Publications

2013

• Image-based data assimilation methods for the personalization of mechanical models - Application to cardiac mechanics and tagged-MRI
  
  • Imperiale Alexandre
  , 2013. This thesis aims at incorporating complex data derived from images into a data assimilation strategy available for mechanical systems. Our work relies on some recent developments that propose a sequential data assimilation method made of a Luenberger filter for the state space and an optimal filter reduced to the remaining parameter space. We aim at performing parameter identification for a biomechanical model of the heart and, within the scope of this application, we formalize the construction of shape discrepancy measurements for two types of data sets: first, the data expected of a processing step of tagged Magnetic Resonance Imaging (tagged-MRI) and, second, more standard data composed by the contours of the object. Initially based on simple distance measurements we enrich these discrepancy measures by incorporating the formalism of currents which enables to embed the contours of the object within the dual of an appropriate space of test functions. For each discrepancy operators we analyze its impact on the observability of the system and, in the case of tagged-MRI, we prove that they are equivalent to a direct measurement of the displacement. From a numerical standpoint, taking into account these complex data sets is a great challenge that motivates the creation of new numerical schemes that provide a more flexible management of the various observation operators. We assess these new means of extracting the rich information contained in the image by identifying in realistic cases the position and the intensity of an infarct in the heart tissue.
• On some multi-phase problems in continuum mechanics
  
  • Bosia Stefano
  , 2013. This work discusses a series of modelling problems in continuum mechanics. The first part is devoted to the mathematical analysis of some diffuse interface models in phase separation of binary mixtures (e.g., coarsening of alloys or bistable polymeric fluids). The second part discusses the function of electronic devices (in particular p-n junctions) under mechanical deformations. The third part presents a model for lifetime predictions in polycrystalline metals under periodic loading. A typical phase separation model is the well-known model H, constructed by coupling the convective Cahn-Hilliard equation with the Navier-Stokes system through the so-called Korteweg force. Here we consider some variants of the model which account, e.g., for shear dependent viscosity or chemically reacting components. We first study basic issues like existence, uniqueness and regularity of solutions. Then we analyze the long-time behaviour of the infinite dimensional dissipative dynamical systems generated by the systems studied. More precisely, we prove the existence of global attractors, exponential attractors, pullback attractors and trajectories attractors for the corresponding dynamical systems. Also, we discuss the robustness of such invariant sets with respect to perturbations of some parameters of the model. The results obtained represent natural extensions of the properties known for single fluid flows, whose features are considered a benchmark for all new techniques proposed in the literature. Finally, as a more realistic description of phase separation phenomena, we introduce a Cahn-Hilliard equation accounting for nonlocal interactions through a singular kernel. In this case some well-posedness and regularity results are demonstrated. The second part of this work is devoted to the study of the coupling effects between mechanical and electronic properties in semiconductors. The modelling of the electronic device is based on the drift-diffusion model for electrons and holes. The device is viewed as a standard macroscopic continuum and the objective is to understand the effects of mechanical strain on the electronic properties of the semiconductor and in particular its effects on the characteristic curve of a p-n junction. This permits to propose a variational formulation of the classical drift-diffusion system and to derive a thermodynamically consistent model for the coupled electromechanical phenomena. The strain mainly influences the mobility coefficients and the generation/recombination term. Two approximate solutions are discussed, one based on only physical assumptions and one involving asymptotic expansions. This part of the work is a preliminary step towards the understanding of the properties of flexible electronic devices. The final part of the thesis presents an application of the theory of dynamical systems to predict the lifetime of polycrystalline metals undergoing a high cycle fatigue regime. A new model is proposed and compared with the existing literature.
• Mathematical framework for Traction Force Microscopy
  
  • Michel Richard
  • Peschetola Valentina
  • Vitale Guido
  • Etienne Jocelyn
  • Duperray Alain
  • Ambrosi Davide
  • Preziosi Luigi
  • Verdier Claude
  ESAIM: Proceedings, EDP Sciences, 2013, 42, pp.61-83. This paper deals with the Traction Force Microscopy (TFM) problem. It consists in obtaining stresses by solving an inverse problem in an elastic medium, from known experimentally measured displacements. In this article, the application is the determination of the stresses exerted by a living cell at the surface of an elastic gel. We propose an abstract framework which formulates this inverse problem as a constrained minimization problem. The mathematical constraints express the biomechanical conditions that the cell must satisfy. From this framework, two methods currently used can be derived, the adjoint method (AM) and the Fourier Transform Traction Cytometry (FTTC) method. An improvement of the FTTC method is also derived using this framework. The numerical results are compared and show the advantage of the AM, in particular it can capture details more accurately. (10.1051/proc/201342005)
  DOI : 10.1051/proc/201342005
• Cooperative folding of muscle myosins: I. Mechanical model
  
  • Caruel Matthieu
  • Allain Jean-Marc
  • Truskinovsky Lev
  , 2013. Mechanically induced folding of passive cross-linkers is a fundamental biological phenomenon. A typical example is a conformational change in myosin II responsible for the power-stroke in skeletal muscles. In this paper we present an athermal perspective on such folding by analyzing the simplest purely mechanical prototype: a parallel bundle of bi-stable units attached to a common backbone. We show that in this analytically transparent model, characterized by a rugged energy landscape, the ground states are always highly coherent, single-phase configurations. We argue that such cooperative behavior, ensuring collective conformational change, is due to the dominance of long- range interactions making the system non-additive. The detailed predictions of our model are in agreement with experimentally observed non-equivalence of fast force recovery in skeletal muscles loaded in soft and hard devices. Some features displayed by the model are also recognizable in the behavior of other biological systems with passive multi-stability and long-range interactions including detaching adhesive binders and pulled RNA/DNA hairpins.
• Morphogenesis and propagation of complex cracks induced by thermal shocks
  
  • Bourdin Blaise
  • Marigo Jean-Jacques
  • Maurini Corrado
  • Sicsic Paul
  Physical Review Letters, American Physical Society, 2013. We study the genesis and the selective propagation of complex crack networks induced by thermal shock or drying of brittle materials. We use a quasi-static gradient damage model to perform large scale numerical simulations showing that the propagation of fully developed cracks follows Griffith criterion and depends only on the fracture toughness, while crack morphogenesis is driven by the material's internal length. Our numerical simulations feature networks of parallel cracks and selective arrest in two dimensions and hexagonal columnar joints in three dimensions, without any hypotheses on cracks geometry and are in good agreement with available experimental results.
• Étude de l'évolution hydromécanique d'un carbonate après altération chimique. Application des méthodes de corrélation d'images 2D et 3D à la mesure des champs locaux de déformation lors d'essais mécaniques à différentes échelles.
  
  • Zinsmeister Louis
  , 2013. Devant le défi grandissant de diminuer la libération dans l'atmosphère des gaz à effet de serre plusieurs solutions ont été envisagées. Une des plus étudiée est le stockage géologique du CO2 par injection dans des aquifères salins profonds. Ces structures géologiques sont présentes dans les bassins sédimentaires et se caractérisent par leur épaisseur (plusieurs centaines de mètres) et leur étendue géographique (l'ensemble du bassin parisien, par exemple, pour la couche du Dogger). Le second interêt de ces structures est que les roches carbonatées qui les composent sont généralement très poreuses (10 à 30% de porosité) et présentent une perméabilité suffisamment élevée pour faciliter l'injection de fluides. Cependant, le comportement et l'évolution à long terme de ces réservoirs, après l'injection de CO2 dans la saumure en place, est incertain. Aussi, la résistance mécanique et les propriétés d'écoulement de ces roches sont étudiées pour réaliser des modèles prédictifs d'évolution, afin de garantir l'intégrité et la pérennité des sites de stockage. Le travail de cette thèse repose sur un protocole d'acidification retardée, développé et utilisé à l'IFPEN, permettant une altération homogène d'échantillons de carbonates. Notre objectif a été de déterminer les effets de ce type d'altération sur le comportement hydromécanique de la roche calcaire de Lavoux. La caractérisation des propriétés d'écoulement de la roche a été combinée à une étude microstructurale. Les paramètres étudiés sont la porosité et la perméabilité avant et après altération. Les résultats d'évolution de la perméabilité ont été analysés en s'appuyant sur une méthode de soustraction d'images de microtomographie RX, qui permet de visualiser la mobilité de particules fines au sein du réseau poreux. Ces dernières sont aussi révélées à postériori dans les effluents par diffraction laser. L'étude microstructurale de la roche après altération a permis la mise en évidence des mécanismes de dissolution et d'altération. Le comportement mécanique de notre roche à différents stades d'altération a été étudié à l'aide d'essais triaxiaux classiques réalisés à différents états de confinement. Ces essais ont montré que l'altération induit une transition du domaine de rupture fragile vers le domaine de rupture ductile. Afin de mieux comprendre ce comportement, nous avons mené des essais mécaniques uniaxiaux avec observation et acquisition d'images numériques " in-situ " : i) sur une presse électromécanique classique équipée de caméras CCD et de microscopie optique, ii) à l'aide d'un dispositif de compression miniaturisé adapté à l'enceinte d'un microscope électronique à balayage. La technique de corrélation d'images numériques a pu être appliquée tout au long du chargement mécanique à différentes échelles, depuis celle de l'échantillon, jusqu'à celle de la microstructure. Les mesures de champs de déformation ainsi réalisées ont permis de mettre en évidence les modes d'endommagement et de rupture selon l'état d'altération. Une cellule triaxiale miniaturisée pour être montée dans l'enceinte du microtomographe a spécialement été développée à l'IFPEN pour cette étude. Cette cellule nous a permis de mener des essais à 5 et 10 MPa de confinement avec observation volumique " in-situ " par tomographie. Nous avons pu identifier pour trois états d'altération, trois types de comportements à la rupture, avec l'observation i) d'une bande de cisaillement en régime de rupture fragile, ii) d'une étroite bande de cisaillement en compaction, iii) d'une large bande de compaction en cisaillement. L'ensemble des résultats obtenus sur les propriétés mécaniques et sur les propriétés d'écoulement (notamment la variation de perméabilité et la présence de fines dans le réseau poreux) tend à montrer que l'altération affecte fortement la microporosité du calcaire, avec des conséquences quant au comportement cisaillant compactant de notre roche.
• Perfectly matched transmission problem with absorbing layers : application to anisotropic acoustics in convex polygonal domains
  
  • Demaldent Edouard
  • Imperiale Sébastien
  International Journal for Numerical Methods in Engineering, Wiley, 2013, 96 (11), pp.689–711. This paper presents an original approach to design perfectly matched layers (PML) for anisotropic scalar wave equations. This approach is based, first, on the introduction of a modified wave equation and, second, on the formulation of general "perfectly matched" transmission conditions for this equation. The stability of the transmission problem is discussed by way of the adaptation of a high frequency stability (necessary) condition, and we apply our approach to construct PML suited for any convex domain with straight boundaries. A new variational formulation of the problem, including a Lagrange multiplier at the interface between the physical and the absorbing domains, is then set and numerical results are presented in 2D and 3D. These results show the efficiency of our approach when using constant damping coefficients combined with high order elements.
• Homogenised constitutive model coupling damage and debonding for reinforced concrete structures under cyclic solicitations
  
  • Combescure Christelle
  • Dumontet Hélène
  • Voldoire François
  International Journal of Solids and Structures, Elsevier, 2013, 50 (24), pp.3861 - 3874. A new stress resultant constitutive model for reinforced concrete plates under cyclic solicitations is presented. This model is built by the periodic homogenisation approach using the averaging method and couples damage of concrete and periodic debonding between concrete and steel rebar. In one-dimensional situations, we derive a closed-form solution of the local problem useful to verify and set up the plate problem. The one dimensional macroscopic constitutive model involves a limited number of parameters, the sensibility of which is studied. Comparison to experimental results underlines the per-tinence of the model by considering internal debonding in order to properly represent the mechanical dissipation occurring during cyclic loadings on reinforced concrete panels. (10.1016/j.ijsolstr.2013.07.021)
  DOI : 10.1016/j.ijsolstr.2013.07.021
• Cohesive Mohr–Coulomb interface effects on the strength criterion of materials with granular-based microstructure
  
  • He Z.
  • Dormieux L.
  • Lemarchand E.
  • Kondo D.
  European Journal of Mechanics - A/Solids, Elsevier, 2013, 42, pp.430-440. (10.1016/j.euromechsol.2013.06.009)
  DOI : 10.1016/j.euromechsol.2013.06.009
• Non-smooth model of the grand piano action
  
  • Thorin Anders
  , 2013. The grand piano action aims at propelling the hammer up to the strings. This mechanism provides the pianist with a high-controllability of the time of impact of the hammer with the strings and the hammer's velocity at the impact. This controllability is believed to be due to the dynamic behaviour of the piano action. The present thesis proposes a simulation method of a complete model of the mechanism, which opens doors to improvements of the haptic rendering of digital keyboards. The sound following the impact of the hammer on the strings is not analysed. In the last fifteen years, various models of the piano action including several degrees of freedom, friction and intermittent contacts, have been proposed. Our approach differs from existing work in that it is based on a new viewpoint for model validation and simulation. Indeed, using a in-depth study of a model with a single degree of freedom, it is shown that the simulation of a complete dynamic model must be driven with a displacement whilst, until now, only force driven simulations have been presented. Velocity discontinuities, occurring during the descent of the key, raise numerical issues which are analysed. They are overcome by non-smooth numerical methods that have been implemented in the computer program XDE. The results of the simulation are presented and compared to experimental measurements. For both piano and forte keystrokes, most of the irregularities in the measured force are reflected in the simulated force. The kinematics of the bodies is also correctly predicted. Eventually, a sensitivity analysis of the dynamic behaviour to the model's parameters is proposed.
• Modeling and simulation of the nucleation and propagation of damage in quasi-brittle materials: Contribution of the variational approach
  
  • Sicsic Paul
  , 2013. This thesis explores the use of damage models to predict the onset and propagation of cracks in a coherent manner. The results are based on the definition of a bulk energy density and a stability principle. Firstly, we study the nucleation of cracks in a notched domain. The limit loading can be decomposed as the product of three stress intensity factors: a scale effect, a geometry induced factor, function of the angle of the notch, and one due to the damage model. The cracks that appear have a characteristic profile whose width is of the order of the internal length. When the latter is small in front of the dimensions of the structure, by separating scales, and using a local minimum principle, we prove that the length of these damage bands follow Griffith's law. This fundamental results extends those based on global minimization but with a sounder physical base. A thorough investigation of the thermal shock problem leads to a better understanding of the nucleation of cracks. Especially the global property of crack periodicity is exhibited. These results are based on the variational approach and the properties would probably be lost for models developed in an other framework. Finally, numerical results based on an alternate minimization algorithm are established. The nucleation phase is controlled by the critical stress whereas the propagation is governed by the toughness. Size effects in two and three dimensions are captured. These numerical simulations are then confronted to experimental results.
• Reciprocity principle and crack identification in transient thermal problems
  
  • Ben Abda Abda
  • Bui Huy Duong
  Journal of Inverse and Ill-posed Problems, De Gruyter, 2013, 9, pp.1-6. We consider the inverse problem of crack determination related to the nondestructive thermal testing of materials process. Using arbitrary transient heat flux applied to the external boundary and measuring the induced temperature, we prove that the overspecified datum suffice to reconstruct planar cracks. (10.1515/jiip.2001.9.1.1)
  DOI : 10.1515/jiip.2001.9.1.1
• Influence of strain-induced crystallization on the crack driving force in fracture behavior of rubber
  
  • Guilie Joachim
  • Le Tallec Patrick
  , 2013. Fatigue crack growth resistance increases with the loading ratio due to strain-induced crystallization, even if the peak stress increases. Recent experiments have allowed determination of the strain-induced crystallization (SIC) distribution around a crack tip during uninterrupted fatigue tests. The purpose of this work is to compare spatial distributions of crystallinity in a cracked sample at different elongations between one experiment and simulation results based on our model.
• Inverse problems in fluid-structure interaction: Application to hemodynamics
  
  • Bertoglio Cristobal
  • Moireau Philippe
  • Gerbeau Jean-Frédéric
  • Fernández Miguel Angel
  • Chapelle Dominique
  • Barber David
  • Gaddum Nicholas
  • Valverde Israel
  • Beerbaum Philipp
  • Hose Rodney
  , 2013. In this talk we deal with the simulation of fluid-structure interaction (FSI) problems in hemodynamics, with the emphasis in data assimilation and simulation in physiological regimes.
• Calcul des structures
  
  • Thorin Anders
  • Forêt Gilles
  , 2013, pp.56. Polycopié du cours MEC441 de l'École polytechnique, France.
• Influence of strain localization on IASCC of proton irradiated 304L stainless steel in simulated PWR primary water
  
  • Le Millier Morgane
  • Calonne Olivier
  • Crépin Jérôme
  • Duhamel Cecilie
  • Fournier Lionel
  • Gaslain Fabrice
  • Héripré Eva
  • Toader Ovidiu
  • Vidalenc Yoann
  • Was Gary
  , 2013, pp.16 p.. Crack initiation has been correlated to crystallographic orientations and strain field measurements using SEM digital image correlation technique coupled with EBSD cartography in order to gain insight into the influence of strain localization on IASCC of proton-irradiated 304L stainless steel in simulated primary water. Type 304L specimens were irradiated with 2 MeV protons at 360°C to 5 and 10 dpa and subsequently strained in simulated primary water under monotonic and pseudo-cyclic loadings. Intergranular cracks occurring on random high angle boundaries nearly perpendicular to the tensile axis were observed. Cracking features differences between monotonic and pseudo-cyclic tests are discussed. Increasing the macroscopic strain for monotonic loading results in a higher density of cracks but does not change the crack length whereas the increase of strain for pseudo-cyclic test induces an important increase of the cracks propagation. Strain field analysis shows a strong heterogeneity, with localization in narrow lines and near some grain boundaries in the irradiated areas. On the contrary, no localization in narrow lines is observed for the non-irradiated material.
• Initiation of SCC crack in nickel based weld metals : influence of microstructure features
  
  • Chaumun Elizabeth
  • Crépin Jérôme
  • de Curières Ian
  • Duhamel Cecilie
  • Guerre Catherine
  • Héripré Eva
  • Sennour Mohamed
  , 2013, pp.1-18. Alloy 82, a nickel based alloy, is used as deposited metal in dissimilar metal welds in Pressurized Water Reactors (PWR). Previous studies dealing with PWSCC (Primary Water Stress Corrosion Cracking) crack growth rate measurement in Alloy 82 highlighted that the crack front consists of isolated intergranular cracks. These uneven crack fronts introduce the question of the effect of the microstructural features on Stress Corrosion Cracking (SCC) behavior. The influence of several parameters, including the chemical composition, metallurgical condition and welding process, on the SCC behavior is studied in order to understand which microstructural parameters are critical for SCC initiation. Microstructural and local mechanical behavior characterization was performed using Electron Back Scattered Diffraction (EBSD) and Digital Imaging Correlation (DIC), and further correlated with SCC initiation sites, showing that SCC cracks are intergranular and propagate perpendicular to the loading direction and that Alloy 82 with lower chromium and carbon contents is more susceptible to SCC. The DIC performed on Alloy 82 welds reveals that the strain field on the surface of U-bend specimens is heterogeneous at several scales (strain field distributions and localization pattern).
• Variational approach to fracture mechanics with plasticity
  
  • Alessi Roberto
  , 2013. In the framework of rate-independent systems, an elastic-plastic-damage model, aimed at the description of ductile fracture processes, is proposed and investigated through a variational formulation. A cohesive, or ductile, crack occurs when the displacement field suffers a discontinuity whilst still being associated to a non-vanishing tensile stress. To predict and effectively describe ductile fracture phenomena is a crucial task for many engineering materials (metals, polymers, ...), as testified by the great interest of the scientific community on the subject. Gradient damage models have been fruitfully used for the description of brittle fractures: in such cases, once the damage level reaches its maximum value, a crack is created where the traction between the two opposite lips immediately drops to zero. On other other hand, the perfect plasticity model could describe the formation of plastic slips at constant stress level. Hence, in order to describe the typical effects of a cohesive fracture, the main idea is to couple, through a variational approach, the perfect plasticity model and a softening gradient damage model. The use of a variational approach results in a weak and derivative-free formulation, gives effective means to deal with the concepts of bifurcation and stability, is intrinsically discrete and indicates a natural and rational way to define efficient numerical algorithms. Embedding damage effects in a plasticity model is not a new idea. Nevertheless the proposed model presents many original aspects as the coupling between plasticity and damage and the way the governing equations of the variables are found. The variational approach relies simply on three concepts: the irreversibility condition, a global, local or differential stability condition and the energy balance. The resulting model possesses a great flexibility in the possible coupled responses, depending on the constitutive parameters. These various responses are first considered by investigating in a one-dimensional quasi-static traction bar test a homogeneous evolution which highlights the main features of the model. The discussion about the stability of the homogeneous solutions leads to the existence of a critical bar length which in turn depends on the characteristic internal material length. For bars that are longer than this critical value the homogeneous response is proven to become unstable and a localization must appear. A construction of localization is then proposed which explicitly takes into account the irreversibility condition on the damage field. This allows the non-homogeneous evolution and the global response to be investigated. It turns out that in general a cohesive crack appears at the center of the damage zone before the complete rupture. At this point the plastic strain localises as a Dirac measure which becomes responsible for this cohesive crack. The associated cohesive law is obtained in a closed form in terms of the parameters of the model and it recovers the cohesive fracture law postulated by Barenblatt. Finally, a numeric scheme is proposed, which is based on an alternate minimization algorithm, and implemented through a finite element library only for the one-dimensional traction bar test. Although the adopted finite element spaces do not embed discontinuities, the numeric results agree perfectly with the analytic solutions. This is due to a kind of numeric regularisation. Nevertheless, future developments aim to extend the simulations in a two/three-dimensional setting and test a generalized finite element method.
• Coupling boundary integral and shell finite element methods to study the fluid structure interactions of a microcapsule in a simple shear flow
  
  • Dupont Claire
  • Salsac Anne-Virginie
  • Barthes-Biesel Dominique
  • Vidrascu Marina
  • Le Tallec Patrick
  , 2013. We simulate the motion of an initially spherical capsule in a simple shear flow in order to determine the influence of the bending resistance on the formation of wrinkles on the membrane. The fluid structure interactions are obtained numerically coupling a boundary integral method to solve for the Stokes equation with a nonlinear finite element method for the capsule wall mechanics. The capsule wall is discretized with MITC linear triangular shell finite elements. We find that, at low flow strength, buckling occurs in the central region of the capsule. The number of wrinkles on the membrane decreases with the bending stiffness and, above a critical value, wrinkles no longer form. For thickness to radius ratios below 5%, the bending stiffness does not have any significant effect on the overall capsule motion and deformation. The mean capsule shape is identical whether the wall is modeled as a shell or a two-dimensional membrane, which shows that the dynamics of thin capsules is mainly governed by shear elasticity and membrane effects.
• Micro-scale experimental investigation of deformation of argillaceous rocks under hydric and mechanical loads
  
  • Wang Linlin
  • Bornert Michel
  • Chanchole S.
  , 2013.
• The bending of an elastic beam by a liquid drop: A variational approach
  
  • Neukirch Sebastien
  • Antkowiak Arnaud
  • Marigo Jean-Jacques
  Proceedings of the Royal Society A: Mathematical, Physical and Engineering Sciences, Royal Society, The, 2013, 469 (2157), pp.20130066. We study the interaction of a liquid drop with an elastic beam in the case where bending effects dominate. We use a variational approach to derive equilibrium equations for the system in the presence of gravity and in the presence or absence of contact line pinning. We show that the derived equilibrium equations for the beam subsystem reveal the external forces applied on the beam by the liquid and vapor phases. Among these, the force applied at the triple line (the curve where the three phases meet) is found to lie along the liquid-vapor interface. (10.1098/rspa.2013.0066)
  DOI : 10.1098/rspa.2013.0066
• Receptor displacement in the cell membrane by hydrodynamic force amplification through nanoparticles.
  
  • Türkcan Silvan
  • Richly Maximilian
  • Bouzigues Cedric I.
  • Allain Jean-Marc
  • Alexandrou Antigoni
  Biophysical Journal, Biophysical Society, 2013, 105 (1), pp.116-126. We introduce an intrinsically multiplexed and easy to implement method to apply an external force to a biomolecule and thus probe its interaction with a second biomolecule or, more generally, its environment (for example, the cell membrane). We take advantage of the hydrodynamic interaction with a controlled fluid flow within a microfluidic channel to apply a force. By labeling the biomolecule with a nanoparticle that acts as a kite and increases the hydrodynamic interaction with the fluid, the drag induced by convection becomes important. We use this approach to track the motion of single membrane receptors, the Clostridium perfringens ε-toxin (CPεT) receptors that are confined in lipid raft platforms, and probe their interaction with the environment. Under external force, we observe displacements over distances up to 10 times the confining domain diameter due to elastic deformation of a barrier and return to the initial position after the flow is stopped. Receptors can also jump over such barriers. Analysis of the receptor motion characteristics before, during, and after a force is applied via the flow indicates that the receptors are displaced together with their confining raft platform. Experiments before and after incubation with latrunculin B reveal that the barriers are part of the actin cytoskeleton and have an average spring constant of 2.5 ± 0.6 pN/μm before vs. 0.6 ± 0.2 pN/μm after partial actin depolymerization. Our data, in combination with our previous work demonstrating that the ε-toxin receptor confinement is not influenced by the cytoskeleton, imply that it is the raft platform and its constituents rather than the receptor itself that encounters and deforms the barriers formed by the actin cytoskeleton. (10.1016/j.bpj.2013.05.045)
  DOI : 10.1016/j.bpj.2013.05.045
• A Testing Technique of Confined Compression for Concrete at High Rates of Strain
  
  • Forquin Pascal
  • Gary G.
  • Gatuingt Fabrice
  , 2013. In order to characterise the behaviour of concrete under high pressures and high strain-rates, dynamic 1D-strain compression tests were performed. A cylindrical specimen is embedded in a steel confinement ring and compressed between 2 cylindrical plugs with a SHPB (Split Hopkinson Pressure Bars) device. Moreover, a new method was used to process the experimental data. It is based on numerical simulations of the cell loaded by an internal pressure that allow to build a relation between the pressure applied by the concrete to the inner surface of the cell and the external hoop strain measured by gauges. Stresses and strains in the specimen are computed at any loading time and the evolution of the deviatoric stress versus the pressure (deviatoric behaviour) and of the pressure versus volumetric strain (spherical behaviour) is deduced. The method is validated by several numerical simulations of the test involving different friction coefficients between the cell and the specimen. Three 1D-strain compression tests were performed and processed with the MB50 high-performance concrete. They showed that the deviatoric and spherical behaviours are almost independent of the strain rate in the observed range of strain rates (80-221 s-1). The deviatoric strength is seen to increase regularly with the hydrostatic pressure. The spherical behaviour indicates a fairly constant dynamic modulus of compressibility (around 5 to 6 GPa) up to a pressure of 900 MPa.
• Fissuration du verre de confinement des déchets radioactifs
  
  • Doquet Véronique
  • Ben Ali Neji
  • Constantinescu Andrei
  • Chabert Emmanuelle
  Flash X - La revue scientifique de l'Ecole polytechnique, Ecole polytechnique, 2013, 15 (Energies), pp.39-42.
• Simulation of a non-smooth dynamical model of the piano key
  
  • Thorin Anders
  • Boutillon Xavier
  • Lozada José
  • Merlhiot Xavier
  • Micaelli Alain
  , 2013, pp.2 p.. Following the literature, two kinds of models for the grand piano action are presented here: one single-degree-of-freedom model and one model based on 6 rotating bodies, 13 contact zones with nonlinear springs, 3 of them (hammer-jack, jack-escapement button, hammer-check) being also subject to Coulomb friction. The latter is simulated by a non-smooth dynamics approach. It appears that force-driven simulations of the key position compare equally well for both models. This is not true for position-driven simulations of the reaction force exerted by the piano key on the pianist's finger.