Publications

2017

• Improving the experimental protocol for a more accurate identification of a given mechanical behavior in a single assay: application to skin
  
  • Affagard Jean-Sébastien
  • Wijanto Florent
  • Allain Jean-Marc
  Strain, Wiley-Blackwell, 2017. Mechanical properties of the skin, the external organ of the human body, are important for many applications such as surgery or cosmetics. Due to the highly hierarchical structure of the tissue, it is interesting to develop microstructural models which have a better predictability and should reduce the consequences of the sample variability. However, these models generally include a quite large number of mechanical parameters. Therefore, complex assays are required to achieve a proper identification of the microstructural models. We investigated here the best experimental protocol to identify a non-linear, anisotropic, model of skin behavior, namely the Holzapfel's law, using displacement field and force measurements. This was done through a sensitivity analysis of the different parameters. We determined first the 1 optimal assay, which appears to be a biaxial test with an alternated loading: first a stretch in one direction, then in the perpendicular one, and so on. To further improve the quality of the assay, we also determined the optimal geometry. Interestingly, slightly asymmetric geometries are more adequate than symmetric ones, while being easier to realize. (10.1111/str.12236)
  DOI : 10.1111/str.12236
• Stress corrosion cracking initiation of alloy 82 in hydrogenated steam
  
  • Chaumun Elizabeth
  • Crépin Jérôme
  • Duhamel Cecilie
  • Guerre Catherine
  • Héripré Eva
  • Sennour Mohamed
  • de Curieres Ian
  , 2018, pp.175-189. Experiments in hydrogenated steam were performed on several U-bend specimens extracted from two Alloy 82 welds. Results demonstrated that Alloy 82 is susceptible to SCC in hydrogenated steam at 400 °C and its susceptibility depends on its chemical composition, welding process and thermal treatment. The microstructure was characterized in the apex of the U-bend specimens. Chemical analysis were performed by electron probe microanalyser (EPMA) and secondary ion mass spectrometry (SIMS) on several areas in the weld in order to correlate crack initiation with chemical heterogeneities. It was concluded that there are more cracks in the roots of the weld passes where the impurity content (sulfur, titanium and aluminum) is higher. (10.1007/978-3-030-04639-2_11)
  DOI : 10.1007/978-3-030-04639-2_11
• In situ TEM observations of dislocation dynamics in α titanium: Effect of the oxygen content
  
  • Barkia Bassem
  • Couzinié Jean-Philippe
  • Lartigue-Korinek Sylvie
  • Guillot Ivan
  • Doquet Véronique
  Materials Science and Engineering, Elsevier, 2017, 703, pp.331-339. Plastic deformation micro-mechanisms, dislocation structures and glide kinetics in two titanium batches with moderate and high oxygen contents (450 and 3200 wppm, respectively) are investigated by in situ tensile tests in a transmission electron microscope, at room temperature. In both materials, plastic deformation is accommodated with <a> type screw dislocations gliding mainly in prismatic planes. The movement of screw segments is jerky, oxygen-dependent and strongly controlled by pinning on localized obstacles if the oxygen content is high. Dislocation multiplication is mainly controlled by the opening of loops produced by a double-cross slip mechanism at super-jogs. Evidences of composite glide between prismatic, first order pyramidal and basal planes are pointed out, proving the existence of intensive cross slip. (10.1016/j.msea.2017.07.040)
  DOI : 10.1016/j.msea.2017.07.040
• Prestrain dependent viscoelastic model: Structural computation under dynamic loading
  
  • Jalocha Dimitri
  • Constantinescu Andrei
  • Nevière Robert
  International Journal of Solids and Structures, Elsevier, 2017, 120, pp.1-6. The aim of this paper is to explore the effect of a prestrain dependent viscoelastic model on a complete structural computation under dynamic loading. The structure under consideration is a propellant based motor, used in launchers and boosters. The modeling takes into account the complete cycle life of the structure including a manufacturing and an in-service step. The computations are performed using standard finite elements and realized both with a linear and a prestrain dependent viscoelastic model in order to highlight the different prediction capabilities. The results exhibit a modification of the response of the structure under dynamic loading. (10.1016/j.ijsolstr.2016.12.011)
  DOI : 10.1016/j.ijsolstr.2016.12.011
• Effective response of classical, auxetic and chiral magnetoelastic materials by use of a new variational principle
  
  • Danas Kostas
  Journal of the Mechanics and Physics of Solids, Elsevier, 2017, 105, pp.25 - 53. This work provides a rigorous analysis of the effective response, i.e., average magnetization and mag-netostriction, of magnetoelastic composites that are subjected to overall magnetic and mechanical loads. It clarifies the differences between a coupled magnetomechanical analysis in which one applies a Eulerian (current) magnetic field and an electroactive one where the Lagrangian (reference) electric field is usually applied. For this, we propose an augmented vector potential variational formulation to carry out numerical periodic homogenization studies of magnetoelastic solids at finite strains and magnetic fields. We show that the developed variational principle can be used for bottom-up design of microstructures with desired magne-tomechanical coupling by properly canceling out the macro-geometry and specimen shape effects. To achieve that we properly treat the average Maxwell stresses arising from the medium surrounding the magnetoelastic representative volume element (RVE) while at the same time we impose a uniform average Eulerian–and not Lagrangian–magnetic field. The developed variational principle is then used to study a large number of ideal as well as more realistic two-dimensional microstructures. We study the effect of particle volume fraction, particle distribution and particle shape and orientation upon the effective magnetoelastic response at finite strains. We consider also unstructured isotropic microstructures based on random adsorption algorithms and we carry out a convergence study of the representativity of the proposed unit cells. Finally, three-phase two-dimensional auxetic microstructures are analyzed. The first consists of a periodic distribution of voids and particle chains in a polymer matrix, while the second takes advantage of particle shape and chirality to produce negative and positive swelling by proper change of the chirality and the applied magnetic field. (10.1016/j.jmps.2017.04.016)
  DOI : 10.1016/j.jmps.2017.04.016
• The mechanism of monomer transfer between two structurally distinct PrP oligomers
  
  • Armiento Aurora
  • Moireau Philippe
  • Martin Davy
  • Lepejova Nad’a
  • Doumic Marie
  • Rezaei Human
  PLoS ONE, Public Library of Science, 2017, 12 (7). In mammals, Prion pathology refers to a class of infectious neuropathologies whose mechanism is based on the self-perpetuation of structural information stored in the pathological conformer. The characterisation of the PrP folding landscape has revealed the existence of a plethora of pathways conducing to the formation of structurally different assemblies with different biological properties. However, the biochemical interconnection between these diverse assemblies remains unclear. The PrP oligomerisation process leads to the formation of neurotoxic and soluble assemblies called O1 oligomers with a high size heterodispersity. By combining the measurements in time of size distribution and average size with kinetic models and data assimilation, we revealed the existence of at least two structurally distinct sets of assemblies, termed Oa and Ob, forming O1 assemblies. We propose a kinetic model representing the main processes in prion aggregation pathway: polymerisation, depolymerisation, and disintegration. The two groups interact by exchanging monomers through a disintegration process that increases the size of Oa. Our observations suggest that PrP oligomers constitute a highly dynamic population. (10.1371/journal.pone.0180538)
  DOI : 10.1371/journal.pone.0180538
• Crack nucleation in variational phase-field models of brittle fracture
  
  • Tanne Erwan
  • Li Tianyi
  • Bourdin Blaise
  • Marigo J.-J
  • Maurini Corrado
  , 2017. Phase-field models, sometimes refered to as gradient damage or smeared crack models, are widely used methods for the numerical simulation of crack propagation in brittle materials. Theoretical results and numerical evidences show that they can predict the propagation of a pre-existing crack according to Griffith' criterion. For a one-dimensional problem, it has been shown that they can predict nucleation upon a critical stress, provided that regularization parameter be identified with the material's internal or characteristic length. In this article, we draw on numerical simulations to study crack nucleation in commonly encountered geometries for which closed-form solutions are not available. We use U-and V-notches to show that the nucleation load varies smoothly from that predicted by a strength criterion to that of a toughness criterion, when the strength of the stress concentration or singularity varies. We present validation and verifications numerical simulations for both types of geometries. We consider the problem of an elliptic cavity in an infinite or elongated domain to show that variational phase field models properly account for structural and material size effects. We conclude that variational phase-field models can accurately predict crack nucleation through energy minimization in a nonlinear damage model instead of introducing ad-hoc criteria.
• Propellant cohesive fracture during the peel test of a propellant/liner structure
  
  • Toulemonde Paul-Aymé
  • Diani Julie
  • Gilormini Pierre
  • Desgardin Nancy
  • Nevière Robert
  The Journal of Adhesion, Taylor & Francis, 2017, 94 (8), pp.657 - 666. (10.1080/00218464.2017.1332999)
  DOI : 10.1080/00218464.2017.1332999
• Parallel Hammerstein Models Identification using Sine Sweeps and the Welch Method
  
  • Roggerone Vincent
  • Rebillat Marc
  • Corteel Étienne
  , 2017, 50 (1), pp.14040-14045. Linearity is a common assumption for many real life systems. But in many cases, the nonlinear behavior of systems cannot be ignored and has to be modeled and estimated. Among the various classes of nonlinear models present in the literature, Parallel Hammertein Models (PHM) are interesting as they are at the same time easy to understand as well as to estimate when using exponential sine sweeps (ESS) based methods. However, the classical EES-based estimation procedure for PHM relies on a very specific input signal (ESS), which limits its use in practice. A method is proposed here based on the Welch method that allows for PHM estimation with arbitrary sine sweeps (ASS) which are a much broader class of input signals than ESS. Results show that for various ASS, the proposed method provides results that are in excellent agreement with the ones obtained with the classical ESS method. (10.1016/j.ifacol.2017.08.2434)
  DOI : 10.1016/j.ifacol.2017.08.2434
• Development of multi-physics and multi-scale Best Effort Modelling of pressurized water reactor under accidental situations
  
  • Targa Alexandre
  , 2017. The safety analysis of nuclear power plants requires a deep understanding of underlying key physical phenomena that determine the integrity of the physical containment barriers. At the present time, cutting edge models focus on a single aspect (discipline) of the physical system coupled with rough models of the other aspects needed to simulate the global system. But, safety analyses can be carried out based on Multiphysics and Multiscales modelling. This Best Effort approach would give a full and accurate (High Fidelity) comprehension of the reactor core under standard and accidental situations. In this approach, the physical phenomena are simulated as accurately as possible (according to present knowledge) by coupled models in the most efficient way. For example, codes exists that are accurate modellings of Neutronics, or modellings of thermal fluid mechanics inside the core, or modellings of thermal fluid mechanics over the whole system, or modellings of thermal mechanics of the fuel pin or over the whole device structure. A Best Estimate approach would couple these models in order to realize a global and accurate modelling of the Nuclear reactor. This approach requires to define well the models that are used in order to exactly specify their limits, and hence, specify uncertainties of the coupled model results in order to assume and optimize them.It is in this context that this PhD thesis work is being under taken. It consists in the development of a Multi-physics and multi-scale Best Estimate modelling in order to obtain an accurate analysis of Pressurized Water Reactor under standard and accidental operating situations. It mainly involves the understanding of each model and their interactions, followed by the implementation of multiphysics algorithms coupling Neutronics and Thermohydraulics at reactor scale to an accurate Thermomechanics at the elementary scale of the fuel pin. In addition, a work project has been carried out in order to prepare or improve the access to the local physical informations that are needed for the implementation of multiscale coupling scheme, at the elementary scale of the fuel pin.
• Norepinephrine reduces arterial compliance less than phenylephrine when treating general anesthesia-induced arterial hypotension
  
  • Vallée Fabrice
  • Passouant Olivier
  • Le Gall Arthur
  • Joachim Jona
  • Mateo Joaquim
  • Mebazaa Alexandre
  • Gayat Etienne
  Acta Anaesthesiologica Scandinavica Supplementum, Wiley-Blackwell, 2017, 61 (6), pp.590 - 600. Introduction: During general anesthesia, arterial hypotension is frequent and may be an important contributor to peri-operative morbidity. We assessed the effect of a 5µg bolus of Norepinephrine (NA) when compared with 50 µg bolus of Phenylephrine (PE) administered to treat hypotension during maintenance anesthesia, on MAP, derived cardiac output and arterial stiffness parameters. Methods: Patients scheduled for a neurosurgical procedure under general anesthesia were prospectively included. Monitoring included invasive blood pressure, esophageal Doppler and arterial tonometer used to estimate central aortic pressure with arterial stiffness parameters, such as augmentation index (Aix). After initial resuscitation, hypotensive episodes were corrected by a bolus administration of NA or PE in a peripheral venous line. Results: There were 269 bolus administrations of vasopressors (149 NA, 120 PE) in 47 patients with no adverse effects detected. A decrease in stroke volume (SV) was observed with PE compared with NA (-18±9% vs -14±7%, p<0.001). This decrease was associated with an increase in Aix, which was greater for PE than for NA (+10±8% vs +6±6%, p<0.0001), and a decrease in total arterial compliance greater for PE compared to NA (Ctot=SV/Central Pulse Pressure) (-35±9% vs. -29±10%, p<0.001). Discussion: This study suggests that 5 µg of NA administered as a bolus in a peripheral venous line could treat general-anesthesia-induced arterial hypotension with a smaller decrease in SV and arterial compliance when compared to PE. (10.1111/aas.12905)
  DOI : 10.1111/aas.12905
• New indices from microneurography to investigate the arterial baroreflex
  
  • Laurin Alexandre
  • Lloyd Matthew G.
  • Hachiya Tesshin
  • Saito Mitsuru
  • Claydon Victoria E.
  • Blaber Andrew
  Physiological Reports, Wiley, 2017, 5 (12). Baroreflex‐mediated changes in heart rate and vascular resistance in response to variations in blood pressure are critical to maintain homeostasis. We aimed to develop time domain analysis methods to complement existing cross‐spectral techniques in the investigation of the vascular resistance baroreflex response to orthostatic stress. A secondary goal was to apply these methods to distinguish between levels of orthostatic tolerance using baseline data. Eleven healthy, normotensive males participated in a graded lower body negative pressure protocol. Within individual neurogenic baroreflex cycles, the amount of muscle sympathetic nerve activity (MSNA), the diastolic pressure stimulus and response amplitudes, diastolic pressure to MSNA burst stimulus and response times, as well as the stimulus and response slopes between diastolic pressure and MSNA were computed. Coherence, gain, and frequency of highest coherence between systolic/diastolic arterial pressure (SAP/DAP) and RR‐interval time series were also computed. The number of MSNA bursts per low‐frequency cycle increased from 2.55 ± 0.68 at baseline to 5.44 ± 1.56 at −40 mmHg of LBNP. Stimulus time decreased (3.21 ± 1.48–1.46 ± 0.43 sec), as did response time (3.47 ± 0.86–2.37 ± 0.27 sec). At baseline, DAP‐RR coherence, DAP‐RR gain, and the time delay between decreases in DAP and MSNA bursts were higher in participants who experienced symptoms of presyncope. Results clarified the role of different branches of the baroreflex loop, and suggested functional adaptation of neuronal pathways to orthostatic stress. (10.14814/phy2.13220)
  DOI : 10.14814/phy2.13220
• Identification haute résolution de nombres d'ondes pour la caractérisation des stratifiés anisotropes
  
  • Margerit Pierre
  • Lebée Arthur
  • Caron Jean-François
  • Ege Kerem
  • Boutillon Xavier
  , 2017, pp.151985. Une méthode originale d’extraction haute résolution de nombres d’onde (HRWA pour High Resolution Wavenumber Analysis), qui permet la caractérisation large bande et locale du comportement viscoélastique linéaire de plaques composites anisotropes, est présentée. Cette méthode, indépendante des conditions aux limites et du type d’excitation, est héritée de méthodes de traitement du signal, ESPRIT (Estimation of Signal Parameters via Rotational Invariance Techniques) et ESTER (ESTimation of ERror), robustes et complètement automatisées. L’implémentation de la méthode HRWA est développée. Trois cas de mesure sur échantillons sont présentés et permettent de montrer les différents potentiels de la méthode. On montre la possibilité d’identifier différents types d’onde (de flexion, de compression, de cisaillement) sur une même mesure. Un phénomène de band-gap est identifié sur un sandwich nid d’abeille. Les différentes directions de fibres dans une plaque à anisotropie variable sont identifiées.
• PROBABILISTIC THERMOMECHANICAL FATIGUE CRITERIA FOR SPHEROIDAL GRAPHITE CAST-IRONS
  
  • Szmytka Fabien
  • Charkaluk E.
  • Constantinescu A.
  , 2017. This paper proposes a method to establish and identify a probability density function characterizing the low-cycle fatigue lifetime for a cast-iron between 300 and 600°C. The method is initiated with a quantitative analysis of the microstructure of the material, which provides the initial probability distribution of graphite particles size. After identifying a given probability density function of particles, one can transport it into a lifetime probability density function using a growth law involving a macroscopic measure of the inelastic loading over a cycle. Several parameters of the growth law are finally estimated from a given set of fatigue experiments on specimens providing a criterion enabling a very satisfying matching between computed and experimental lifetime.
• Isothermal low cycle fatigue of a lost foam cast Al-Si-Cu alloy: study of the damage mechanisms with synchrotron X-ray tomography and Digital Volume Correlation
  
  • Limodin Nathalie
  • Wang Long
  • Dahdah Nora
  • El Bartali Ahmed
  • Witz Jean-Francois
  • Seghir Rian
  • Charkaluk Eric
  • Buffiere Jean-Yves
  , 2017, pp.41. Al-Si-Cu alloys manufactured by the Lost Foam Casting process have a complex microstructure. In order to study its role on damage under loading conditions representative of the in-service conditions of automotive cylinder heads, in situ Low Cycle Fatigue tests monitored by synchrotron X-ray tomography have been performed at room and high temperature (250°C). The initiation and the first propagation stages of short fatigue cracks have been observed in the interior of the samples. Those observations were correlated with the 3D strain fields obtained from Digital Volume Correlation. Cracks nucleate usually at the first loading cycle at the sharp edges of subsurface pores due to their notch effect that induces a strain localization but also at hard inclusions located in the pores neighbourhood. Then, crack propagation proceeds through hard inclusions where the von Mises cumulated strain increases. This scenario is however temperature dependent. At room temperature, crack initiation is porosity driven while propagation is correlated with the presence of hard phases in the interdendritic space. At high temperature, massive fracture of eutectic Silicon was observed. Crack propagation is then dictated by the coalescence of these microcracks in the most strained area of the specimen, i.e. between or close to large pores.
• Characterisation of 3D damage mechanisms of a cast Al alloy during in‐situ high temperature low cycle fatigue tests using Synchrotron X‐ray tomography and digital volume correlation
  
  • Dahdah Nora
  • Limodin N
  • El Bartali Ahmed
  • Witz Jean-Francois
  • Charkaluk Eric
  • Buffiere Jean-Yves
  , 2017. The damage mechanisms of a cast Al alloy (A319) are studied in-situ during low cycle fatigue at high temperature (250 ➦C) using synchrotron X-ray tomography. 3D images of the development of damage under load are obtained and correlated with the local strain levels obtained with Digital Volume Correlation
• Equilibrated Warping: Finite Element Image Correlation with Mechanical Regularization
  
  • Genet Martin
  • Lee Lik Chuan
  • Kozerke Sebastian
  , 2017. Image correlation/registration is playing an increasing role in many domains such as biomedical engineering. Despite significant progress in the past decades, robustness, efficiency and precision of existing methods and tools must still be improved to translate them into medical and engineering applications. This abstract describes a finite element-based image correlation method with, as regularization, a novel continuum large deformation formulation of the equilibrium gap principle (introduced in [Claire et al., 2004] at the discrete level for linearized elasticity).
• Assessment of atrioventricular valve regurgitation using biomechanical cardiac modeling
  
  • Chabiniok Radomir
  • Moireau Philippe
  • Kiesewetter Christoph
  • Hussain Tarique
  • Razavi Reza
  • Chapelle Dominique
  , 2017, 10263, pp.401-411. In this work we introduce the modeling of atrioventricular valve regurgitation in a spatially reduced order biomechanical heart model. The model can be fast calibrated using non-invasive data of cardiac magnetic resonance imaging and provides an objective measure of contractile properties of the myocardium in the volume overloaded ven-tricle, for which the real systolic function may be masked by the significant level of the atrioventricular valve regurgitation. After demonstrating such diagnostic capabilities, we show the potential of modeling to address some clinical questions concerning possible therapeutic interventions for specific patients. The fast running of the model allows targeting specific questions of referring clinicians in a clinically acceptable time.
• Fatigue crack growth law identification by Digital Image Correlation and electrical potential method for ductile cast iron
  
  • Hosdez Jerome
  • Witz Jean-François
  • Martel Corentin
  • Limodin N.
  • Najjar Denis
  • Charkaluk E.
  • Osmond Pierre
  • Szmytka F.
  Engineering Fracture Mechanics, Elsevier, 2017. In this paper, a comparison between two methods used to identify fatigue crack propagation law is conducted: Digital Image Correlation (DIC) and Direct Current Potential Drop (DCPD). For this purpose, fatigue tests were conducted at R-ratio of 0.1 on a ductile cast iron commonly used for exhaust manifolds manufacturing. Results show a good agreement between the methods illustrating the accuracy of each technique for the analysis of fatigue crack growth. Moreover, an interest of DIC is also to allow studying the plasticity that occurs at the crack tip during the fatigue test.
• Joint-state and parameters estimation using nudging and SEIK filters for HIV mechanistic models
  
  • Prague Mélanie
  • Thiébaut Rodolphe
  • Moireau Philippe
  • Collin Annabelle
  , 2017. Différentes méthodes ontétéontété utilisées dans le domaine de la statistique pour estimer les paramètres dans les modèles mécanistes. En particulier, l'approche basée sur la vraissemblance pénalisée pour l'estimation des paramètres dans leséquationsleséquations différentielles ordinaires avec des modèles non linéaireslinéairesà effets mixtes sur les paramètres (ODE-NLME) est souvent employée. Nous utilisons ici le programme NIMROD [Prague2013] comme référence pour l'estimation dans ces modèles. Cependant, une telle approche prend beaucoup de temps de calcul. Nous proposons d 'envisager l'assimilation de données, historiquement utilisée dans le contexte de la géophysique. Nous proposons un observateur d'´ etat de Luenberger couplé couplé`coupléà un observateur de Kalman (filtre RoUKF, ´ egalement appelé filtre SEIK) pour effectuer une estimation conjointe desétatsdesétats et des paramètres sur un ensemble de données composé d'observations longitudinales de biomarqueurs pour de multiples patients. Nous comparons ces méthodes en termes de performances et de temps de calcul. Nous discutons comment le concept d'effets aléatoires peutêtrepeutêtre modélisé en utilisant les filtres de Kalman et ses limites. Nous illustrons les deux méthodes en simulation et sur deux ensembles de données (l'essai clinique randomisé ALBI ANRS 070 et les données d'observation de la cohorte d'Aquitaine ANRS CO3) en utilisant un modèle mécaniste du VIH. Mots-clés. Bayésien; Equation Différentielle ordinaire; Kalman and Luenberger filters; Modèles m/'ecanistes; Modèles non linéairè a effets mixtes; VIH; Vraissemblance pénalisée. Abstract. Various methods have been used in the statistical field to estimate parameters in mechanistic models. In particular, approach based on penalised likelihood for estimation of parameters in ordinary differential equations with non linear models on parameters (ODE-NLME) has proven successful. We will consider the NIMROD program [Prague2013] as a benchmark for estimation in these models. However, such approach is time consuming. We propose to consider data assimilation which historically arose in the context of geophysics. We propose a Luenberger (also called nudging) state observer coupled with a parameter Kalman-based observer (RoUKF filter, also called SEIK filter) to perform a joint state and parameter estimation on a dataset composed of longitudinal observations of biomarkers for multiples patients. We compare these methods in term of performances and computation time. We discuss how the concept of random effect can be modelled using Kalman-based filter and its limitations. We illustrate both methods in simulation and on two datasets 1
• A spheroid model for the sound radiation of a loudspeaker on a sound bar
  
  • Roggerone Vincent
  • Corteel Étienne
  • Boutillon Xavier
  , 2017, 141 (5), pp.3968 - 3968. The sound radiation of a loudspeaker on a sound bar with a slender shape is analyzed. Measurements and boundary element method (BEM) simulations of a rectangular rigid enclosure with a flat piston turn out to be in close agreement up to the frequency limit imposed by the discretization chosen for the BEM. Looking up for a shorter computation time, we consider an analytic model based on a geometrical approximation of the sound bar by a prolate spheroid. The corresponding spheroidal coordinate system allows for an analytical solution of the sound-radiation problem. The following parameters are adjusted: geometry of the ellipse-based spheroid, size and location of the circular piston, minimum order of the spheroidal wave functions that ensures convergence. In the light of the BEM results, we also predict the frequency validity of the analytic model. In order to improve the control of the acoustical field radiated by a sound bar, we discuss the influence of the enclosure edges on the regularity of the sound field pattern. [Work supported by the ANR-13-CORD-0008 EDISON 3D grant from the French National Agency of Research.] (10.1121/1.4989036)
  DOI : 10.1121/1.4989036
• Hierarchical honeycomb material design and optimization: Beyond linearized behavior
  
  • Combescure Christelle
  • Elliott Ryan
  International Journal of Solids and Structures, Elsevier, 2017, 115-116, pp.161 - 169. This paper explores the importance of nonlinear material properties in the design of hierarchical honeycomb materials. The recent literature on the design and optimization of linear material properties for hierarchical honeycombs is reviewed. Then a full nonlinear post-bifurcation numerical analysis is performed for five representative hierarchical honeycomb structures. Particular attention is paid to the following four nonlinear material properties: the critical load λ c at which the structure first experiences an instability; the plastic critical load λ p at which the onset of plasticity would occur (if no elastic instability occurred); the stable post-bifurcated structure of the honeycomb; and the purely elastic resilience of the nonlinear material. It is found that although the honeycomb's linear Young's modulus is optimally maximized at a hierarchy ratio of γ 1 ≈ 30%, the critical load is reduced by a factor of two (relative to the standard honeycomb) at this ratio. Further, the critical load displays a monotone decreasing trend with increasing hierarchy ratio. A similar trend is found for the plastic critical load. A non-monotone trend for the resilience is discovered and explained by a qualitative change in the stable post-bifurcated structure for the hierarchical honeycombs which occurs as the hierarchy ratio is increased. The observed loss of strength (decreased critical load) is significant and may negate any advantages of the increased Young's modulus. This result demonstrates the importance of considering nonlinear properties and their implications in the design and optimization of hierarchical materials. (10.1016/j.ijsolstr.2017.03.011)
  DOI : 10.1016/j.ijsolstr.2017.03.011
• Brittle and semibrittle creep of Tavel limestone deformed at room temperature
  
  • Nicolas A
  • Fortin J
  • Regnet J B
  • Verberne B A
  • Plümper O
  • Dimanov A
  • Spiers C J
  • Guéguen Y
  Journal of Geophysical Research : Solid Earth, American Geophysical Union, 2017, 122, pp.4436 - 4459. Deformation and failure mode of carbonate rocks depend on the confining pressure. In this study, the mechanical behavior of a limestone with an initial porosity of 14.7% is investigated at constant stress. At confining pressures below 55 MPa, dilatancy associated with microfracturing occurs during constant stress steps, ultimately leading to failure, similar to creep in other brittle media. At confining pressures higher than 55 MPa, depending on applied differential stress, inelastic compaction occurs, accommodated by crystal plasticity and characterized by constant ultrasonic wave velocities, or dilatancy resulting from nucleation and propagation of cracks due to local stress concentrations associated with dislocation pileups, ultimately causing failure. Strain rates during secondary creep preceding dilative brittle failure are sensitive to stress, while rates during compactive creep exhibit an insensitivity to stress indicative of the operation of crystal plasticity, in agreement with elastic wave velocity evolution and microstructural observations. (10.1002/2016jb013557)
  DOI : 10.1002/2016jb013557
• Buckling response of architectured columns: controlling instability across the scales through a rational design.
  
  • Tarantino Gabriella
  • Caruel Matthieu
  • Danas Kostas
  , 2017. In the present study, we combine numerical simulations with compressive testing of 3D printed polymer structures to investigate the buck ling response of a slender column, whose architecture employs unit-cell lattices which in turn consist of a sequence of columns uniformly spaced. The lower-scale columns are both of comparable size with the macro-geometry and a priori susceptible to buckling. Within the accuracy of data, the experimental trends are consistent with the numerical simulations and show that both the buckling and the post-buckling response at the macroscopic (i.e. continuum) level are dependent on the lower-scale microstructure.
• Développement d'une technique de couplage de Corrélation d'Images et de Méthode des Éléments Finis appliquée à la fissuration en fatigue
  
  • Hosdez Jérôme
  • Langlois Médéric
  • Witz Jean-Francois
  • Limodin Nathalie
  • Najjar Denis
  • Charkaluk Eric
  , 2017. Dans cette étude, des couplages de techniques de corrélation d’images avec des calculs par éléments finis ont permis d’étudier la plasticité se produisant en pointe de fissure durant un essai de propagation de fissure de fatigue. La méthodologie consiste en l’application, sous forme de conditions aux limites de type Dirichlet, des champs de déplacement expérimentaux obtenus par corrélation d’images à un modèle éléments finis élasto-plastique. L’évolution de la plasticité, se produisant en pointe de fissure, passant d’un état confiné à généralisé, a pu être observée et quantifiée