored energy. It leads to a higher energy dissipated by the crack propagation process. Experimentally, we evidence the possibility to reproduce the theoretical results using 3D printing. A good quantitative comparison is obtained between numerical and experimental approaches. This study shows that it is possible to improve crack propagation resistance while lightening the component by addition of architected zones outside the crack path. This opens up the way to tune finely, through the use of optimization tools, the crack propagation response. (10.1007/s10704-024-00811-5)
  DOI : 10.1007/s10704-024-00811-5
• Extending the Discrete Element Method to Account for Dynamic Confinement and Strain-Rate Effects for Simulating Hard Impacts on Concrete Targets
  
  • Antoniou A.
  • Daudeville L.
  • Marin P.
  • Potapov S.
  Journal of Dynamic Behavior of Materials, Springer Verlag, 2024. Concrete plays a pivotal role as a foundational material in critical infrastructure, particularly in nuclear plants. Given the imperative for robustness and safety in such contexts, the design of concrete structures necessitates methodologies capable of precisely predicting damage resulting from impacts. When subjected to impact, concrete experiences high loading rates and significant triaxial stresses in the vicinity of the impacting object, potentially resulting in fragmentation, pore closure and projectile penetration. In addressing these challenges, the discrete element method (DEM) emerges as a suitable approach primarily due to its inherent ability to model discontinuities such as cracking and fragmenta-tion. Within this framework, DEM employing spherical discrete elements (DE) has been implemented into Europlexus, a fast transient dynamics finite element (FE) code. This paper presents a refined constitutive DEM model for concrete, especially accounting for porosity closure under high confine-ment and the effect of strain-rate on tensile strength and fracture energy. The calibration of constitutive parameters is conducted through the simulations of a series of quasi-static (QS) tests encompassing tension, compression and triaxial compression. The strain-rate dependency parameters are identified through dynamic tensile tests conducted using a split-Hopkinson pressure bar apparatus. The whole constitutive model is presented as well as its calibration. Finally, the validation of the DEM approach is demonstrated through simulations of penetration and perforation tests conducted on concrete targets. (10.1007/s40870-024-00438-6)
  DOI : 10.1007/s40870-024-00438-6
• Caractérisation et modélisation de la tenue à la fatigue et à la corrosion de l’acier inoxydable super duplex 2507 obtenu par fabrication additive LP-DED
  
  • Ballésio Sébastien
  , 2024. L'objectif principal de cette thèse est de proposer une étude approfondie du couple "Acier inoxydable super duplex 2507 – procédé Laser Powder-Direct Energy Deposition (LP-DED)" et d'établir une démarche d'optimisation du processus de fabrication (avec ou sans traitement thermique) en termes de propriétés mécaniques et électrochimiques. Pour atteindre cet objectif, plusieurs étapes sont entreprises.Tout d'abord, des critères pour évaluer la qualité du matériau déposé par fabrication additive sont définis. Cela aboutit à l'optimisation des paramètres d'impression en utilisant la machine LP-DED BeAM Mobile-004 et la poudre d'acier inoxydable super duplex SAF2507.Ensuite, l'impact du procédé sur la microstructure est évalué ainsi que de divers traitements thermiques appliqués à la matière brute fabriquée par procédé LP-DED.Une comparaison avec un acier forgé de même nuance est également proposé et permet de mettre en perspective les différences et points communs entre les procédés conventionnels et la fabrication additive métallique.Des essais de traction uniaxiale, de dureté, de corrosion par piqûre et des campagnes d'essais de fatigue sont alors menés sur les différents matériaux obtenus. Cette phase expérimentale permet d'étudier l'influence de la microstructure et d'observer les variations des propriétés examinées.Enfin, en ouverture, un modèle est proposé pour décrire le comportement en fatigue observé sur l'acier inoxydable super duplex 2507. Ce modèle tient compte du caractère biphasique du matériau, chaque phase présentant un comportement spécifique. La proportion de chaque phase est intégrée pour tenter de refléter les contrastes observés lors des campagnes d'essais de fatigue par auto-échauffement en fonction de la microstructure du matériau.Cette thèse contribue ainsi à une meilleure compréhension et optimisation du procédé de fabrication additive LP-DED pour l’acier inoxydable super duplex 2507, en mettant en lumière l'importance de la microstructure sur les propriétés mécaniques et électrochimiques du matériau.Elle propose ainsi les procédés et leurs limites pour aboutir à un matériau imprimé avec les propriétés attendues. (10.70675/7b7ffc53z7dc4z426azb39dzba3f31535526)
  DOI : 10.70675/7b7ffc53z7dc4z426azb39dzba3f31535526
• Fissuration par champ de phase : Suivre le chemin d'équilibre de la structure
  
  • Loiseau Flavien
  • Lazarus Veronique
  , 2024.
• Quantification of microscale factors for fatigue failure in NiTi shape memory alloys
  
  • Ju Xiaofei
  • Moumni Ziad
  • Borbély András
  • Zhang Yahui
  • Zhong Shengyi
  Journal of Materials Research and Technology, Elsevier, 2024, 31, pp.1 à 5. Fatigue behavior is intrinsically linked to microstructural alterations induced by cyclic loading. However, the quantification of microstructural defects associated with fatigue damage of NiTi shape memory alloys (SMAs) is lacking, which hinders the development of a physically based fatigue criterion. To this end, a multi-scale experimental analysis was conducted on cyclically deformed NiTi SMAs, which evidenced a strong correlation between microstructural inhomogeneity and localized deformation behavior. The microstructural change associated with fatigue was quantified in terms of stored strain–energy, with the highest values observed in the regions where fatigue cracks initiate. Consequently, stored energy is deemed as an effective fatigue indicator, offering valuable insights for future work in the design and optimization of SMAs’ structures against fatigue. (10.1016/j.jmrt.2024.05.260)
  DOI : 10.1016/j.jmrt.2024.05.260
• Capacité portante des fondations superficielles sur sol renforcé par inclusions rigides
  
  • Shen Yuxiang
  • Pérez-Herreros Jesús
  • Cuira Fahd
  • Semblat Jean-François
  • Burlon Sébastien
  , 2024. Cette étude porte sur la détermination de la capacité portante des fondations superficielles sur sol renforcé par inclusions rigides via l'approche cinématique du calcul à la rupture. Une comparaison entre la capacité portante estimée et la somme des capacités portantes individuelles des inclusions et de la semelle révèle que cette dernière donne une surestimation significative. Des facteurs correcteurs liés à l'inclinaison et à l'excentrement de la charge sont également étudiés.
• A damage-based wear model using the thick level set approach
  
  • Caradec Quentin
  • Maitournam Habibou
  • Breuzé Matthieu
  • Stolz Claude
  , 2024.
• Développement d'une méthode DEM polyédrique pour la simulation de la relocalisation du combustible nucléaire lors d'un APRP
  
  • Bessiere Thibault
  • Potapov Serguei
  • Lafon Philippe
  • Ambard Antoine
  • Radjai Farhang
  , 2024. Lors d’un Accident de Perte de Réfrigérant Primaire (APRP), les crayons combustibles sont soumis à des sollicitations thérmo-mécaniques intenses. Celles-ci peuvent provoquer un ballonnement ponctuel de la gaine dans lequel le combustible, présent dans un état fragmenté, peut se relocaliser radialement et axialement et induire un état de température élevé conduisant à la rupture de la gaine, donc de la première barrière de sûreté. Des recherches visent à étudier cet effet grâce à des modèles numériques avancés. Ici, on propose d’assimiler le combustible fragmenté à un milieu granulaire et de mettre en œuvre une modélisation par éléments discrets de la relocalisation des fragments. L’objectif final étant de prendre en compte l’interaction entre fragments et gaz de fission ainsi que la rupture de la gaine et l’expulsion des fragments, un premier objectif est de reprendre la méthode des éléments discrets (DEM) disponible dans le code Europlexus, et de l’étendre à des géométries polyédriques, en s’inspirant d’une méthode utilisée dans le code Rockable basée sur des sphéro-polyèdres.
• Numerical analysis of an enriched beam model
  
  • Sharifi Mahshid
  • Daude Frédéric
  • Stolz Claude
  , 2024. The finite element for a thin-walled tube developped in [1] is considered. A classical EulerBernoulli beam element has been enriched in order to take into account the influence of the rotations of the mid-surface on the local deformations of the tube cross-sections. The latter is a semi-analytical model using a Fourier expansion based on the circumferential variable. A grid convergence study of the enriched model is carried out for different mesh discretisations. Next, the stability of the explicit time integration is studied using the lumped mass matrix and the critical time steps associated to the mass matrix lumping are estimated.
• Application of a generic path-following method to phase-field fracture
  
  • Loiseau Flavien
  • Lazarus Veronique
  , 2024. The phase-field approach to fracture has emerged as a powerful tool to simulate the nucleation and growth of cracks in a structure. In the past two decades, it has been extensively applied to fracture problems as it captures crack initiation, propagation, and interaction without explicitly tracking the crack path. One of the most popular algorithms to solve phase-field problems is alternate minimization. However, it can suffer from slow convergence, especially when dealing with unstable crack propagation. Moreover, force-controlled loading often leads to unstable crack propagation and the lack of equilibrium solution after the crack propagation, preventing their use. Path-following methods offer a promising solution to those limitations, enabling the tracking of unstable crack propagation while preserving the equilibrium during the whole loading (Rastiello et al., 2022). Based on various control strategies, these methods also improve the solver stability. Singh et al. (2016) and May et al. (2016) proposed path-following approaches specifically tailored to the resolution scheme of Miehe et al. (2010) based on crack surface and fracture dissipation. Additionally, Wu (2018) adapted the nodal displacement control (Borst, 1987) and the fracture surface control (Singh et al., 2016) to the alternate minimization. Nevertheless, the first approach is problem-dependent, and the second approach may fail under force loading (Rastiello et al., 2022). This work proposes a generic path-following method applicable to various fracture problems, regardless of geometry, boundary conditions, or fracture model complexity, by leveraging the maximum strain increment control (Chen &amp; Schreyer, 1990). This method is model-independent, as it relies solely on the displacement field, and problem-independent, it does not rely on a specific choice of control DOF. After presenting the modified alternate minimization solver, we demonstrate its effectiveness through simulations of crack propagation in the SENT test. The results are compared to a semi-analytical solution based on LEFM and to the alternate minimization solution. Notably, the classic alternate minimization fails to capture the snap-back (instability under displacement control) observed in the semi-analytical method. The proposed approach correctly capturesthis phenomenon, which converges towards the semi-analytical solution. Then, this method is also applied to the simulation of Compact Tension (CT) experiments, in which the selection of numerical boundary conditions at the pinhole significantly influences the fracture behavior (Triclot et al., 2023). The proposed solver renders the application of force boundary conditions possible, better representing the experimental conditions. References Borst, R. de. (1987). Computation of post-bifurcation and post-failure behavior of strain-softening solids. Computers &amp; Structures, 25(2), 211–224. https://doi.org/10.1016/0045-7949(87)90144-1 Chen, Z., &amp; Schreyer, H. L. (1990). A numerical solution scheme for softening problems involving total strain control. Computers &amp; Structures, 37 (6), 1043–1050. https://doi.org/10.1016/0045-7949(90)90016-U May, S., Vignollet, J., &amp; Borst, R. de. (2016). A new arc-length control method based on the rates of the internal and the dissipated energy. Engineering Computations, 33(1), 100–115. https://doi.org/10.1108/EC-02-2015-0044 Miehe, C., Hofacker, M., &amp; Welschinger, F. (2010). A phase field model for rate-independent crack propagation: Robust algorithmic implementation based on operator splits. Computer Methods in Applied Mechanics and Engineering, 199(45), 2765–2778. https://doi.org/10.1016/j.cma.2010.04.011 Rastiello, G., Oliveira, H. L., &amp; Millard, A. (2022). Path-following methods for unstable structural responses induced by strain softening: A critical review. Comptes Rendus. Mécanique, 350, 205–236. https://doi.org/10.5802/crmeca.112 Singh, N., Verhoosel, C. V., Borst, R. de, &amp; Brummelen, E. H. van. (2016). A fracture-controlled path-following technique for phase-field modeling of brittle fracture. Finite Elements in Analysis and Design, 113, 14–29. https://doi.org/10.1016/j.finel.2015.12.005 Triclot, J., Corre, T., Gravouil, A., &amp; Lazarus, V. (2023). Key role of boundary conditions for the 2D modeling of crack propagation in linear elastic compact tension tests. Engineering Fracture Mechanics, 277, 109012. https://doi.org/10.1016/j.engfracmech.2022.109012 Wu, J.-Y. (2018). Robust numerical implementation of non-standard phase-field damage models for failure in solids. Computer Methods in Applied Mechanics and Engineering, 340, 767–797. https://doi.org/10.1016/j.cma.2018.06.007
• Manufacturing of 2507 super duplex stainless steel by laser powder-directed energy deposition: process optimization and microstructure analyses
  
  • Ballésio Sébastien
  • Le Hong Thai
  • Dhondt Matthieu
  • Doudard Cédric
  • Szmytka Fabien
  International Journal of Advanced Manufacturing Technology, Springer Verlag, 2024, 132 (11-12), pp.5663-5682. This paper presents a thorough manufacturability study of 2507 super duplex stainless steel (SDSS) produced by the Laser Powder-Directed Energy Deposition (LP-DED) process. First, experimental observations on single-track surface morphology and geometrical characteristics are described. Then, dimensional and porosity analyses of thin walls and bulk samples are presented and discussed in order to identify the optimal process parameter combinations, from a process point of view, for further characterizations to determine the general properties of LP-DED-printed samples. Finally, the results obtained on wrought steel of the same grade are compared to those of the printed material in terms of microstructure, texture, and ferrite/austenite ratio. A set of meticulously chosen process parameters with regard to the process and the machine (nozzle size, maximum power deliverable) gives the optimal results for the as-built LP-DED material in terms of porosity and microstructure quality but with smaller grains (irrespective of the phase studied) and a higher proportion of ferrite (about 75%). (10.1007/s00170-024-13673-x)
  DOI : 10.1007/s00170-024-13673-x