Publications

2015

• A Luenberger observer for reaction-diffusion models with front position data
  
  • Collin Annabelle
  • Chapelle Dominique
  • Moireau Philippe
  Journal of Computational Physics, Elsevier, 2015, 300, pp.20. We propose a Luenberger observer for reaction-diffusion models with propagating front features, and for data associated with the location of the front over time. Such models are considered in various application fields, such as electrophysiology, wild-land fire propagation and tumor growth modeling. Drawing our inspiration from image processing methods, we start by proposing an observer for the eikonal-curvature equation that can be derived from the reaction-diffusion model by an asymptotic expansion. We then carry over this observer to the underlying reaction-diffusion equation by an "inverse asymptotic analysis", and we show that the associated correction in the dynamics has a stabilizing effect for the linearized estimation error. We also discuss the extension to joint state-parameter estimation by using the earlier-proposed ROUKF strategy. We then illustrate and assess our proposed observer method with test problems pertaining to electrophysiology modeling, including with a realistic model of cardiac atria. Our numerical trials show that state estimation is directly very effective with the proposed Luenberger observer, while specific strategies are needed to accurately perform parameter estimation – as is usual with Kalman filtering used in a nonlinear setting – and we demonstrate two such successful strategies. (10.1016/j.jcp.2015.07.044)
  DOI : 10.1016/j.jcp.2015.07.044
• SCC crack initiation in nickel based alloy welds in hydrogenated steam at 400°C
  
  • Chaumun Elizabeth
  • Crépin Jérôme
  • Duhamel Cecilie
  • Guerre Catherine
  • Heripre Eva
  • Sennour Mohamed
  • Curières Ian De
  , 2015, pp.19 p..
• Prestrain-dependent viscosity of a highly filled elastomer: experiments and modeling
  
  • Jalocha Dimitri
  • Constantinescu Andrei
  • Nevière Robert
  Mechanics of Time-Dependent Materials, Society for Experimental Mechanics, 2015, 19 (3). Highly filled elastomers exhibit a complex microstructure made up of rigid fillers bounded by a thin layer polymeric matrix. The interactions between the fillers and the binder amplify locally the applied strains and induce a nonlinear viscoelastic behavior. The aim here is to analyze the influence of prestrain on the viscoelastic behavior. This paper proposes a prestrain-dependent viscoelastic constitutive model. The model is a superposition of three relaxation spectra, each corresponding to a family of polymer chains, and can be regarded in either its continuous or discrete expression. More specifically, one of these relaxation spectra is modified to assure the prestrain sensitivity. The parameters of the discrete model are identified from relaxation and DMA experiments performed on a solid propellant, and the obtained predictions match closely the experiments. The novelty of the analysis proposed in this paper is threefold. On the one hand, we report a new series of experimental measures, performed for a large range of frequencies for the DMA experiment and relaxation times for the relaxation experiment, and, on the other hand, we propose a constitutive law compatible with the principles of thermodynamics, which predicts closely the measurements. Finally, the analysis is performed comparing both relaxation and DMA experiments using the spectrum of relaxation times. A peculiarity of the present discussion is the novel identification method used, which identifies directly the relaxation times. This technique leads to models with smaller and optimum numbers of parameters than classical methods based on a logarithmic distribution of relaxation times. (10.1007/s11043-015-9262-z)
  DOI : 10.1007/s11043-015-9262-z
• Revisiting the identification of generalized Maxwell models from experimental results
  
  • Jalocha D.
  • Constantinescu A.
  • Neviere R.
  International Journal of Solids and Structures, Elsevier, 2015, 67-68 (August), pp.169–181. Linear viscoelastic material behavior is often modeled using a generalized Maxwell model. The material parameters, i.e. relaxation times and elastic moduli, of the Maxwell elements are determined from either a relaxation or a Dynamical Mechanical Analysis (DMA) experiments. The underlying mathematical problem is known to be ill-posed, which means that uniqueness of the identification is not assured and that small errors in the initial data will conduct to high discrepancies in the identified parameters. The standard technique to remove the ill-posedness is to chose a priori a series of relaxation times and to identify only the moduli. The aim of this paper is to propose two techniques to identify an optimal series of relaxation times. In the case of the relaxation experiment relaxation times will be optimized from the numerical integration of the measured relaxation spectrum. In the case of the DMA experiments we show that mathematical results obtained by Krein and Nudelmann can be used to determine the complete series of relaxation times. The methods are illustrated by identification examples using both artificial and experimental data. The results show that the methods provide a good match of the identified models in term of relaxation or complex moduli. (10.1016/j.ijsolstr.2015.04.018)
  DOI : 10.1016/j.ijsolstr.2015.04.018
• Analysis of passive cardiac constitutive laws for parameter estimation using 3D tagged MRI
  
  • Hadjicharalambous Myrianthi
  • Chabiniok Radomir
  • Asner Liya
  • Sammut Eva
  • Wong James
  • Carr-White Gerald
  • Lee Jack
  • Razavi Reza
  • Smith Nicolas
  • Nordsletten David
  Biomechanics and Modeling in Mechanobiology, Springer Verlag, 2015, 14 (4), pp.807-828. An unresolved issue in patient-specific models of cardiac mechanics is the choice of an appropriate constitutive law, able to accurately capture the passive behavior of the myocardium, while still having uniquely identifiable parameters tunable from available clinical data. In this paper, we aim to facilitate this choice by examining the practical identifiability and model fidelity of constitutive laws often used in cardiac mechanics. Our analysis focuses on the use of novel 3D tagged MRI, providing detailed displacement information in three dimensions. The practical identifiability of each law is examined by generating synthetic 3D tags from in silico simulations, allowing mapping of the objective function landscape over parameter space and comparison of minimizing parameter values with original ground truth values. Model fidelity was tested by comparing these laws with the more complex transversely isotropic Guccione law, by characterizing their passive end-diastolic pressure–volume relation behavior, as well as by considering the in vivo case of a healthy volunteer. These results show that a reduced form of the Holzapfel–Ogden law provides the best balance between identifiability and model fidelity across the tests considered. (10.1007/s10237-014-0638-9)
  DOI : 10.1007/s10237-014-0638-9
• Instruments à cordes : problématiques et avancées récentes
  
  • Boutillon Xavier
  , 2015.
• Thermodynamical framework for modeling chemo-mechanical coupling in muscle contraction – Formulation and preliminary results
  
  • Caruel M.
  • Moireau P
  • Chapelle D
  , 2015. Nous proposons un modèle multi-échelle de la contraction cardiaque dans lequel les moteurs moléculaires à l'origine du processus contractile sont représentés par des élé-ments mécaniques multistables paramétrés à la fois par des degrés de liberté géométriques et par des états chimiques. Ce modèle permet de poser les fondements thermody-namiques permettant de décrire l'interaction complexe entre les phénomènes mécaniques et chimiques a l'échelle sub-cellulaire. Ce travail a pour objet de représenter les car-actéristiques physiologiques du dispositif contractile observées expérimentalement et en particulier (i) le mécanisme passif de récupération rapide de force, (ii) la relation entre la vitesse de contraction et la charge appliquée et (iii) le cycle dit de Lymn-Taylor décrivant l'activité métabolique. Abstract : We propose a multiscale model of cardiac contraction in which the molecular motors at the origin of the contractile process are considered as multistable mechanical entities endowed with internal degrees of freedom of both mechanical and chemical nature. This model provides a thermodynamical basis for modeling the complex interplay of chemical and mechanical phenomena at the sub-cellular level. Important motivations for this work include the ability to represent the experimentally observed physiological characteristics of the contractile apparatus such as (i) the passive quick force recovery mechanism, (ii) the relation between the contraction velocity and the applied force and (iii) the so called Lymn-Taylor cycle describing the metabolism.
• Molecular Origin of the Influence of the Temperature on the Loss Factor of a Solid Propellant
  
  • Azoug Aurélie
  • Nevière Robert
  • Constantinescu Andrei
  Propellants, Explosives, Pyrotechnics, Wiley-VCH Verlag, 2015, 40 (4). This study focuses on the viscoelastic behavior of an industrial hydroxyl-terminated polybutadiene (HTPB) based solid propellant. The analysis of the loss factor as function of temperature enables the investigation of the molecular mechanisms participating in the nonlinear viscoelastic behavior. A design of experiments determines the influences of the filler fraction, of the NCO/OH ratio, of the plasticizer content, and of the presence or absence of filler-binder bonding agents. For all the tested materials, the loss factor as function of temperature exhibits two distinct peaks when measured by Dynamic Mechanical Analysis. Exponentially modified Gaussian distributions are applied on each peak to characterize the behavior. While the first peak is commonly associated with the rubber-glass transition of the material, the second peak has not been clearly associated with a molecular mechanism. This study shows that the second peak of the loss factor in HTPB-based solid propellants originates from the flow of free polymer chains in the polymer network with a reptation mechanism. The sol polymer fraction controls the area of the second peak, whereas its temperature at the maximum corresponds to an activation temperature determined by the molar masses of the sol polymer. Finally, when the propellant is stretched, a decrease in area and an increase in the temperature of the peak show that the reptation of the sol polymer chains is constrained by the network. (10.1002/prep.201400060)
  DOI : 10.1002/prep.201400060
• Microsphere model for strain-induced crystallization and three dimensional applications
  
  • Guilie Joachim
  • Lê Thien-Nga
  • Le Tallec Patrick
  Journal of the Mechanics and Physics of Solids, Elsevier, 2015, 81, pp.58-74. Strain-crystallising rubber exhibits interesting properties: for instance, fatigue lifetime is known to be modified by this microstructural evolution which dissipates energy and creates a strong anisotropic reinforcement. We develop herein a micro-sphere 3D constitutive model for such strain-crystallising rubber. It is based on a simplified 1D micromechanical model that we extend with a micro-sphere approach to a full thermodynamically consistent evolutive anisotropic model. A specific numerical strategy is then proposed. The model is assessed on several significative configurations and reproduces the main experimental features while predicting the evolution of anisotropy as a function of the loading history. We finally show that it can also predict the crystallised zone in front of a mode I crack. (10.1016/j.jmps.2015.05.004)
  DOI : 10.1016/j.jmps.2015.05.004
• Development of human corneal epithelium on organized fibrillated transparent collagen matrices synthesized at high concentration
  
  • Tidu Aurelien
  • Ghoubay-Benallaoua Djida
  • Lynch Barbara
  • Haye Bernard
  • Illoul Corinne
  • Allain Jean-Marc
  • Borderie Vincent M.
  • Mosser Gervaise
  Acta Biomaterialia, Elsevier, 2015, 22, pp.50-58. Several diseases can lead to opacification of cornea requiring transplantation of donor tissue to restore vision. In this context, transparent collagen I fibrillated matrices have been synthesized at 15, 30, 60 and 90 mg/mL. The matrices were evaluated for fibril organizations, transparency, mechanical properties and ability to support corneal epithelial cell culture. The best results were obtained with 90 mg/mL scaffolds. At this concentration, the fibril organization presented some similarities to that found in corneal stroma. Matrices had a mean Young's modulus of 570 kPa and acellular scaffolds had a transparency of 87% in the 380-780 nm wavelength range. Human corneal epithelial cells successfully colonized the surface of the scaffolds and generated an epithelium with characteristics of corneal epithelial cells (i.e. expression of cytokeratin 3 and presence of desmosomes) and maintenance of stemness during culture (i.e. expression of Delta Np63 alpha and formation of holoclones in colony formation assay). Presence of cultured epithelium on the matrices was associated with increased transparency (89%). (C) 2015 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. (10.1016/j.actbio.2015.04.018)
  DOI : 10.1016/j.actbio.2015.04.018
• Variational Approach to Dynamic Brittle Fracture via Gradient Damage Models
  
  • Li Tianyi
  • Marigo Jean-Jacques
  • Guilbaud Daniel
  • Potapov Serguei
  Applied Mechanics and Materials, Trans Tech Publications, 2015, 784, pp.334-341. In this paper we present a family of gradient-enhanced continuum damage models which can be viewed as a regularization of the variational approach to fracture capable of predicting in a unified framework the onset and space-time dynamic propagation (growth, kinking, branching, arrest) of complex cracks in quasi-brittle materials under severe dynamic loading. The dynamic evolution problem for a general class of such damage models is formulated as a variational inequality involving the action integral of a generalized Lagrangian and its physical interpretation is given. Finite-element based implementation is then detailed and mathematical optimization methods are directly used at the structural scale exploiting fully the variational nature of the formulation. Finally, the link with the classical dynamic Griffith theory and with the original quasi-static model as well as various dynamic fracture phenomena are illustrated by representative numerical examples in quantitative accordance with theoretical or experimental results. (10.4028/www.scientific.net/AMM.784.334)
  DOI : 10.4028/www.scientific.net/AMM.784.334
• Fourth-order energy-preserving locally implicit time discretization for linear wave equations
  
  • Chabassier Juliette
  • Imperiale Sebastien
  , 2015. A family of fourth order locally implicit schemes is presented as a special case of fourth order coupled implicit schemes for linear wave equations. The domain of interest is decomposed into several regions where different (explicit or implicit) fourth order time discretization are used. The coupling is based on a Lagrangian formulation on the boundaries between the several non conforming meshes of the regions. Fourth order accuracy follows from global energy identities. Numerical results in 1d and 2d illustrate the good behavior of the schemes and their potential for the simulation of realistic highly heterogeneous media or strongly refined geometries, for which using everywhere an explicit scheme can be extremely penalizing. Fourth order accuracy reduces the numerical dispersion inherent to implicit methods used with a large time step, and makes this family of schemes attractive compared to classical approaches. (10.1002/nme.5130)
  DOI : 10.1002/nme.5130
• Glucose Induces Slow-Wave Sleep by Exciting the Sleep-Promoting Neurons in the Ventrolateral Preoptic Nucleus: A New Link between Sleep and Metabolism
  
  • Varin Christophe
  • Rancillac Armelle
  • Geoffroy Hélène
  • Arthaud Sébastien
  • Fort Patrice
  • Gallopin Thierry
  Journal of Neuroscience, Society for Neuroscience, 2015, 35 (27), pp.9900-9911. Sleep-active neurons located in the ventrolateral preoptic nucleus (VLPO) play a crucial role in the induction and maintenance of slow-wave sleep (SWS). However, the cellular and molecular mechanisms responsible for their activation at sleep onset remain poorly understood. Here, we test the hypothesis that a rise in extracellular glucose concentration in the VLPO can promote sleep by increasing the activity of sleep-promoting VLPO neurons. We find that infusion of a glucose concentration into the VLPO of mice promotes SWS and increases the density of c-Fos-labeled neurons selectively in the VLPO. Moreover, we show in patch-clamp recordings from brain slices that VLPO neurons exhibiting properties of sleep-promoting neurons are selectively excited by glucose within physiological range. This glucose-induced excitation implies the catabolism of glucose, leading to a closure of ATP-sensitive potassium (KATP) channels. The extracellular glucose concentration monitors the gating of KATP channels of sleep-promoting neurons, highlighting that these neurons can adapt their excitability according to the extracellular energy status. Together, these results provide evidence that glucose may participate in the mechanisms of SWS promotion and/or consolidation. SIGNIFICANCE STATEMENT: Although the brain circuitry underlying vigilance states is well described, the molecular mechanisms responsible for sleep onset remain largely unknown. Combining in vitro and in vivo experiments, we demonstrate that glucose likely contributes to sleep onset facilitation by increasing the excitability of sleep-promoting neurons in the ventrolateral preoptic nucleus (VLPO). We find here that these neurons integrate energetic signals such as ambient glucose directly to regulate vigilance states accordingly. Glucose-induced excitation of sleep-promoting VLPO neurons should therefore be involved in the drowsiness that one feels after a high-sugar meal. This novel mechanism regulating the activity of VLPO neurons reinforces the fundamental and intimate link between sleep and metabolism. (10.1523/JNEUROSCI.0609-15.2015)
  DOI : 10.1523/JNEUROSCI.0609-15.2015
• Dynamics of a chain of permanent magnets
  
  • Lee Joosung
  • Boisson Jean
  • Rouby Corinne
  • Doaré Olivier
  • Ducceschi Michele
  • Bodelot Laurence
  , 2015. An arrangement of several spherical or cylindrical magnets presents different stable configurations. One of them is the straight chain, whose dynamics is studied in the present work. This structure behaves similarly to a beam, but here the rigidity is exclusively due to magnetic forces. Theoretically, the dynamical equations of the structure are obtained by first providing an expression of the energies involved in the system. At this stage, the magnetic interactions are either modelized by considering dipoles approximations for the magnets or by computing numerically the magnetic field around the magnets. Conditions of contact are introduced in the model thanks to Lagrange multipliers and a dynamical system governing the displacement of each magnet is finally obtained. An analogy with the eigenfrequencies of elastic beams allows to provide a model of an equivalent flexural rigidity induced by magnetic forces. Good agreement is found with the equivalent rigidity obtained for circular rings of magnets. An experimental study of rigid assembly composed with neodymium permanent magnets is also performed. Free oscillations and forced oscillations experiments are realized. A good agreement is found between experimental and theoretical eigenfrequencies and eigenmodes. Next, the effect of an external magnetic field on the dynamics of a clamped-free chain of cylindrical magnets is studied. Here, the magnetic field is even able to modify the stability properties of the system. The clamped-free chain can now buckle in the same way as a beam would buckle when submitted to gravity. The comparison between theoretical and experimental results emphasizes the limitations of the dipolar description for cylindrical magnets. Using full computations of the magnetic field, we now develop dynamical models of other systems involving differents shapes for the magnets (spheres, cylinders, plates...), as well as corresponding experiments.
• In situ fatigue tests under X-ray tomography: damage mechanisms in a cast Aluminium Silicon alloy
  
  • Limodin Nathalie
  • Wang Long
  • Dahdah Nora
  • El Bartali Ahmed
  • Witz Jean-Francois
  • Seghir Rian
  • Charkaluk Eric
  • Buffiere Jean-Yves
  , 2015, pp.676.
• Some remarks on standard gradient models and gradient plasticity
  
  • Nguyen Quoc Son
  Mathematics and Mechanics of Solids, SAGE Publications, 2015, 20 (6), pp.760-769. The standard gradient models, originally introduced in the works of Gurtin ( Physica D 1996; 92: 178) and Frémond and Nedjar ( Int J Solids Struct 1996; 33: 1083) have been studied intensively in the last few decades for various applications in plasticity, damage mechanics and multi-phase analysis. In this paper, these models are revisited and discussed in relation with some classical descriptions of solids such as plasticity and visco-plasticity. The constitutive equations of such a model and the governing equations for a solid have been initially derived by these authors from an extended virtual work principle. Without this starting point, it is shown here that these equations appear as a generalized Biot equation for the solid and can be obtained directly from the global expression of the energy and dissipation potentials. This result gives the possibility to write the governing equations for standard gradient models of any order. These models also appear as a simple generalization of classical descriptions in solid mechanics. As an example, for a time-independent process such as incremental plasticity, our attention is focused on the constitutive modeling and on the governing equations of the response. It is shown that an elastic regularization can be introduced to avoid some theoretical and numerical difficulties which are well known in the classical theory of rigid plasticity. (10.1177/1081286514551499)
  DOI : 10.1177/1081286514551499
• A model for porous single crystals with cylindrical voids of elliptical cross-section
  
  • Mbiakop A.
  • Constantinescu A.
  • Danas Kostas
  International Journal of Solids and Structures, Elsevier, 2015, 64-65 (July), pp.100-119. This work presents a rate-dependent constitutive model for porous single crystals with arbitrary number of slip systems and orientations. The single crystal comprises cylindrical voids with elliptical cross-section at arbitrary orientations and is subjected to general plane-strain loadings. The proposed model, called modified variational model (MVAR), is based on the nonlinear variational homogenization method, which makes use of a linear comparison porous single crystal material to estimate the response of the nonlinear porous single crystal. The MVAR model is validated by periodic finite element simulations for a large number of parameters including general in-plane crystal anisotropy, general in-plane void shapes and orientations, various creep exponents (i.e., nonlinearity) and general plane strain loading conditions. The MVAR model, which at the present state involves no calibration parameters, is found to be in good agreement with the finite element results for all cases considered in this work. The model is then used in a predictive manner to investigate the complex response of porous single crystals in several cases with strong coupling between the anisotropy of the crystal and the (morphological) anisotropy induced by the shape and orientation of the voids. (10.1016/j.ijsolstr.2015.03.017)
  DOI : 10.1016/j.ijsolstr.2015.03.017
• Microstructural insight into the nonlinear swelling of argillaceous rocks
  
  • Wang L.L.
  • Bornert Michel
  • Yang D.S.
  • Héripré E.
  • Chanchole S.
  • Halphen B.
  • Pouya A.
  • Caldemaison D.
  Engineering Geology, Elsevier, 2015, 193, pp.435-444. Argillaceous rocks are chosen as possible host rocks for underground radioactive nuclear waste disposal. These rocks exhibit complex coupled thermo–hydro–chemo-mechanical behavior, the description of which would strongly benefit from an improved experimental insight on micro-scale. In this work we present some recent observations of the evolution of these rocks upon swelling on the scale of their composite microstructure, essentially made of a clay matrix with embedded grains of calcite and quartz with sizes ranging from a few to several hundreds of micrometers. The micro-scale experimental investigation was based on the combination of high definition and high resolution imaging in an environmental scanning electron microscope (ESEM) and digital image correlation techniques. Samples were held at a constant temperature of 2 °C while the vapor pressure in the ESEM chamber was varied from a few to several hundreds of Pascals, generating a relative humidity (RH) ranging from about 10% up to 99%. Results on micro-scale showed strongly heterogeneous deformation fields, which result from complex hydromechanical interactions between different components of argillaceous rocks. The swelling of argillaceous rocks is moderate at low RH but becomes significant at high RH. The observations demonstrated that the nonlinearity is related not only to the micro-cracking upon wetting, but also to the nonlinear swelling of the clay matrix itself that is governed by different mechanisms. (10.1016/j.enggeo.2015.05.019)
  DOI : 10.1016/j.enggeo.2015.05.019
• HUXLEY-SIMMONS MODEL REVISITED
  
  • Caruel Matthieu
  • Truskinovsky Lev
  , 2015. Muscle contraction is a largely mechanical process taking place at the sub-cellular level. While being an intrinsically active system the contractile apparatus also displays some intriguing passive mechanical properties including negative stiffness and a fundamental nonequivalence of isometric and isotonic loading protocols. We reveal the origin of this unusual behavior by analyzing a conceptual model which represents a delicate generalization of the Huxley-Simmons model. Our analytically explicit study sheds light on the crucial role of long-range interactions in this system. The model can be easily adapted to a wide class of biological phenomena involving cooperative switching mediated by effective backbones, from muscle power-stroke to gating, binding and folding.
• Microstructural changes and in-situ observation of localization in OFHC copper under dynamic loading
  
  • Bodelot Laurence
  • Escobedo-Diaz Juan P.
  • Trujillo Carl P.
  • Martinez Daniel T.
  • Cerreta Ellen K.
  • Iii George T. Gray
  • Ravichandran Guruswami
  International Journal of Plasticity, Elsevier, 2015. (10.1016/j.ijplas.2015.06.002)
  DOI : 10.1016/j.ijplas.2015.06.002
• Sequential State Estimation for Electrophysiology Models with Front Level-Set Data Using Topological Gradient Derivations
  
  • Collin Annabelle
  • Chapelle Dominique
  • Moireau Philippe
  , 2015, 9126, pp.402-411. We propose a new sequential estimation method for making an electrophysiology model patient-specific, with data in the form of level sets of the electrical potential. Our method incorporates a novel correction term based on topological gradients, in order to track solutions of complex patterns. Our assessments demonstrate the effectiveness of this approach, including in a realistic case of atrial fibrillation. (10.1007/978-3-319-20309-6_46)
  DOI : 10.1007/978-3-319-20309-6_46
• Patient-Specific Biomechanical Modeling of Cardiac Amyloidosis – A Case Study
  
  • Chapelle Dominique
  • Felder Alessandro
  • Chabiniok Radomir
  • Guellich Aziz
  • Deux Jean-François
  • Damy Thibaud
  , 2015, 9126, pp.295-303. We present a patient-specific biomechanical modeling framework and an initial case study for investigating cardiac amyloidosis (CA). Our patient-specific heartbeat simulations are in good agreement with the data, and our model calibration indicates that the major effect of CA in the biophysical behavior lies in a dramatic increase of the passive stiffness. We also conducted a preliminary trial for predicting the effects of pharmacological treatments – which is an important clinical challenge – based on the model combined with a simple venous return representation. This requires further investigation and validation, albeit provides some valuable preliminary insight. (10.1007/978-3-319-20309-6_34)
  DOI : 10.1007/978-3-319-20309-6_34
• Steps Towards Quantification of the Cardiological Stress Exam
  
  • Chabiniok Radomir
  • Sammut Eva
  • Hadjicharalambous Myrianthi
  • Asner Liya
  • Nordsletten David
  • Razavi Reza
  • Smith Nicolas
  , 2015, 9126, pp.12-20. In this work we aim to advance the translation of model-based myocardial contractility estimation to the clinical problem of quantitative assessment of the dobutamine stress exam. In particular, we address the question of limited spatial resolution of the observations obtained from cine MRI during the stress test, in which typically only a small number of cine MRI slices are acquired. Due to the relative risk during the dobutamine infusion, a safe acquisition protocol with a healthy volunteer under the infusion of a beta-blocker is applied in order to get a better insight into the contractility estimation using such a type of clinical data. The estimator is compared for three types of observations, namely the processed short axis cine stack contiguously covering the ven-tricles, the short axis stack limited to only 3 slices and the combination of 3 short and 3 long axis slices. A decrease of contractilities in AHA regions under the beta-blocker infusion was estimated for each observation. The corrected model (by using the estimated parameters) was then compared with the displacements extracted from 3D tagged MRI. (10.1007/978-3-319-20309-6_2)
  DOI : 10.1007/978-3-319-20309-6_2
• PhD THESIS Experimental Characterization, Modeling and Simulation of Magneto-Rheological Elastomers
  
  • Pössinger Tobias
  , 2015. In this thesis, we study a class of active materials named Magneto-Rheological Elastomers (MREs) with a main focus on their coupled magneto-mechanical response up to large strains and up to high magnetic fields. With the purpose of achieving a coupled characterization of MREs behavior for the design of haptic interface devices, this work encompasses experimental, theoretical and numerical developments. The first part of this work is dedicated to aspects pertaining to sample fabrication. Isotropic and magnetic field-cured MREs, composed of soft silicone rubber and micrometric carbonyl iron powder, are manufactured using a reliable and repeatable process. A special sample geometry is designed in order to obtain both homogeneous mechanical and magnetic fields during the coupled-field characterization. The interfacial adhesion between the iron fillers and the silicone matrix in MREs submitted to large deformations is investigated and a critical strain threshold is identified beyond which a primer treatment of the particles is needed to prevent debonding between the particles and the matrix. The second part of this thesis focuses on the coupled magneto-mechanical characterization of MREs and involves both theoretical and experimental developments. Based on the general theoretical framework for transversely isotropic magneto-elastic continua proposed by Kankanala, Danas and Triantafyllidis [Kan04, Dan12, Dan14], the coupled magneto-mechanical constitutive laws for both isotropic and anisotropic MREs are used to determine experimentally the corresponding constitutive model’s material parameters. The actual characterization of MREs is conducted thanks to a specially designed and novel experimental setup allowing tensile tests up to large strains and under high magnetic fields. The experimental data thus obtained provide the constitutive models for the isotropic and anisotropic MREs needed as input for the subsequent numerical simulations. The third part of this work pertains to the experiments, modeling and numerical calculations for boundary value problems corresponding to the design of a haptic interface prototype. A coupled variational formulation for a non-uniform applied magnetic field, using displacement, magnetic vector potential and magnetization as independent variables, is proposed and subsequently applied to the solution of the boundary value problem of an MRE layer subjected to the spatially localized magnetic field produced by an electromagnetic coil. The axisymmetric problem is solved numerically using finite element analysis. The device has been built and experimental results are compared to numerical simulations, thus providing a benchmark for the validation of the axisymmetric simulations as well as a proof of concept for the design of haptic interface applications.
• In situ monitoring of the deformation mechanisms in titanium with different oxygen contents.
  
  • Barkia Bassem
  • Doquet Véronique
  • Couzinie Jean-Philippe
  • Guillot Ivan
  • Héripré Eva
  Materials Science and Engineering: A, Elsevier, 2015, 636, pp.91-102. The deformation mechanisms of two titanium batches with different oxygen contents were monitored during tensile tests performed along the rolling and transverse directions under an optical or scanning electron microscope, after EBSD mappings of grain orientations. Whereas the contribution of mechanical twinning was very limited, grain boundary sliding, sometimes leading to intergranular decohesion, as well as kink bands formation were observed. Based on the identification of the primary slip traces in a significant number of grains, the critical resolved shear stresses (CRSSs) for prismatic, basal and pi1<a> were estimated. Transmission electron microscopy was used to identify unambiguously dislocations of pi1<c+a> systems and to estimate the corresponding CRSS. The difference in oxygen content between T40 and T60 was found to modify the magnitude of the CRSSs, but to leave their relative values nearly unchanged. The evolutions in the work hardening rate were correlated with the active deformation mechanisms. (10.1016/j.msea.2015.03.044)
  DOI : 10.1016/j.msea.2015.03.044