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Cycloadditions of π-allylpalladium(II) intermediates towards medium-sized N-heterocycles
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Dynamics of shock induced bubble formation at the bottom of a liquid filled tank
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Impact of curing on residual stresses formation and fatigue behaviour of carbon/epoxy laminate composites - application to racing yachts
No full textInternational audienceOne of the levers to gain in performance of composite structures used for racing yachts (IMOCA, America's Cup) is by a better knowledge between the manufacturing process and the final properties. This study addresses one aspect of this objective. The objective is to study the impact of the manufacturing process on the fatigue behaviour of carbon/epoxy laminate. More precisely, the work aimed at understanding the link between the curing parameters, the residual stresses in the composite laminate and the fatigue behaviour. This work is carried on in collaboration with two companies from the Brittany Sailing Valley dealing with the design and manufacturing of composite parts
Fatigue life study of laminated composites carbon-epoxy manufactured from unidirectional plies and 2D-woven plies combining thermo-mechanical analysis and a residual strength model
No full textInternational audienceLong-term durability under repeated mechanical loading is a major criterion for design of composite structures. The development of a fatigue criterion faces two major difficulties: (i) the proposal of a load/life curve (S-N curve) describing the natural dispersion of the fatigue tests and (ii) the duration of these tests. The goal of the study is to combine the Sendeckyj's approach based on a so-called residual strength that fits S-N curve and associates a failure probability [1], and a self-heating-based method that estimate a mean S-N curve from the thermal dissipation, determined from temperature measurements on a single sample [2]. The fatigue lifetime for high cycles loading is estimated by the heat-build up protocol. Quasi-static tests and a few fatigue tests are used to estimate the failure distribution based on a two-parameters Weibull model
Mixité. 25% de filles en Numérique et sciences informatiques : une construction sociale ?
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Stochastic incremental mirror descent algorithms with Nesterov smoothing
No full textInternational audienceWe propose a stochastic incremental mirror descent method constructed by means of the Nesterov smoothing for minimizing a sum of finitely many proper, convex and lower semicontinuous functions over a nonempty closed convex set in a Euclidean space. The algorithm can be adapted in order to minimize (in the same setting) a sum of finitely many proper, convex and lower semicontinuous functions composed with linear operators. Another modification of the scheme leads to a stochastic incremental mirror descent Bregman-proximal scheme with Nesterov smoothing for minimizing the sum of finitely many proper, convex and lower semicontinuous functions with a prox-friendly proper, convex and lower semicontinuous function in the same framework. Different to the previous contributions from the literature on mirror descent methods for minimizing sums of functions, we do not require these to be (Lipschitz) continuous or differentiable. Applications in Logistics, Tomography and Machine Learning modelled as optimization problems illustrate the theoretical achievements
Dynamic properties and toughening mechanisms of GNPs reinforced carbon fibers/epoxy textile composites under an SHPB impact load
No full textInternational audienceIn the recent years, the mechanical and damping properties of fiber reinforced polymer composites have been improved by the addition of nanofillers such as titanium oxide, alumina oxide, carbon nanotube, and nanoclay. Recently, graphene nanoplatelets (GNPs) due to its exceptional characteristics, is getting significant attention. The literature that is currently available shows that adding graphene nanoplatelets can improve the stiffness, strength, and fracture toughness of fiber reinforced polymer composites when they are subjected to static and quasi-static loadings. However, there is currently a lack of information regarding how the incorporation of graphene nanoplatelets (GNPs) to the epoxy matrix can affect the dynamic response of carbon-epoxy composites, such as dynamic stress-strain behavior, stiffness, strength, and damage kinetics. The aim of this study is to examine the dynamic behavior of a composite made of carbon fibers and epoxy matrix filled with different mass fractions (0% as a reference, 0.5%, 1%, and 2%) of GNPs using Split Hopkinson Pressure Bar (SHPB). When compared to the findings obtained with carbon fiber reinforced polymer (CFRP) composites, the experimental results showed that adding up to 1% of GNPs resulted in significant improvements in strength and stiffness as well as decreases in delaminated area. However, a further increase in the mass fraction of GNPs in the epoxy matrix could lead to the presence of agglomerations and the non-uniform distribution of the filled resin between the carbon fibers, resulting in weak interfacial bonding, reducing the performance of the final composite, and leading to delamination failure
Modèles mécaniques de poutre enrichis pour la simulation de tubes minces sous pression
No full textThe phenomenon of whipping is one of the potential consequences of the accidental of a high energy ligne break (HELB), it is part of the loading cases taken into account in the framework of internal aggression files of nuclear power plants. Whipping consequences must be evaluated for the safety of industrial installations since it involves large displacements of the severed piping which could, depending on the size, impact the neighbouring structures and components. As these are complex physical phenomena, the characterization of the pipe rupture and the associated whipping is currently based on simplified approaches using conservative assumptions. In some cases, these assumptions make difficulties, which can lead to costly modifications. In order to answer these stakes, it is proposed in this research project to improve the tools and methods currently available for the simulation of the whipping phenomenon in order to make more realistic the modeling of the preponderant physical phenomena and in particular their coupling. The final objective is thus to quantify the displacements of the broken piping as well as its impact on the structures and on the materials implying a potential loss of integrity or operability. For this, it is necessary to take into account in the modeling a large number of phenomena. In particular, it is necessary to model the following events: the rupture of the pipe, the force of the jet initiated at the breach, the displacement of the severed pipe, the impact of the latter with the neighbouring structures, the plastic deformations following this impact and the possible formation of a plastic hinge as well as the interaction between the fluid contained in the pipe and the structure. Numerical studies have been carried out using three-dimensional modeling, which is costly in terms of calculation and simulation time. It is therefore desired to model in a simplified way the dynamics of the ruptured pipe in order to allow a quick evaluation of the safety conditions. In particular, it has been shown that, in the current state of modelling, it is necessary to model the impact zone of the pipe with non-linear shell elements in order to find a correct estimation of the impact force of the pipe on the obstacle. This is why it is first proposed to develop a simplified model of enriched beam type (i.e. one-dimensional wire type), taking into account the ovalization of the pipe according to different loading cases. Thus an enriched beam type finite element taking into account the ovalization of its transverse section is developed, whether it is a straight beam or one with an initial curvature as is the case of an elbow. The kinematics retained is a classical Euler-Bernoulli beam kinematics to which is added a Love-Kirchhoff type shell kinematics, developed to the first order in the thickness of the tube and in Fourier series according to the tangential variable in order to remain in a wire formalism. Moreover, a non-linear coupling, between the beam rotations and the shell displacements of the section, is introduced in order to allow the tube to ovalize even on a straight part. The described model is then discretized and implemented in the industrial dynamic explicit calculation code Europlexus. In order to verify and validate the model, the pipe is subjected to different loading cases such as simple extension, pure bending, the pipe under internal pressure or the application of a localized surface force on the transverse section allowing large deformations of the cross-section. The numerical results obtained are in good agreement with the corresponding analytical or numerical solutions obtained with other simulation codes.Le phénomène de fouettement est une des conséquences potentielles de la rupture accidentelle d'une tuyauterie haute énergie (RTHE). Le fouettement doit être pris en compte dans les études sur la sûreté des installations industrielles puisqu'il suppose de grands déplacements de la tuyauterie sectionnée qui pourrait, en fonction de l'encombrement, impacter les structures et composants avoisinants. S'agissant de phénomènes physiques complexes, la caractérisation de la rupture de la tuyauterie et du fouettement associé est fondée actuellement sur des approches simplifiées reposant sur des hypothèses conservatives. Ces hypothèses peuvent alors conduire à des difficultés, ce qui est résolu par l'introduction de modifications au niveau de ces installations, modifications qui peuvent s'avérer coûteuses. De façon à répondre à ces enjeux, il est proposé dans ce projet de recherche un travail d'amélioration des outils et méthodes actuellement disponibles pour la simulation du phénomène de fouettement afin de rendre plus réaliste la modélisation des phénomènes physiques prépondérants et en particulier leur couplage. Pour cela, il est nécessaire de prendre en compte dans la modélisation un grand nombre de phénomènes. Il s'agit, en particulier, de modéliser les évènements suivants : la rupture de la tuyauterie, la force du jet initié à la brèche, le déplacement de la tuyauterie sectionnée, l'impact de celle-ci avec les structures avoisinantes, les déformations plastiques suite à cet impact et la formation éventuelle d'une rotule plastique ainsi que l'interaction entre le fluide contenu dans la tuyauterie et la structure. De nombreuses études ont été réalisées en modélisation tridimensionnelle, laquelle s'avère coûteuse en temps de calcul et de simulation. Il est donc souhaité de modéliser de façon simplifiée la dynamique de la tuyauterie rompue afin de permettre une évaluation rapide des conditions de sûreté. Il a en particulier été montré, qu'en l'état actuel des modélisations, il était nécessaire de modéliser la zone d'impact de la tuyauterie en éléments coques non-linéaires pour retrouver une estimation correcte de la force d'impact de la tuyauterie sur l'obstacle. Ainsi un élément fini de type poutre enrichi permettant de prendre en considération l'ovalisation de sa section transverse est développé, que ce soit sur une poutre droite ou sur une poutre ayant une courbure initiale comme dans le cas d'un coude. La cinématique retenue est une cinématique classique de poutre d'Euler-Bernoulli à laquelle s'ajoute une cinématique de coque de type Love-Kirchhoff, développée au premier ordre dans l'épaisseur du tube et en séries de Fourier selon la variable tangentielle afin de rester dans un formalisme filaire. De plus un couplage non linéaire, entre les rotations de type poutre et les déplacements de la section de type coque, est introduit afin de permettre au tube de s'ovaliser même sur une partie droite. Le modèle décrit est alors discrétisé et implémenté dans le code de calcul industriel de dynamique rapide explicite Europlexus. Afin de vérifier et de valider le modèle, la tuyauterie est soumise à différents cas de chargement tels que l'extension simple, la flexion pure, le tube sous pression interne ou encore l'application d'une force surfacique localisée sur la section transverse permettant de grandes déformations de la section. Les résultats numériques obtenus sont en bonne adéquation avec les solutions analytiques ou numériques issues d'autres codes industriels
Windowed Green function method for wave scattering by periodic arrays of 2D obstacles
No full textInternational audienceThis paper introduces a novel boundary integral equation (BIE) method for the numerical solution of problems of planewave scattering by periodic line arrays of two-dimensional penetrable obstacles. Our approach is built upon a direct BIE formulation that leverages the simplicity of the free-space Green function but in turn entails evaluation of integrals over the unit-cell boundaries.Such integrals are here treated via the window Green function method. The windowing approximation together with a finite-rank operator correction—used to properly impose the Rayleigh radiation condition—yield a robust second-kind BIE that produces superalgebraically convergent solutions throughout the spectrum,including at the challenging Rayleigh–Wood anomalies. The corrected windowed BIE can be discretized by means of off-the-shelf Nyström and boundary element methods, and it leads to linear systems suitable for iterative linear algebra solvers as well as standard fast matrix–vector product algorithms. A variety of numerical examples demonstrate the accuracy and robustness of the proposed methodology
Multi-objective hull form optimization of a SWATH configuration using surrogate models
No full textInternational audienceThe present study introduces a surrogate-based multi-objective hull form optimization of a SWATH configuration, enabling optimal hull design compromises between seakeeping performance and ship resistance. A parametric model of a SWATH ship is built, which has variable horizontal torpedoes semi-axis and strut angle of inclination. The displacement is assumed to be constant and is balanced with the vertical semi-axis of torpedoes. The objective seakeeping function is the amplitude of vertical movement on the ship's gangway, calculated in irregular waves. As the energy dissipation of SWATH ships are mainly generated by viscous effects, these are estimated using empirical formulas and are added to the equations of motion. The ship resistance is computed with a finite volume solver using a RANS model. Three levels of fidelity, having increasing computation costs, are considered to model ship resistance. The first low fidelity level concerns the wetted surface of the hull. Due to their geometry, SWATH ships can be destabilized by the Munk moment and be dynamically unstable, which can lead to instabilities in resistance calculations. The medium-fidelity level then considers a free sinkage and a fixed pitch to ensure the stability of the calculations. The third higher-fidelity level considers stabilizing fins to counterbalance the destabilizing moment. Trim angles of fins are solved to reach moment equilibrium and the fins drag is included in the total ship resistance, which is to be minimized. The multi-objective optimization problem is solved for these three degrees of fidelity. Results differences between fidelity level approaches are also compared, in order to highlight the impact on the optimal hull designs of using a low-fidelity method, with regard to the computational costs