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
• 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
• 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
• 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
• Mise au point d'un essai de compaction dynamique et statique sur matériaux fragiles
  
  • Safa Kamal
  • Gary Gérard
  • Forquin Pascal
  , 2009.
• Influence de l'eau libre sur la résistance en compression dynamique confinée des bétons
  
  • Forquin Pascal
  • Safa Kamal
  • Gary Gérard
  , 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
• Robust filtering for joint state parameter estimation for distributed mechanical systems
  
  • Chapelle Dominique
  • Moireau Philippe
  • Le Tallec Patrick
  Discrete and Continuous Dynamical Systems - Series A, American Institute of Mathematical Sciences, 2009, 23 (1-2), pp.65-84. We present an effective filtering procedure for jointly estimating state variables and parameters in a distributed mechanical system. This method is based on a robust, low-cost filter related to collocated feedback and used to estimate state variables, and an $H^\infty$ setting is then employed to formulate a joint state-parameter estimation filter. In addition to providing a tractable filtering approach for an infinite-dimensional mechanical system, the $H^\infty$ setting allows to consider measurement errors that cannot be handled by Kalman type filters, e.g. for measurements only available on the boundary. For this estimation strategy a complete error analysis is given, and a detailed numerical assessment - using a test problem inspired from cardiac biomechanics - demonstrates the effectiveness of our approach. (10.3934/dcds.2009.23.65)
  DOI : 10.3934/dcds.2009.23.65
• Higher-order topological sensitivity for 2-D potential problems. Application to fast identification of inclusions
  
  • Bonnet Marc
  International Journal of Solids and Structures, Elsevier, 2009, 46, pp.2275-2292. This article concerns an extension of the topological derivative concept for 2D potential problems involving penetrable inclusions, whereby a cost function $J$ is expanded in powers of the characteristic size $\varepsilon$ of a small inclusion. The $O(\varepsilon^{4})$ approximation of $J$ is established for a small inclusion of given location, shape and conductivity embedded in a 2-D region of arbitrary shape and conductivity, and then generalized to several such inclusions. Simpler and more explicit versions of this result are obtained for a centrally-symmetric inclusion and a circular inclusion. Numerical tests are performed on a sample configuration, for (i) the O(\varepsilon^{4})$ expansion of potential energy, and (ii) the identification of a hidden inclusion. For the latter problem, a simple approximate global search procedure based on minimizing the $O(\varepsilon^{4})$ approximation of $J$ over a dense search grid is proposed and demonstrated. (10.1016/j.ijsolstr.2009.01.021)
  DOI : 10.1016/j.ijsolstr.2009.01.021
• Improving the reliability of material databases using multiscale approaches
  
  • Rollet Y.
  • Bonnet Marc
  • Carrere Nicolas
  • Leroy F.-H.
  • Maire J.-F.
  Composites Science and Technology, Elsevier, 2009, 69, pp.73-80. This article addresses the propagation of constitutive uncertainties between scales occurring in the multiscale modelling of fibre-reinforced composites. The amplification of such uncertainties through upward or downward transitions by a homogenisation model is emphasized and exemplified with the Mori-Tanaka model. In particular, the sensitivity to data uncertainty in the inverse determination of constituent parameters based on downward transitions is stressed on an example. Then a database improvement method, which exploits simultaneously the available information on constitutive uncertainties at all scales instead of just propagating those associated with one scale, is presented and shown to yield substantial reductions in uncertainty for both the constitutive parameters and the response of structures. The latter finding is demonstrated on two examples of structures, with significant gains in confidence obtained on both. (10.1016/j.compscitech.2007.10.049)
  DOI : 10.1016/j.compscitech.2007.10.049
• High-resolution modal analysis
  
  • Ege Kerem
  • Boutillon Xavier
  • David Bertrand
  Journal of Sound and Vibration, Elsevier, 2009, 325 (4-5), pp.852-869. Usual modal analysis techniques are based on the Fourier transform. Due to the Delta T .Delta f limitation, they perform poorly when the modal overlap mu exceeds 30%. A technique based on a high-resolution analysis algorithm and an order-detection method is presented here, with the aim of filling the gap between the low- and the high-frequency domains (30% < mu < 100%). A pseudo-impulse force is applied at points of interests of a structure and the response is measured at a given point. For each pair of measurements, the impulse response of the structure is retrieved by deconvolving the pseudo-impulse force and filtering the response with the result. Following conditioning treatments, the reconstructed impulse response is analysed in different frequency-bands. In each frequency-band, the number of modes is evaluated, the frequencies and damping factors are estimated, and the complex amplitudes are finally extracted. As examples of application, the separation of the twin modes of a square plate and the partial modal analyses of aluminium plates up to a modal overlap of 70% are presented. Results measured with this new method and those calculated with an improved Rayleigh method match closely. (10.1016/j.jsv.2009.04.019)
  DOI : 10.1016/j.jsv.2009.04.019
• Le sous-sol peut-il être étanche ?
  
  • Berest Pierre
  • Bérest P.
  , 2009.
• Multi-level fast multipole BEM for 3-D elastodynamics
  
  • Bonnet Marc
  • Chaillat Stéphanie
  • Semblat Jean-François
  , 2009, pp.15-28. To reduce computational complexity and memory requirement for 3-D elastodynamics using the boundary element method (BEM), a multi-level fast multipole BEM (FM-BEM) based on the diagonal form for the expansion of the elastodynamic fundamental solution is proposed and demonstrated on numerical examples involving single-region and multi-region configurations where the scattering of seismic waves by a topographical irregularity or a sediment-filled basin is examined.
• Filtering for distributed mechanical systems using position measurements: perspectives in medical imaging
  
  • Moireau Philippe
  • Chapelle Dominique
  • Le Tallec Patrick
  Inverse Problems, IOP Publishing, 2009, 25 (3), pp.++. We propose an effective filtering methodology designed to perform estimation in a distributed mechanical system using position measurements. As in a previously introduced method, the filter is inspired from robust control feedback, but here we take full advantage of the estimation specificity to choose a feedback law that can act on displacements instead of velocities and still retain the same kind of dissipativity property which guarantees robustness. This is very valuable in many applications for which positions are more readily available than velocities, as in medical imaging. We provide an in-depth analysis of the proposed procedure, as well as detailed numerical assessments using a test problem inspired from cardiac biomechanics, as medical diagnosis assistance is an important perspective for this approach. The method is formulated first for measurements based on Lagrangian displacements, but we then derive a nonlinear extension allowing to instead consider segmented images, which of course is even more relevant in medical applications. (10.1088/0266-5611/25/3/035010)
  DOI : 10.1088/0266-5611/25/3/035010