Publications

2009

• A Brief Review of Important Features of Compression Tests with Hopkinson Bars
  
  • Gary Gérard
  , 2009, pp.21-31.
• Shearing effects on the breathing mechanism of a cracked beam section in bi-axial flexure
  
  • El Arem Saber
  European Journal of Mechanics - A/Solids, Elsevier, 2009, 28 (6), pp.1079-1087. The main purpose of this paper is to complete the works presented by Andrieux and Varé (2002) and El Arem et al. (2003) by taking into account the effects of shearing in the constitutive equations of a beam cracked section in bi-axial flexure. The paper describes the derivation of a lumped cracked beam model from the three-dimensional formulation of the general problem of elasticity with unilateral contact conditions on the crack lips. Properties of the potential energy and convex analysis are used to reduce the three-dimensional computations needed for the model identification, and to derive the final form of the elastic energy that determines the nonlinear constitutive equations of the cracked transverse section. We aim to establish a relation of behavior between the applied forces and the resulting displacements field vectors, which is compatible with the beams theory in order to allow the model exploitation for shafts dynamics analysis. The approach has been applied to the case of a cracked beam with a single crack covering the half of its circular cross section.
• SMART-I²: Spatial Multi-users Audio-visual Real Time Interactive Interface, a broadcast application context
  
  • Rébillat Marc
  • Corteel Etienne
  • Katz Brian F.G.
  , 2009, pp.1-4. SMART-I2 is a high quality 3D audio-visual interactive rendering system. In SMART-I2, the screen is also used as a multichannel loudspeaker. The spatial audio rendering is based on Wave Field Synthesis, an approach that creates a coherent spatial perception of sound over a large listening area. The azimuth localization accuracy of the system has been verifed by a perceptual experiment. Contrary to conventional systems, SMART-I2 is able to realize a high degree of 3D audio-visual integration with almost no compromise on either the audio or the graphics rendering quality. Such a system can provide benefits to a wide range of applications. Index Terms-- Audio-visual integration (10.1109/3DTV.2009.5069682)
  DOI : 10.1109/3DTV.2009.5069682
• Dissipative aspects in high cycle fatigue
  
  • Charkaluk Eric
  • Constantinescu Andrei
  Mechanics of Materials, Elsevier, 2009, 41 (5), pp.483-494. The fatigue crack initiation phenomenon in metals is essentially associated to the gliding of dislocations and the creation of slip bands which characterize irreversible damaging mechanisms at the grains scale. A multiscale approach is then interesting to establish a link between the scale of the structure submitted to cyclic loading and the scale of damage. Some stress based criteria were previously proposed in this framework based on the shakedown theories, as Dang Van or Papadopoulos ones. The same approach is developed in this paper in the usual Thermodynamics of Irreversible Processes (TIP) framework in order to study the link between dissipation, shakedown and fatigue damage. Some recent results in metals fatigue coming from infrared thermography experiments are then interpreted through the TIP; the proposed framework seems to be particularly relevant to define a more general feature for the study of the fatigue phenomenon. (10.1016/j.mechmat.2009.01.018)
  DOI : 10.1016/j.mechmat.2009.01.018
• Elasticity and Electrostatics of Plectonemic DNA
  
  • Clauvelin Nicolas
  • Audoly Basile
  • Neukirch Sébastien
  Biophysical Journal, Biophysical Society, 2009, 96 (9), pp.3716-3723. We present a self-contained theory for the mechanical response of DNA in single molecule experiments. Our model is based on a one-dimensional continuum description of the DNA molecule and accounts both for its elasticity and for DNA-DNA electrostatic interactions. We consider the classical loading geometry used in experiments where one end of the molecule is attached to a substrate and the other one is pulled by a tensile force and twisted by a given number of turns. We focus on configurations relevant to the limit of a large number of turns, which are made up of two phases, one with linear DNA and the other one with superhelical DNA. The model takes into account thermal fluctuations in the linear phase and electrostatic interactions in the superhelical phase. The values of the torsional stress, of the supercoiling radius and angle, and key features of the experimental extension-rotation curves, namely the slope of the linear region and thermal buckling threshold, are predicted. They are found in good agreement with experimental data. (10.1016/j.bpj.2009.02.032)
  DOI : 10.1016/j.bpj.2009.02.032
• Construction des lois de fatigue à partir de modèles de forces cohésives : cas de fissures en mode I
  
  • Abdelmoula Radhi
  • Marigo Jean-Jacques
  • Weller Thibaut
  Comptes Rendus. Mécanique, Académie des sciences (Paris), 2009, 337, pp.166-172. Nous reprenons la méthode de construction de lois de fatigue à partir de modèles de forces cohésives et nous l'étendons au cas d'une fissure soumise à un chargement cyclique de mode I. En prenant pour variable mémoratrice l'ouverture cumulée et pour densité d'énergie de surface celle associée au modèle de Dugdale, nous construisons explicitement la loi de fatigue reliant le taux d'avancée de fissure par cycle d /dN au facteur d'intensité des contraintes KI. En particulier on retrouve une loi de Paris avec un exposant 4, i.e. d /dN = CK4 I, pour des faibles valeurs de KI. On montre enfin que ce résultat se généralise à toute une famille d'énergies de surface de type Barenblatt, seule la constante C variant d'un modèle à l'autre. (10.1016/j.crme.2009.04.002)
  DOI : 10.1016/j.crme.2009.04.002
• Influence de l'eau libre sur la résistance en compression dynamique confinée des bétons
  
  • Forquin Pascal
  • Safa Kamal
  • Gary Gérard
  , 2009.
• Mise au point d'un essai de compaction dynamique et statique sur matériaux fragiles
  
  • Safa Kamal
  • Gary Gérard
  • Forquin Pascal
  , 2009.
• Extended topological sensitivity for defect identification
  
  • Bonnet Marc
  , 2009.
• Amélioration de la précision de la mesure de la température par thermographie. Application aux essais dynamiques
  
  • Negreanu Ionut
  • Cédelle Julie
  • Gary Gérard
  • Hervé P.
  • Mohr Dirk
  , 2009.
• The influence of indenter bluntness on the apparent contact stiffness of thin coatings
  
  • Korsunsky Alexander M.
  • Constantinescu Andrei
  Thin Solid Films, Elsevier, 2009, 517 (17), pp.4835-4844. n the present paper, the influence of punch tip sharpness on the interpretation of indentation measurements is considered. Firstly, in order to obtain analytical insight into the nature of the problem, closed form solutions are presented for the indentation of a homogeneous elastic half-space by an axisymmetric indenter of arbitrary shape, including Hertzian, conical, and conical indenter with a rounded tip. Next, a fast and efficient numerical implementation of a semi-analytical approach to the solution of problems about frictionless axisymmetric indentation of an elastic layer on a dissimilar substrate is described. The approach allows rapid determination of the load–displacement curves for an arbitrary punch shape. The aim of the study was to explore the implications of a finite indenter tip radius for the problem of property identification of thin films. The variation of the apparent substrate stiffness with indentation depth was established for several generic indenter shapes, namely for flat, conical and parabolic punches, and for a conical punch with a rounded tip. It is demonstrated that in each of these cases the depth variation can be described by a simple curve belonging to a family of two-parametric functions. On the basis of these findings we assess the efficiency of using different punch shapes for property determination. A procedure is proposed for this purpose which relies on the use of the depth variation profiles established here. Finally, the influence of imperfect punch shape on the accuracy of analysis is considered. (10.1016/j.tsf.2009.03.018)
  DOI : 10.1016/j.tsf.2009.03.018
• Crack initiation under thermal fatigue: An overview of CEA experience. Part I: Thermal fatigue appears to be more damaging than uniaxial isothermal fatigue
  
  • Fissolo A.
  • Amiable S.
  • Ancelet O.
  • Mermaz F.
  • Stelmaszyk J.M.
  • Constantinescu Andrei
  • Robertson C.
  • Vincent L.
  • Maillot V.
  • Bouchet F.
  International Journal of Fatigue, Elsevier, 2009, 31 (3), pp.587-600. For nuclear reactor components, uniaxial isothermal fatigue curves are used to estimate the crack initiation under thermal fatigue. However, such approach would be not sufficient in some cases where cracking was observed. To investigate differences between uniaxial and thermal fatigue damage, tests have been carried out using the thermal fatigue devices SPLASH and FAT3D: a bi-dimensional (2D) loading condition is obtained in SPLASH and crack initiation is defined as the first 150-μm surface cracks, whereas a tri-dimensional (3D) loading condition is obtained in FAT3D and crack initiation refers to the first 2-mm surface crack. All the analysed tests clearly show that for identical levels of strain, the number of cycles required to achieve crack initiation is significantly lower in thermal fatigue than in uniaxial isothermal fatigue. The enhanced damaging effect probably results from a pure mechanical origin: a nearly perfect biaxial state corresponds to an increased hydrostatic stress. In that frame, a Part II accompanying paper will be dedicated to investigate accurately on multiaxial effect, and to improve thus estimation of crack initiation under thermal fatigue. (10.1016/j.ijfatigue.2008.03.038)
  DOI : 10.1016/j.ijfatigue.2008.03.038
• Investigation of the dwell period's influence on the fatigue crack growth of a titanium alloy
  
  • Lefranc Pierre
  • Sarrazin-Baudoux Christine
  • Doquet Véronique
  • Petit Jean
  Scripta Materialia, Elsevier, 2009, 60 (5), pp.281-284. The dwell effect, which is known to induce a reduction in the fatigue life of titanium alloys at room temperature, is related to early crack initiation. The present results support faster crack growth rates. The governing mechanisms are identified by mean of scanning electron microfractographic observations. The potential role of the atmosphere is examined through comparative testing performed in air and in high vacuum in order to distinguish the specific contributions of cold creep and environment assistance. (10.1016/j.scriptamat.2008.09.033)
  DOI : 10.1016/j.scriptamat.2008.09.033
• Bifurcation analysis and defects nucleation
  
  • Stolz Claude
  , 2009.
• Cyclic behavior and energy approach of the fatigue of Shape Memory Alloys
  
  • Moumni Ziad
  • Zaki Wael
  • Maitournam Habibou
  Journal of Mechanics of Materials and Structures, Mathematical Sciences Publishers, 2009, 4 (2), pp.pp. 395-411. We present an energy-based low-cycle fatigue criterion that can be used in analyzing and designing structures made from shape memory alloys subjected to cyclic loading. Experimentally, a response similar to plastic shakedown is observed. During the first cycles the stress-strain curve shows a hysteresis loop which evolves during the first few cycles before stabilizing. By adopting an analogy with plastic fatigue, it is shown that the dissipated energy of the stabilized cycle is a relevant parameter for estimating the number of cycles to failure of such materials. Following these observations, we provide an application of the cyclic model, previously developed by the authors within the framework of generalized standard materials with internal constraints in order to evaluate such parameter. Numerical simulations are presented along with a validation against experimental data in case of cyclic superelasticity.
• Dimensionnement à la fatigue des structures soudées par laser
  
  • Lê Thi Thuy Trang
  , 2009. Les travaux que nous présentons dans cette thèse s'inscrivent dans le cadre des études portant sur le dimensionnement à la fatigue des structures soudées par laser. Ces études sont les résultats d'une collaboration entre PSA Peugeot Citroën et le Laboratoire de Mécanique des Solides de l'école Polytechnique. La technique de soudage par laser devient une méthode d'assemblage de plus en plus utilisée par les constructeurs automobiles, navals, ferroviaires, etc. Elle permet de réaliser des assemblages en utilisant une source laser de forte puissance, donnant des cordons continus plus profonds et de plus faible largeur par rapport à d'autres techniques. Elle améliore donc le temps de soudage, la qualité, les propriétés mécaniques des soudures, et réduit le coût de production et le poids des véhicules. La première partie de cette thèse consiste d'une part à analyser l'influence de certains paramètres du procédé sur la tenue en service des structures soudées par laser, d'autre part en une étude bibliographique sur les méthodes actuelles de dimensionnement des structures soudées. Les limitations de ces dernières ont mis en évidence la nécessité d'une nouvelle méthode pour les structures soudées par laser. La méthodologie proposée s'appuie sur la démarche initiée par Fayard, Bignonnet et Dang Van. Elle se fonde sur la recherche d'un volume élémentaire représentatif et d'une contrainte de dimensionnement déterminée à partir des contraintes moyennées sur ce volume. La contrainte de dimensionnement est utilisée pour estimer la durée de vie à l'amorçage des fissures de fatigue dans les structures soumises à des sollicitations multiaxiales. Pratiquement, dans un calcul d'éléments finis, cette contrainte est déterminée au point critique en utilisant une règle de maillage précise. La deuxième partie de la thèse a pour l'objectif de donner une méthode numérique qui d'une part facilite la mise en oeuvre de la règle de maillage proposée pour les calculs, d'autre part permet la multi-utilisation d'un seul maillage pour des calculs de fatigue et de vibration-acoustique. La solution adoptée est la méthode de calcul multi-échelle Arlequin. Cette dernière permet de calculer les structures modélisées par deux maillages de différentes échelles superposés. Elle présente la facilité d'implémentation et d'utilisation ainsi que la flexibilité, et la possibilité d'automatisation dans la modélisation des structures soudées. Nous présentons dans cette thèse une méthode d'implémentation de la méthode Arlequin dans la plate-forme du code de calcul Abaqus en utilisant des éléments créés par l'utilisateur (User Element). Sa validité est montrée par des calculs en fatigue réalisés sur des sous-structures.
• COMPORTEMENT HYDRO-MECANIQUE DE ROCHES RESERVOIR SOUS CONTRAINTES : Relations entre évolution de perméabilité et échelles des mécanismes d'endommagement
  
  • Dautriat Jérémie
  , 2009. La compréhension et la prévention des mécanismes d'endommagement ayant un impact sur les cinétiques de production et les taux de récupération reste un problème ouvert en ingénierie de réservoir. L'objectif de cette étude repose sur la caractérisation du comportement hydromécanique de roches gréseuses et carbonatés en 'condition de réservoir' et l'identification des mécanismes locaux responsables des évolutions de perméabilité mesurables macroscopiquement. L'essentiel du travail expérimental s'est articulé autour d'un dispositif triaxial original, permettant de mesurer en cours de chargement les évolutions de perméabilité dans les directions principales du tenseur de contraintes. La validation de notre protocole effectuée, une première campagne d'essais en compression hydrostatique et uniaxiale sur les grès de Fontainebleau et de Bentheim permet, d'une part, une analyse plus fine du comportement intrinsèque de ces matériaux par la mise en évidence et la prise en compte d'effets expérimentaux, et d'autre part d'étudier l'impact de l'endommagement fragile sur les perméabilités directionnelles. Une seconde série d'expériences de compression, selon différents chemins de chargement, a été réalisée sur un carbonate, le calcaire d'Estaillades. En régime de déformation élastique, la modélisation de la réduction de perméabilité a été appréhendée par une approche Réseau de Pores, basée sur une représentation simplifiée de l'espace poreux et extraite des reconstructions 3D par µ-tomographie RX. Si la rupture fragile des échantillons ne perturbe que très peu la perméabilité initiale, à plus haute pression effective, les diminutions de perméabilité peuvent atteindre 90% et sont alors associées aux mécanismes de pore collapse. L'analyse des échantillons déformés, couplée aux techniques de corrélation d'images acquises in-situ sur des dispositifs de compression simple sous MEB et sous Optique, permet une meilleure prise compte du rôle des échelles d'hétérogénéités dans la localisation des déformations et de mieux définir leur impact sur les évolutions de perméabilités à l'échelle de l'échantillon.
• X-ray tomographic characterization of the macroscopic porosity of CVI SiC/SiC composites - Effects on the elastic behavior
  
  • Gélébart Lionel
  • Chateau Camille
  • Bornert Michel
  • Crépin Jérôme
  • Boller Elodie
  , 2010, 30 (10), pp.33-46. In the context of the development of the next generation of nuclear reactors, SiCiSiC composites are candidate for structural applications. Because of their complex thermo-mechanical behaviour, due to their complex microstructure, a multi-scale approach is under development. An important microstructural parameter of the CVI composite is the complex distribution of the residual porosity inherent to the CVI process. This paper focuses on the characterization of the macroporosity (the porosity among the tows) and on its effect on the thermo-mechanical behaviour. The experimental characterization of the macroporosity is performed using an X-ray tom0 aphy technique on the beamline ID19 at the ESRF synchrotron (France) with a resolution of 5.023pm . The numerical 3D images are used to describe the distribution of macroporosity with respect to the position of the plies. It is clearly established that the stacking of the plies has a significant effect on the porosity distribution. As a consequence for the micromechanical modelling, a unique element that contains only one ply is not representative of the porosity distribution and is not sufficient to evaluate the "effective" mechanical properties: several volume elements, called "statistical volume elements", with at least 2 plies per volume element have to be used in order to account for the variability of the stacking of the plies. Finally, such "statistical volume elements" (SVE) are directly extracted from the tomographic image and the "effective" elastic behaviour is evaluated from the average of the "ap arent" behaviour evaluated on each SVE. In spite of their quite important size (3.2x3.2x0.45mm ), the "apparent" behaviours evaluated for each SVE exhibit important fluctuations. (10.1002/9780470584002.ch3)
  DOI : 10.1002/9780470584002.ch3
• Crack identification by 3D time-domain elastic or acoustic topological sensitivity
  
  • Bellis Cédric
  • Bonnet Marc
  Comptes Rendus. Mécanique, Académie des sciences (Paris), 2009, 337, pp.124-130. The topological sensitivity analysis, based on the asymptotic behaviour of a cost functional associated with the creation of a small trial flaw in a defect-free solid, provides a computationally-fast, non-iterative approach for identifying flaws embedded in solids. This concept is here considered for crack identification using time-dependent measurements on the external boundary. The topological derivative of a cost function under the nucleation of a crack of infinitesimal size is established, in the framework of time-domain elasticity or acoustics. The simplicity and efficiency of the proposed formulation is enhanced by the recourse to an adjoint solution. Numerical results obtained on a 3-D elastodynamic example using the conventional FEM demonstrate the usefulness of the topological derivative as a crack indicator function. (10.1016/j.crme.2009.03.015)
  DOI : 10.1016/j.crme.2009.03.015
• Plasticity and Fracture of Martensitic Boron Steel Under Plane Stress Conditions
  
  • Ebnöther Fabien
  • Mohr Dirk
  International Journal of Solids and Structures, Elsevier, 2009, pp.3535-3547.
• A new fast multi-domain BEM to model seismic wave propagation and amplification in 3D geological structures
  
  • Chaillat Stéphanie
  • Bonnet Marc
  • Semblat Jean-François
  Geophysical Journal International, Oxford University Press (OUP), 2009, 177, pp.509-531. The analysis of seismic wave propagation and amplification in complex geological structures raises the need for efficient and accurate numerical methods. The solution of the elastodynamic equations using traditional boundary element methods (BEMs) is greatly hindered by the fully-populated nature of the matrix equations arising from the discretization. In a previous study limited to homogeneous media, the present authors have established that the Fast Multipole (FM) method reduces the complexity of a 3-D elastodynamic BEM to $N \log N$ per GMRES iteration and demonstrated its effectiveness on 3-D canyon configurations. In this article, the frequency-domain FM-BEM methodology is extented to 3-D elastic wave propagation in piecewise-homogeneous domains in the form of a FM-accelerated multi-region BE-BE coupling approach. This new method considerably enhances the capability of the BEM for studying the propagation of seismic waves in 3-D alluvial basins of arbitrary geometry embedded in semi-infinite media. Several fully 3-D examples (oblique SV-waves) representative of such configurations validate and demonstrate the capabilities of the multi-domain fast multipole approach. They include comparisons with available (low-frequency) results for various types of incident wavefields, and time-domain results obtained by means of Fourier synthesis. (10.1111/j.1365-246X.2008.04041.x)
  DOI : 10.1111/j.1365-246X.2008.04041.x
• Influence of free water on the quasi-static and dynamic strength of concrete in confined compression tests
  
  • Forquin Pascal
  • Safa Kamal
  • Gary Gérard
  Cement and Concrete Research, Elsevier, 2009, 40 (2), pp.321-333. The behaviour of concrete under high pressure and dynamic loadings is experimentally investigated in the present paper. The specimen is confined in a cylindrical elastic steel ring that insures a quasi-uniaxial strain state of loading. It is subjected to static and dynamic (with strain rates in the range from 1e−6/s to 200/s) axial compressive loadings. Transverse gauges glued on the lateral surface of the ring allow for the measurement of the confining pressure so that the volumetric and the deviatoric response of the specimen can be computed. At high or intermediate strain rates, water saturated and dried specimens show strongly different results: i.e. a continuous increase of strength with pressure in dried specimens and a quasi nil strength enhancement in water-saturated specimens. This difference is not observed with quasi-static loadings. As explained through a basic poromechanics analysis, this dissimilarity is mainly attributed to an increase of pore pressure inside the saturated concrete during fast (quasi-static or dynamic) experiments. (10.1016/j.cemconres.2009.09.024)
  DOI : 10.1016/j.cemconres.2009.09.024
• Dynamic Behavior of Concrete: Experimental aspects
  
  • Toutlemonde François
  • Gary Gérard
  , 2009, pp.1-47. (10.1002/9780470611555.ch1)
  DOI : 10.1002/9780470611555.ch1
• Modélisation et Calcul des Milieux Continus
  
  • Le Tallec Patrick
  , 2009, pp.550.
• Multiscale approach of mechanical behaviour of SiC/SiC composites : elastic behaviour at the scale of the tow
  
  • Chateau Camille
  • Gélébart Lionel
  • Bornert Michel
  • Crépin Jérôme
  • Caldemaison Daniel
  , 2010, 30 (1-3), pp.45-55. SiC/SiC composites are candidates for structural applications at elevated temperatures in the context of the development of the 4th generation of nuclear reactors. A multiscale approach is under development to construct a predictive modelling of their complex mechanical behaviour due to their heterogeneous microstructure. This approach is based on two scale transitions: from the fibres/matrix microstructure to the tow and from the tow to the woven composite, each scale presenting a significant residual porosity. This paper focuses on the first scale transition and on the modelling of the elastic behaviour of the tow at room temperature. A microstructural investigation of several tows in a 2D SiC/SiC specimen has been conducted using scanning electron microscopy to get statistical data on microstructural characteristics by image analysis in order to generate a virtual microstructure. The elastic problem of homogenisation is numerically solved by means of finite element techniques. The simulations performed on various volumes show noticeable fluctuations of the apparent behaviour: so separation of length scales is not satisfied in this material. Nevertheless, this problem is neglected in a first approximation and the homogeneous equivalent behaviour is evaluated by averaging the apparent behaviours of several volume elements - smaller than the Representative Volume Element (RVE) – called Statistical Volume Elements (SVEs). Finally, influence of porosity and pores' morphology is quantified.