Publications

2018

• A generalisation to cohesive cracks evolution under effects of non-uniform stress field
  
  • Pham Tuan-Hiep
  • Laverne Jérôme
  • Marigo Jean-Jacques
  Vietnam Journal of Mechanics, Viện Hàn Lâm Khoa học và Công nghệ Việt Nam, 2018, 40 (4), pp.349-375. The aim of the present work is to study the stabilizing effect of the non-uniformity of the stress field on the cohesive cracks evolution in two-dimensional elastic structures. The crack evolution is governed by Dugdale's or Barenblatt's cohesive force models. We distinguish two stages in the crack evolution: the first one where all the crack is submitted to cohesive forces, followed by a second one where a non cohesive part appears. Assuming that the material characteristic length d c associated with the cohesive model is small by comparison to the dimension L of the body, we develop a two-scale approach, and using the complex analysis method, we obtain the entire crack evolution with the loading in a closed form for the Dugdale's case and in semi-analytical form for the Barenblatt's case. In particular, we show that the propagation is stable during the first stage, but becomes unstable with a brutal jump of the crack length as soon as the non cohesive crack part appears. We discuss also the influence of all the parameters of the problem, in particular the non-uniform stress and cohesive model formulations, and study the sensitivity to imperfections. (10.15625/0866-7136/13499)
  DOI : 10.15625/0866-7136/13499
• Extracellular Matrix acts as pressure detector in biological tissues:
  
  • Dolega Monika
  • Brunel Benjamin
  • Le Goff Magali
  • Greda Magdalena
  • Verdier Claude
  • Joanny Jean-François
  • Recho Pierre
  • Cappello Giovanni
  , 2018. Imposed deformations play an important role in morphogenesis and tissue homeostasis, both in normal and pathological conditions 1-5. To perceive mechanical perturbations of different types and magnitudes, tissues need a range of appropriate detectors 6-8 , with a compliance that has to match the perturbation amplitude. As a proxy of biological tissues, we use multicellular aggregates, a composite material made of cells, extracellular matrix and permeating fluid. We compare the effect of a selective compression of cells within the aggregate, leaving the extracellular matrix unstrained, to a global compression of the whole aggregate. We show that the global compression strongly reduces the aggregate volume 9-13 , while the same amount of selective compression on cells has almost no effect 14,15. We support this finding with a theoretical model of an actively pre-stressed composite material, made of incompressible and impermeable cells and a poroelastic interstitial space. This description correctly predicts the emergent bulk modulus of the aggregate as well as the hydrodynamic diffusion coefficient of the percolating interstitial fluid under compression. We further show that, on a longer timescale, the extracellular matrix serves as a sensor that regulates cell proliferation and migration in a 3D environment through its permanent deformation and dehydration following the global compression. (10.1101/488635)
  DOI : 10.1101/488635
• Caractérisation et modélisation de la rupture de l’alliage Ti-6Al-4V sous chargement dynamique complexe
  
  • Ruiz de Sotto Miguel
  • Doquet Véronique
  • Longère Patrice
  • Papasidero Jessica
  , 2018.
• Electromechanical couplings and growth instabilities in semiconductors
  
  • Guin Laurent
  , 2018. In the last decades, solid mechanics has gone beyond its original issues of mechanical properties of materials and structures to embrace problems coming from other scientific fields and in particular physics. Semiconductors, the base materials of all electronic devices, are a prime example where crystalline solids show multiphysics couplings. Indeed, mechanics plays there an important role both in the fabrication process and in the operation of electronic devices. In this work, we examine these two aspects by studying first the couplings between electronic transport phenomena and mechanical deformations and second the morphological instabilities that develop in semiconductor epitaxial growth.First, developing a fully-coupled theory of deformable semiconductors that includes mechanical, electrical and electronic fields, we show for the first time the existence of an electronic contribution to mechanical stress. While for crystalline semiconductors this contribution is weak, the effect of strains on electronic transport remains significant through their modification on band energy levels, density of states and mobility of electrons and holes. Considering the advent of new technologies of flexible electronics, we apply the general theory to compute through asymptotic expansions, the effect of bending -causing non-uniform strains- on the current-voltage characteristic of a p-n junction, the basic device of solar cells. To complete this picture, we measure the changes induced by uniaxial stresses on the electronic characteristic of a silicon heterojunction solar cell.In the second part of this work, going down to the atomic scale, we consider the problem of epitaxial growth on vicinal surfaces. On these surfaces, the crystal grows through the propagation of the atomic steps, which may develop step bunching, an instability whereby the regular step spacing breaks down, resulting in an alternating pattern of wide atomic terraces and step bunches. Through a comprehensive linear stability analysis of the step dynamics governing equations, we discuss the influence of each physical mechanism on the step bunching instability. In particular, we clarify the impact on stability of the dynamics, of the recently pointed out adatom jump effect, and of elasticity, beyond the assumption of nearest-neighbor interactions. In addition, we show that our general stability results, i.e., obtained without neglecting the dynamics terms, are significantly different from those derived with the quasistatic approximation, even in regimes of slow deposition or evaporation where the latter was considered sufficient. Not only valuable from a theoretical prospective, these new results provide possible explanations for some cases of step bunching observed in silicon and gallium arsenide. In view of these new aspects, we reexamine the problem of step bunching under electromigration and show that the adatom jump and dynamics effects do not affect the stability dependence on the direction of the electromigration current.Finally, we investigate the mechanical properties at the atomic scale of another crystalline material with semiconducting properties, polycrystalline graphene. Using molecular dynamic simulation, we develop a cohesive zone model for fracture along grain boundaries.
• Creep Fatigue Interaction in Solder Joint Alloys of Electronic Packages
  
  • Zanella Stéphane
  , 2018. Solder joints reliability analysis represents a challenge for the aerospace and defense industries, which are in need of trustworthy equipment with a long lifetime in harsh environments. The evolution of electronic packages, driven by consumer civil applications, introduces new architectures and materials for which reliability needs to be qualified for the constraints of the aerospace and defense applications. One of the most critical elements of an electronic assembly is the solder joint interconnection. In this context, the knowledge of fatigue properties of solder material is required to design the assemblies, to define accelerated tests or to perform lifetime simulations.Fatigue laws used commonly in the industry are generally simplified criteria such as Coffin-Manson relation, based on inelastic deformation, or Morrow relation, based on dissipated energy per cycle. Cyclic creep and plastic strains are mingled and formulated as a unique inelastic strain in these relationships. The underlying assumption is that damage contributions of creep and plasticity phenomena are equivalent. The relevance of these laws in the case of solder joint and the mission profiles of aerospace and defense industries can be discussed. In fact, solder joint materials have low melting temperatures which are required by the assembly manufacturing process, inducing viscous strains even at room temperature. In this context, important viscous strains are developed due to the harsh environment with high temperatures and the long maintain phases of space, defense and avionics industries. Creep-fatigue interaction must be taken into account for solder joint material in order to address these applications requirements.Limitations of the literature are the lack of clear experimental data separating plastic and creep strains during fatigue tests. Representativeness of experimental tests based on bulk samples can be discussed because of the complex microstructure of solder joints. No consensus exists on the mechanical model and the parameters. In this context, an innovative test bench has been developed to perform shear fatigue tests with assembled electronic packages in order to study creep-fatigue interaction in solder joints.Dwell time, temperature and force have an impact on the number of cycles to failure. Combined increase of temperature and dwell time reduces the number of cycles to failure until a factor of 10. The hysteresis response of the package is converted in stress and plastic and viscous strains in order to calibrate a viscoplastic model and a fatigue law. Results show limitations of classic Coffin-Manson fatigue law. Experimental results from the literature have been used to complete our test plan. A frequency modified fatigue model shows increased prediction accuracy for fatigue tests performed at different frequencies. In fact, time-dependent viscous damage is included in the law by the frequency factor. However, limitations of this law have been found in particular for long dwell time configuration. A creep-fatigue model is proposed to dissociate damages from plastic and viscous strains. This fatigue law increases prediction accuracy in the case of high temperature and long dwell time configuration. Microstructure evolutions indicate the destruction of the dendritic structure and replaced by small grains recrystallization in the area close to the fracture. Coalescence of different precipitates is also observed in the damaged area. More investigations on this topic are required in order to evaluate the specific markers of plastic and viscous damages.
• Shape-morphing architected sheets with non-periodic cut patterns
  
  • Celli Paolo
  • Mcmahan Connor
  • Ramirez Brian
  • Bauhofer Anton
  • Naify Christina
  • Hofmann Douglas
  • Audoly Basile
  • Daraio Chiara
  Soft Matter, Royal Society of Chemistry, 2018, 14 (48), pp.9744-9749. We investigate the out-of-plane shape morphing capability of single-material elastic sheets with architected cut patterns that result in arrays of tiles connected by flexible hinges. We demonstrate that a non-periodic cut pattern can cause a sheet to buckle into three-dimensional shapes, such as domes or patterns of wrinkles, when pulled at specific boundary points. These global buckling modes are observed in experiments and rationalized by an in-plane kinematic analysis that highlights the role of the geometric frustration arising from non-periodicity. The study focuses on elastic sheets, and is later extended to elastic-plastic materials to achieve shape retention. Our work illustrates a scalable route towards the fabrication of three-dimensional objects with nonzero Gaussian curvature from initially-flat sheets. (10.1039/c8sm02082e)
  DOI : 10.1039/c8sm02082e
• Transformation paramétrique de distance à vol d’oiseau en distance sur une voirie
  
  • Nguyen Huong
  • Robin Frédérique
  • Vonseel Audrey
  , 2018. Ce rapport présente les résultats obtenus à l’issue de la semaine d’études maths-entreprise (SEME), organisée par l’Agence pour les Mathématiques en Interaction avec les Entreprises (AMIES) et le Laboratoire de Mathématiques et Applications (LMA) de l’Université de Poitiers et s’étant déroulée à Poitiers au printemps 2018. L’étude, proposée par Catherine Gloaguen et encadrée par Hermine Biermé, vise à proposer une estimation d’une distance D au plus court chemin sur une carte aléatoire modélisant des grands réseaux d’accès fixes sachant la distance à vol d’oiseau d. La modélisation aléatoire des rues utilise des techniques de géométrie stochastique (pavages de Poisson type Voronoï, Delaunay ou Ligne) et est considérée comme connue.
• Study on magneto-sensitive solids : Experiments, Theory and Numerics
  
  • Psarra Erato
  , 2018. The present work deals with the stability and post-bifurcation response of isotropic magnetorheological elastomers (MREs). MREs are elastomers comprising a finite volume fraction of magnetizable iron particles, distributed randomly in the volume. Specifically, a nonlinear magnetoelastic film/substrate system is experimentally, numerically and theoretically exploited to obtain active control of surface roughness. The non-intuitive interplay between magnetic field and elastic deformation owes to material and geometry selection, namely, a ferromagnetic particle composite film bonded on a compliant passive foundation. Cooperation of two otherwise independent loading mechanisms--mechanical pre-compression and magnetic field--allows to bring the structure near a marginally stable state and then destabilize it with either magnetic or mechanical fields. We demonstrate for the first time that the critical magnetic field is a decreasing function of pre-compression and vice versa. The experimental results are probed successfully with full-field finite element simulations at large strains and magnetic fields. A theoretical magnetomechanical bifurcation analysis on an infinite magnetoelastic system is further employed to explore the effect of the interlayer combined properties on the critical response and is compared with the available numerical results. With the perspective of applying the principle of surface actuation to new magnetomechanically triggered patterns, we further investigate the post-bifurcation of an entirely magnetorheological bilayer block. The underlying idea is to create different interlayer contrasts of magnetic and mechanical properties allowing us to trigger a larger range of surface patterns than that already obtained when using a MRE film on a passive (magnetically insensitive) foundation. Post-bifurcation calculations of MRE films bonded on MRE substrates allow to reveal novel patterns that lead to significant curvature localisation and crinkling. In all cases studied, the magnetoelastic coupling allows for the reversible on/off control of surface patterning under adjustable critical magnetic and mechanical fields for a single specimen and thus, this study constitutes a first step towards realistic active haptic and morphing devices.
• Variational approach to dynamic fracture and applications to the fragmentation of metals and ceramics
  
  • Geromel Fischer Arthur
  , 2018. The main objective of this work was the study of the fragmentation of a metallic shell. This thesis is divided into four parts: construction of a damage model, numerical implementation, calibration of the model parameters using experimental data and analytical works.In this work, we considered a model that couples the standard gradient damage models with plasticity and dynamics. Using the energy and the action of the system, we could obtain all the equations necessary to describe the dynamic ductile model: the equations of dynamics, the plasticity criterion and the damage criterion. We then detail the numerical implementation of these models.Some qualitative behaviours are then obtained, such as the number and the direction of cracks, and the convergence to the quasi-static model.In order to better understand the influence of the parameters, we studied the problem analytically. By studying the amplitude of the perturbations, we describe how to obtain an analytic approximation for the number of cracks in a ring under expansion.In order to run realistic simulations, it is needed to calibrate the material parameters. We focus here on a simple case of brittle materials. The experimental data were obtained in a series of shockless spalling tests performed by the CEA.We also study other forms of regularization, now applied to the plastic strain, avoiding localization in zero-thickness bands.We considered using the dissipative properties of the temperature field to regularize the model. Finally, we conclude with plasticity models where we add a term depending on the gradient of the plastic strain (gradient plasticity models).
• Caractérisation du mécanisme de glissement aux joints de grains dans l’aluminium à haute température par mesures de champs in situ MEB
  
  • El Sabbagh Alexandre
  , 2018. Dans de nombreuses applications industrielles les matériaux polycristallins sont soumis à de hautes températures, auxquelles le mécanisme de glissement aux joints de grains (GBS pour grain boundary sliding) tient un rôle essentiel. Il est fortement couplé à la plasticité intra cristalline, cependant peu de modèles tiennent compte de ce couplage. Le GBS est encore un mécanisme mal compris pour lequel nous manquons de quantifications expérimentales. Nous avons développé à cette fin un dispositif pour réaliser des expériences de compression in-situ dans un microscope électronique à balayage, équipé d’une mesure de température sans contact. Les essais ont été menés sur un aluminium à gros grains contenant 0.1% de manganèse entre 25°C et 400°C, à faible vitesse de déformation. Les champs cinématiques mesurés par corrélation d’images numérique ont permis d’analyser la mise en place des mécanismes de plasticité durant la déformation et leur évolution en fonction de la température. Nous avons mis en évidence un fort couplage entre les mécanismes plastiques intragranulaires et le GBS. A mesure que la température augmente nous avons constaté une forte évolution de la plasticité. La déformation se localise de plus en plus aux joints de grains, tandis que la plasticité dans les grains se complexifie impliquant de plus en plus de systèmes de glissement. Une méthode de corrélation d’images a été utilisée pour mesurer les discontinuités du champ cinématique aux joints de grains et quantifier la contribution du GBS à la déformation globale à 200°C. Celui-ci s’active dès le début et tout au long de la déformation. Nous avons constaté que malgré une taille de grains importante la contribution du GBS n’est pas négligeable, elle est plus importante en début de déformation puis semble atteindre un palier. Une approche locale a été développée pour quantifier l’amplitude locale du GBS. Cela a permis d’étudier et de discuter l’influence sur celui-ci de paramètres comme l’angle de désoriention du joint, un coefficient caractérisant le transfert du glissement intragranulaire à travers le joint, et l’orientation du joint par rapport à la direction de chargement. Ce dernier paramètre semble le plus influent, mais il ne suffit pas pour caractériser l’amplitude du glissement. Il apparaît que les propriétés locales de la microstructure influencent fortement celui-ci et ne peuvent être négligés.
• Mechanical Behaviour of Ultra fine grain aluminium alloy. Analysis and modelling of the enhanced role of grain boundaries
  
  • Goyal Anchal
  , 2018. Ultrafine grained (UFG) alloys seem promising, based on their high tensile properties and the possibility of superplastic forming at relatively low temperature. However, their deformation mechanisms are not fully understood, and their performance in fatigue has not been thoroughly investigated. This work compares the viscoplastic behavior, and the deformation and damage mechanisms in tension and fatigue of a UFG Al-Mg alloy (600 nm mean grain size) obtained by severe plastic deformation (ECAP process) with that of its coarse-grained (CG) counterpart.The strain rate sensitivity (SRS) of both materials has been measured during creep, relaxation and tensile tests run at various strain rates and temperature. Microstructural refinement is shown to increase the SRS, which rises as the strain rate decreases, and controls the ductility. The UFG material becomes softer and more ductile than the CG material at high temperature. The temperature and strain rate domain for which the UFG alloy is stronger or softer has been determined.Tensile tests run in a SEM, with DIC measurements of strain fields at meso/ micro scales (using gold microgrids printed by electron beam lithography) and at sub-micron scale (using a superfine speckle obtained by film remodelling) have shown that grain boundary sliding is more and more active in both materials as the temperature rises and as the strain rate decreases. Grain boundary sliding is cooperative and occurs mostly at high-angle grain boundaries in the UFG alloy, where the strain field is more heterogeneous, and where very high strain levels (> 100%) are often observed in localized bands.A 2D finite element model taking into account the viscoplastic behaviour inside the grains, and viscous sliding at the grain boundaries has been identified other the whole temperature range investigated. It captures well the observed behaviours and the much larger contribution of grain boundary sliding in the UFG alloy. It also provides the evolution of this contribution during strain hardening.Plastic strain-controlled push-pull tests and stress-controlled push-pull tests were run to investigate the cyclic behaviour and damage mechanisms of the two materials in low and high-cycle fatigue. The tests were followed by fractographic observations, statistical analysis of surface damage, as well as TEM observations of dislocations arrangements. Both materials exhibit cyclic hardening, although it is more modest in the UFG alloy, in which grain growth occurs at high amplitude. While isotropic hardening predominates in the CG alloy where the density of dislocation strongly increases during cyclic tests, kinematic hardening predominates in the UFG alloy, because of its limited capacity to store dislocations and its more heterogeneous plastic deformation. For a given plastic strain range, the UFG alloy has a shorter fatigue life than its CG counterpart, because of a much easier crack initiation, mostly from intermetallic particles. For a given stress range, it has a slightly higher life, due to a slower development of microcracks, which have a transgranular path in the largest grains, with some intergranular growth within the smallest grains.
• Time and Temperature Dependent Creep in Tournemire Shale
  
  • Geng Z.
  • Bonnelye A.
  • Chen M.
  • Jin Y.
  • Dick P.
  • David C.
  • Fang X.
  • Schubnel A.
  Journal of Geophysical Research : Solid Earth, American Geophysical Union, 2018, 123 (11), pp.9658-9675. We conducted a series of triaxial creep experiments on shale specimens coming from Tournemire, France, using the stress-stepping method up to failure, at a confining pressure of 80 MPa, on two orientations (parallel and perpendicular to bedding), and at temperatures of 26 and 75 °C. In these week-long experiments, stress, strains, and P wave ultrasonic velocities were recorded (quasi-) continuously. The strength at creep failure of Tournemire shale was ~70% higher than the peak strength measured during constant strain rate (~10−7/s) experiments, and failure was reached at larger strains. An overall transition from P wave velocity increase at moderate differential stress to P wave velocity decrease closer to brittle failure was also observed. At a smaller timescale, P wave velocities initially decreased and then increased gradually during each step of creep deformation. The magnitude of these variations showed important (i) stress, (ii) orientation, and (iii) temperature dependences larger increase was observed for P wave propagating along the main compressive stress orientation, larger decrease for P wave propagating perpendicular to it, and a changing behavior enhanced at a higher temperature. Scanning electron microscopy performed postmortem revealed evidence of time-dependent pressure solution, localized compaction, crack growth, and sealing/healing. Our data reveal that shale deformation is highly stress sensitive only in a narrow stress domain where stress corrosion cracking-induced brittle dilatant creep deformation is dominant. At stresses below, pressure solution compaction creep dominates the deformation and shales compact, consolidate, and heal. This has important implications for the mechanics of shallow fault zones and accretionary prisms. ©2018. American Geophysical Union. All Rights Reserved. (10.1029/2018JB016169)
  DOI : 10.1029/2018JB016169
• Measuring permeability vs depth in the unlined section of a wellbore using the descent of a fluid column made of two distinct fluids : inversion workflow, laboratory & in-situ tests
  
  • Manivannan Sivaprasath
  , 2018. In wells producing water, oil, gas or geothermal energy, or in access wells to hydrocarbon storage, it is critical to evaluate the permeability of the formation as a function of depth, to improve the reservoir model, and also to identify the zones where additional investigation or special completions are especially useful.A new technique is proposed, consisting of scanning the open hole (uncased section of the wellbore) with an interface between two fluids with a large viscosity contrast. The injection rate into the formation depends on interface location and well pressure history. An inverse problem should be solved: estimate permeability as a function of depth from the evolution of flow rates with time. The wells are usually equipped with a central tube. The scanning is done by injecting a liquid in the central tube at constant wellhead pressure. Injection and withdrawal rates are measured at the wellhead; the difference between these two rates is the formation injection rate.To validate and improve this technique, we used a laboratory model mimicking a multi-layer formation, already available at LMS. We also made use of in-situ tests performed on an ultra-low permeable cap rock above an underground gas storage reservoir. In these tests, a viscous fluid contained in the open hole was displaced by a less-viscous fluid (a method called opening WTLog). The more permeable layers were correctly identified (Manivannan et al. 2017), but a quantitative estimation was challenging due to transient phenomena in the vicinity of the wellbore (near-wellbore zone). In addition, the investigation radius was small.These challenges are addressed by proposing a slightly modified test procedure and a new interpretation workflow. Laboratory tests with a modified test setup showed the advantages of the ‘closing’ method in which the well is filled with a less-viscous fluid at the start of the test. We also added a stabilization period before the injection of viscous fluid to minimize the transient effects; this period is also used to estimate the average permeability of the open hole and the effect of near-wellbore damage (skin).Then the test proper is performed (closing WTLog). The injection profile of the less-viscous fluid is computed from the wellhead flow rate history. A permeability profile is estimated from the injection profile. The permeability estimation considers a monophasic flow in each layer and the same skin value for all the formation layers. Major uncertainties in the permeability estimates are caused by formation pressures and heterogeneities in skin values; they are estimated using an analytical formula. We have verified on the laboratory setup that the estimated permeability profiles are well correlated to the permeabilities measured before the tests.An attempt was made to perform a WTLog in a 1750-m long wellbore opening in a salt formation. The first phase was successful and the average permeability was correctly assessed. However, this permeability was so small (4.0E-21 m² or 4 nD) that the gauges and the flowmeters were not accurate enough to allow a clear distinction between the permeabilities of the various parts of the open hole.
• Caractérisation expérimentale du comportement orthotrope viscoplastique de l’alliage Ti-6Al-4V pour bord d’attaque des aubes du fan d’un moteur d’avion
  
  • Ruiz de Sotto Miguel
  • Doquet Véronique
  • Longère Patrice
  • Papasidero Jessica
  , 2018.
• Modèles d'endommagement à gradient en grandes déformations
  
  • Crabbé Blandine
  , 2018. Les modèles d'endommagement à gradient, aussi dénommés modèles à champs de phases, sont désormais largement utilisés pour modéliser la rupture fragile et ductile, depuis l'initiation de l'endommagement jusqu'à la propagation d'une fissure. Cependant, la majorité des études disponibles dans la littérature ne concerne que le cadre des petites déformations, et très peu d'études poussées ont été menées afin d'étudier leur pertinence dans un contexte de grandes déformations. Ce serait pourtant d'un intérêt primordial, notamment pour l'industrie pneumatique, qui deviendrait alors capable de prédire plus précisément l'initiation de l'endommagement dans ses structures.Dans la première partie de ce travail, nous établissons des solutions analytiques d'évolution de l'endommagement (homogène et localisée) pour des matériaux visqueux, en petites et en grandes déformations. En petites déformations, les modèles rhéologiques de Maxwell et Poynting-Thomson sont étudiés, et en grandes déformations, les modèles de Maxwell et Zener sont choisis. Une étude sur l'évolution de l'endommagement dans un cas purement hyperélastique est aussi menée.A cette première partie analytique succède une partie numérique, qui détaille l'implémentation des modèles d'endommagement à gradient dans des codes éléments finis en grandes déformations. De même qu'en petites déformations, une stratégie de minimisation alternée est adoptée pour résoudre successivement les problèmes d'endommagement et de déplacement. Le matériau suit une loi de Mooney-Rivlin quasi-incompressible, et une méthode mixte en déplacement-pression est utilisée. Des tests en 2D et 3D sont effectués, qui mettent en évidence la capacité des modèles à initier de l'endommagement en grandes déformations.Les modèles d'endommagement utilisés pour la seconde partie ne sont cependant capables d'initier de l'endommagement que dans les zones où la déformation est importante, c'est-à-dire dans les zones de forte contrainte déviatorique. Il a toutefois été montré que certains matériaux polymères, quasi-incompressibles, s'endommagent dans les zones de forte pression hydrostatique. Par conséquent, la recherche et l'étude d'un modèle d'endommagement capable d'initier de l'endommagement dans les zones de forte pression, pour des matériaux quasi-incompressibles lorsqu'ils sont sains, fait l'objet d'une troisième partie.Enfin, la croissance brusque de cavités dans un matériau hyperélastique, appelée phénomène de cavitation, est étudiée, ainsi que son interaction avec l'endommagement. Dans un premier temps, nous considérons la cavitation comme une simple bifurcation hyperélastique d'un matériau néo-hookéen compressible isotrope, et déterminons l'expression analytique de l'élongation critique pour laquelle la cavitation fait son apparition. Dans un second temps, nous montrons qu'il y a une compétition entre la cavitation et l'endommagement, et qu'en fonction de la valeur du ratio des élongations critiques respectives pour chaque phénomène, deux types de rupture apparaissent.
• Harnessing olfactory bulb oscillations to perform fully brain-based sleep-scoring and real-time monitoring of anaesthesia depth
  
  • Bagur Sophie
  • Lacroix Marie Masako
  • de Lavilléon Gaëtan
  • Lefort Julie M
  • Geoffroy Hélène
  • Benchenane Karim
  PLoS Biology, Public Library of Science, 2018, 16 (11), pp.e2005458. Real-time tracking of vigilance states related to both sleep or anaesthesia has been a goal for over a century. However, sleep scoring cannot currently be performed with brain signals alone, despite the deep neuromodulatory transformations that accompany sleep state changes. Therefore, at heart, the operational distinction between sleep and wake is that of immobility and movement, despite numerous situations in which this one-to-one mapping fails. Here we demonstrate, using local field potential (LFP) recordings in freely moving mice, that gamma (50-70 Hz) power in the olfactory bulb (OB) allows for clear classification of sleep and wake, thus providing a brain-based criterion to distinguish these two vigilance states without relying on motor activity. Coupled with hippocampal theta activity, it allows the elaboration of a sleep scoring algorithm that relies on brain activity alone. This method reaches over 90% homology with classical methods based on muscular activity (electromyography [EMG]) and video tracking. Moreover, contrary to EMG, OB gamma power allows correct discrimination between sleep and immobility in ambiguous situations such as fearrelated freezing. We use the instantaneous power of hippocampal theta oscillation and OB gamma oscillation to construct a 2D phase space that is highly robust throughout time, across individual mice and mouse strains, and under classical drug treatment. Dynamic analysis of trajectories within this space yields a novel characterisation of sleep/wake transitions: whereas waking up is a fast and direct transition that can be modelled by a ballistic trajectory, falling asleep is best described as a stochastic and gradual state change. Finally, we demonstrate that OB oscillations also allow us to track other vigilance states. Non-REM (NREM) and rapid eye movement (REM) sleep can be distinguished with high accuracy based on beta (10-15 Hz) power. More importantly, we show that depth of anaesthesia can be tracked in real time using OB gamma power. Indeed, the gamma power predicts and anticipates the motor response to stimulation both in the steady state under constant anaesthetic and dynamically during the recovery period. Altogether, this methodology opens the avenue for multi-timescale characterisation of brain states and provides an unprecedented window onto levels of vigilance. (10.1371/journal.pbio.2005458)
  DOI : 10.1371/journal.pbio.2005458
• Tracer diffusion in crowded narrow channels
  
  • Bénichou O.
  • Illien P.
  • Oshanin G.
  • Sarracino A.
  • Voituriez R.
  Journal of Physics: Condensed Matter, IOP Publishing [1989-....], 2018, 30 (44), pp.443001. We summarise different results on the diffusion of a tracer particle in lattice gases of hard-core particles with stochastic dynamics, which are confined to narrow channels—single-files, comb-like structures and quasi-one-dimensional channels with the width equal to several particle diameters. We show that in such geometries a surprisingly rich, sometimes even counter-intuitive, behaviour emerges, which is absent in unbounded systems. This is well-documented for the anomalous diffusion in single-files. Less known is the anomalous dynamics of a tracer particle in crowded branching single-files—comb-like structures, where several kinds of anomalous regimes take place. In narrow channels, which are broader than single-files, one encounters a wealth of anomalous behaviours in the case where the tracer particle is subject to a regular external bias: here, one observes an anomaly in the temporal evolution of the tracer particle velocity, super-diffusive at transient stages, and ultimately a giant diffusive broadening of fluctuations in the position of the tracer particle, as well as spectacular multi-tracer effects of self-clogging of narrow channels. Interactions between a biased tracer particle and a confined crowded environment also produce peculiar patterns in the out-of-equilibrium distribution of the environment particles, very different from the ones appearing in unbounded systems. For moderately dense systems, a surprising effect of a negative differential mobility takes place, such that the velocity of a biased tracer particle can be a non-monotonic function of the force. In some parameter ranges, both the velocity and the diffusion coefficient of a biased tracer particle can be non-monotonic functions of the density. We also survey different results obtained for a tracer particle diffusion in unbounded systems, which will permit a reader to have an exhaustively broad picture of the tracer diffusion in crowded environments. (10.1088/1361-648X/aae13a)
  DOI : 10.1088/1361-648X/aae13a
• Morphology, static and fatigue behavior of a natural UD composite: The date palm petiole ‘wood’
  
  • Benzidane R.
  • Sereir Z.
  • Bennegadi M.L.
  • Doumalin P.
  • Poilane C.
  Composite Structures, Elsevier, 2018, 203, pp.110-123. (10.1016/j.compstruct.2018.06.122)
  DOI : 10.1016/j.compstruct.2018.06.122
• A modified dissipated energy fatigue criterion to consider the thermo-oxidative ageing of electrically conductive silicone adhesive joints
  
  • Jouan Alexandre
  • Constantinescu Andrei
  International Journal of Fatigue, Elsevier, 2018, 116, pp.68-79. The increasing use of adhesive joints as an assembly technique in various technical fields at the expense of the classical soldering and welding technique brings up the critical questions of their reliability and their lifetime prediction. Fatigue testing of adhesive joints has therefore gained importance in the last few decades, in order to validate the choice of adhesives in structural applications. The influence of environment and ageing on the fatigue life has also gained attention as adhesives are used in wet, or oxidative, or high temperature atmospheres. This paper presents an experimental fatigue study led on a commercial conductive silicone adhesive following the classical stress-life approach. The bonded assemblies were fatigue tested either at virgin state or after thermo-oxidative ageing. Based on results of the fatigue tests, a law relying on two mechanical predictors including the dissipated energy is proposed that considers the influence of a thermo-oxidative ageing of the adhesive joint. (10.1016/j.ijfatigue.2018.06.005)
  DOI : 10.1016/j.ijfatigue.2018.06.005
• Evaluation of cardiac output variations with the peripheral pulse pressure to mean arterial pressure ratio
  
  • Tantot Audrey
  • Caillard Anais
  • Le Gall Arthur
  • Matéo Joaquim
  • Millasseau Sandrine
  • Mebazaa Alexandre
  • Gayat Etienne
  • Vallée Fabrice
  Journal of Clinical Monitoring and Computing, Springer Verlag, 2018. (10.1007/s10877-018-0210-8)
  DOI : 10.1007/s10877-018-0210-8
• Influence of the internal geometry on the elastic properties of materials using 3D-printing of computer-generated random microstructures
  
  • Zerhouni Othmane
  • Tarantino Maria-Gabriella
  • Danas Kostas
  • Hong Fei
  , 2018. Understanding elastic properties of rocks is a scientific challenge due to the complexity of their microstructures. This study combines the numerical tools to generate models for internal geometry of pores with the 3D printing technology in order to control the shape and size of pores as well as the distribution of pore's network inside the sample. Accuracy of the printing has been assessed by optical microscopy. The numerical and experimental tests conducted on generated microstruc-tures show that the elastic properties are independent of the size of the pore. In turn, the shape of the pore has a strong effect on the elastic properties. This study shows that in case of multiple pore types, the ones with small aspect ratios have a strong impact though of minor volume fraction. The methodology that is developped can be extended to investigate the influence of more parameters in case of connected porosity and tests theories that have been proposed to link physical properties of reservoir rocks to their internal geometry. (10.1190/segam2018-2998182.1)
  DOI : 10.1190/segam2018-2998182.1
• Méthodologie de caractérisation et de modélisation d'un joint adhésif sous sollicitations multiaxiales dynamiques.
  
  • Janin Anthony
  , 2018. Les joints adhésifs sont de plus en plus utilisés dans des structures industrielles critiques. Ils sont donc susceptibles de subir des chargements dynamiques complexes. Les méthodes de caractérisation dynamiques existantes ne caractérisent pas seulement le joint adhésif, mais l'assemblage collé tout entier. Cette thèse propose une méthode innovante pour caractériser un joint adhésif sous sollicitations dynamiques multiaxiales. La méthode expérimentale repose sur trois éléments principaux: i) un système de barres d'Hopkinson conventionnel (SHPB), ii) une nouvelle géométrie d'éprouvette, nommée DODECA, qui permet d'appliquer trois chargements multiaxiaux différents et iii) des mesures locales de déformation et de contrainte par corrélation d'images. La contrainte et la déformation dans le joint adhésif sont estimées directement à partir des données expérimentales pendant le chargement jusqu'au point de rupture. Une autre approche basée sur la méthode FEMU (Finite Element Model Updating) a été utilisée pour compléter le modèle du joint adhésif. Une méthode inverse numérique a été développée pour obtenir les paramètres élastiques, plastiques et de rupture du joint adhésif. De plus, des outils qualitatifs ont été proposés pour estimer les incertitudes sur les paramètres identifiés. Ce travail a prouvé l'intérêt de l'imagerie rapide locale pour caractériser les joints adhésifs.Cette méthode innovante a été validée sur une autre éprouvette nommée BIADH45. Cette dernière étude a aussi mis l'accent sur de nouveaux domaines de recherche : en particulier, le rôle des interfaces dans la rupture du joint adhésif et l'intérêt des substrats en CMO dans la caractérisation dynamique des joints adhésifs.
• A diffuse interface model for the analysis of propagating bulges in cylindrical balloons
  
  • Lestringant C
  • Audoly B.
  Proceedings of the Royal Society of London. Series A, Mathematical and physical sciences, Royal Society, The, 2018, 474, pp.20180333. With the aim to characterize the formation and propagation of bulges in cylindrical rubber balloons, we carry out an expansion of the non-linear axisymmetric membrane model assuming slow axial variations. We obtain a diffuse interface model similar to that introduced by van der Waals in the context of liquid-vapor phase transitions. This provides a quantitative basis to the well-known analogy between propagating bulges and phase transitions. The diffuse interface model is amenable to numerical as well as analytical solutions, including linear and non-linear bifurcation analyses. Comparisons to the original membrane model reveal that the diffuse interface model captures the bulging phenomenon very accurately, even for well-localized phase boundaries. (10.1098/rspa.2018.0333)
  DOI : 10.1098/rspa.2018.0333
• Enhanced local maximum-entropy approximation for stable meshfree simulations
  
  • Kumar Siddhant
  • Danas Kostas
  • Kochmann Dennis M
  Computer Methods in Applied Mechanics and Engineering, Elsevier, 2018. We introduce an improved meshfree approximation scheme which is based on the local maximum-entropy strategy as a compromise between shape function locality and entropy in an information-theoretical sense. The improved version is specifically designed for severe, finite deformation and offers significantly enhanced stability as opposed to the original formulation. This is achieved by (i) formulating the quasistatic mechanical boundary value problem in a suitable updated-Lagrangian setting, (ii) introducing anisotropy in the shape function support to accommodate directional variations in nodal spacing with increasing deformation and eliminate tensile instability, (iii) spatially bounding and evolving shape function support to restrict the domain of influence and increase efficiency, (iv) truncating shape functions at interfaces in order to stably represent multi-component systems like composites or polycrystals. The new scheme is applied to benchmark problems of severe elastic and elastoplastic deformation that demonstrate its performance both in terms of accuracy (as compared to exact solutions and, where applicable, finite element simulations) and efficiency. Importantly, the presented formulation overcomes the classical tensile instability found in most mesh-free interpolation schemes, as shown for stable simulations of, e.g., the inhomogeneous extension of a hyperelastic block up to 100% or the torsion of a hyperelastic cube by 200 •-both in an updated Lagrangian setting and without the need for remeshing. (10.1016/j.cma.2018.10.030)
  DOI : 10.1016/j.cma.2018.10.030
• Conversion of Love waves in a forest of trees
  
  • Maurel Agnes
  • Marigo Jean-Jacques
  • Pham Kim
  • Guenneau Sebastien
  Physical Review B: Condensed Matter and Materials Physics (1998-2015), American Physical Society, 2018, 98 (13), pp.134311. We inspect the propagation of shear polarized surface waves akin to Love waves through a forest of trees of the same height atop a guiding layer on a soil substrate. An asymptotic analysis shows that the forest behaves like an infinitely anisotropic wedge with effective boundary conditions. We discover that the foliage of trees brings a radical change in the nature of the dispersion relation of these surface waves, which behave like spoof plasmons in the limit of a vanishing guiding layer, and like Love waves in the limit of trees with a vanishing height. When we consider a forest with trees of increasing or decreasing height, this hybrid “spoof Love wave” is either trapped within the trees or converted into a downward propagating bulk (shear) wave. These mechanisms of wave trapping and wave conversion appear to be robust with respect to perturbations of height or position of trees in the metawedge and with respect to three-dimensional effects such as regarding a potential change of elastic wave polarization. (10.1103/PhysRevB.98.134311)
  DOI : 10.1103/PhysRevB.98.134311