Publications

2018

• Collective effects in muscle contraction and cellular adhesion
  
  • Borja da Rocha Hudson
  , 2018. Two biological systems, a half-sarcomere of a skeletal muscle and an adhesive cluster of a crawling keratocyte, are considered in parallel because of the deep similarity in their structure and functionality. Their passive response can be modeled by a large number of multi-stable units coupled through long-range interactions, frustrated by quenched disorder and exposed to thermal noise. In such systems, long-range interactions lead to synchronization, defying temporal and spatial fluctuations. We use a mean-field description to obtain analytic results and elucidate the remarkable ensemble-dependence of the mechanical behavior of such systems in the thermodynamic limit. Despite important structural differences between muscle cross-bridges and adhesive binders, one can identify a common underlying spin glass structure, which we fully exploit in this work. Our study suggests that the muscle machinery is fine-tuned to operate near criticality, and we argue that in this respect the quenched disorder, reflecting here steric incommensuration, may be functional. We use the analogy between cell detachment and thermal fracture of disordered solids to study the statistics of fluctuations during cellular adhesion. We relate the obtained results to recent observations of intermittent behavior involved in cell debonding, also suggesting near-criticality. In addition to the study of the equilibrium properties of adhesive clusters, we also present the first results on their kinetic behavior in the presence of time-dependent loading.
• Revue critique de l'article « l'enracinement social de la mortalité routière »
  
  • Bavouzet Julia
  • Weisz-Patrault Daniel
  , 2018. Ce travail est une revue critique de l'article « L'enracinement social de la mortalité routière » par Matthieu Grossetête, publié aux Actes de la recherche en sciences sociales (2010/4 n° 184 | pages 38 à 57). Il ne s'agit pas de réévaluer le texte au jour de nouvelles données dans un processus normal d'évolution des connaissances, mais d'analyser la qualité de la démarche selon trois axes principaux : la méthodologie, la rigueur statistique et la stratégie argumentative. Pour chacun de ces trois aspects, nous mettons à jour des failles suffisamment importantes pour établir que les conclusions proposées par l'auteur sont objectivement fausses pour certaines et abusives pour les autres, car non étayées par les données présentées. Nous recommandons donc de retirer l'article des Actes de la recherche en sciences sociales car ce dernier ne satisfait pas aux standards de la méthode scientifique.
• Quantification of Biventricular Strains in Heart Failure With Preserved Ejection Fraction Patient Using Hyperelastic Warping Method
  
  • Zou Hua
  • Xi Ce
  • Zhao Xiaodan
  • Koh Angela S
  • Gao Fei
  • Su Yi
  • Tan Ru-San y
  • Allen John
  • Lee Lik Chuan
  • Genet Martin
  • Zhong Liang
  Frontiers in Physiology, Frontiers, 2018, 9. Heart failure (HF) imposes a major global health care burden on society and sufferingon the individual. About 50% of HF patients have preserved ejection fraction (HFpEF). More intricate and comprehensive measurement-focused imaging of multiple strain components may aid in the diagnosis and elucidation of this disease. Here, we describe the development of a semi-automated hyperelastic warping method forrapid comprehensive assessment of biventricular circumferential, longitudinal, and radial strains that is physiological meaningful and reproducible. We recruited and performed cardiac magnetic resonance (CMR) imaging on 30 subjects [10 HFpEF, 10HF with reduced ejection fraction patients (HFrEF) and 10 healthy controls]. In each subject, a three-dimensional heart model including left ventricle (LV), right ventricle (RV), and septum was reconstructed from CMR images. The hyperelastic warping method was used to reference the segmented model with the target images and biventricular circumferential, longitudinal, and radial strain–time curves were obtained. The peak systolic strains are then measured and analyzed in this study. Intra- and inter-observer reproducibility of the biventricular peak systolic strains was excellent with allICCs>0.92. LV peak systolic circumferential, longitudinal, and radial strain, respectively,exhibited a progressive decrease in magnitude from healthy control→HFpEF→HFrEF : control (−15.5±1.90,−15.6±2.06, 41.4±12.2%); HFpEF (−9.37±3.23,−11.3±1.76, 22.8±13.1%); HFrEF (−4.75±2.74,−7.55±1.75, 10.8±4.61%). A similar progressive decrease in magnitude was observed for RV peak systolic circumferential, longitudinal and radial strain : control (−9.91±2.25,−14.5±2.63,26.8±7.16%); HFpEF (−7.38±3.17,−12.0±2.45, 21.5±10.0%); HFrEF(−5.92±3.13,−8.63±2.79, 15.2±6.33%). Furthermore, septum peak systolic circumferential, longitudinal, and radial strain magnitude decreased gradually from healthy control to HFrEF: control (−7.11±1.81, 16.3±3.23, 18.5±8.64%); HFpEF (−6.11±3.98,−13.4±3.02, 12.5±6.38%); HFrEF (−1.42±1.36,−8.99±2.96,3.35±2.95%). The ROC analysis indicated LV peak systolic circumferential strain to be the most sensitive marker for differentiating HFpEF from healthy controls. Our results suggest that the hyperelastic warping method with the CMR-derived strains may reveal subtle impairment in HF biventricular mechanics, in particular despite a “normal” ventricular ejection fraction in HFpEF. (10.3389/fphys.2018.01295)
  DOI : 10.3389/fphys.2018.01295
• A computational approach based on a multiaxial fatigue criterion combining phase transformation and shakedown response for the fatigue life assessment of Nitinol stents
  
  • Scalet Giulia
  • Menna Costantino
  • Constantinescu Andrei
  • Auricchio Ferdinando
  Journal of Intelligent Material Systems and Structures, SAGE Publications, 2018, 29 (19), pp.3710-3724. Self-expanding stents made of Nitinol, a Nickel–Titanium shape memory alloy, are used in standard medical implants for the treatment of cardiovascular diseases. Despite the increasing success, clinical studies have reported stent failure after the deployment in the human body, thus undermining patient’s safety and life. This study aims to fill the gap of reliable assessment of the fatigue life of Nitinol stents. We propose a global computational design method for preclinical validation of Nitinol stents, which can be extended to patient-specific computations. The proposed methodology is composed of a mechanical finite element analysis and a fatigue analysis. The latter analysis is based on a novel multiaxial fatigue criterion of the Dang Van type, combining the shakedown response of the stent and the complexity of phase transformation taking place within the material. The method is implemented in the case of a carotid artery stent. The implant configuration as well as the applied cyclic loading are shown to affect material phase evolution as well as stent lifetime. The comparison with the results obtained by applying a strain-based constant-life diagram approach allows to critically discuss both fatigue criteria and to provide useful recommendations about their applicability. (10.1177/1045389X18798957)
  DOI : 10.1177/1045389X18798957
• Modeling and identification of the constitutive behaviour of magneto-rheological elastomers
  
  • Voropaieff Jean-Pierre
  , 2018. In this thesis, we study a class of “active materials” called Magnetorheological elastomers (MRE) which are ferromagnetic impregnated rubbers whose mechanical properties are altered by the application of external magnetic fields. With the purpose of characterizing the behavior of MREs up to large strains and high magnetic fields, this work brings a completely novel experimental, theoretical and numerical approach.The first part of this study focuses on an experimental investigation of MRE where multiple microstructures (isotropic and transversely isotropic materials) and multiple particles’ volume fraction are tested. A special sample geometry is designed in order to increase the uniformity of internal magnetic and mechanical fields measured during coupled-field experiments. The interfacial adhesion between the iron fillers and the silicone matrix is investigated and we show that when specimens are subjected to external magnetic fields, a silane primer treatment of the particles is needed to prevent debonding at the interface particle/matrix. Then, we present the magneto-mechanical testing setup that allows simultaneous 3D mechanical and magnetic measurements before discussing the results. Even if is found that instabilities are ubiquitous in MREs, lots of useful data are collected and will be used to compute the parameters proposed in the material model.The second part of the thesis is dedicated to the modeling of isotropic MREs. The continuum description proposed by Kankanala, Triantafyllidis and Danas (2004, 2012, 2014) to derive constitutive laws that account for finite strains is used and, in particular, the energetic approach (that requires an energy density function) is chosen. Multiple equivalent variational formulation alternatives (based on different choices of the independent magnetic variable used in the energy function: B, H or M) are given and implemented into 3D finite element (FEM) codes. Based on the use of FEM simulation in combination with least square optimization methods, the previously collected experimental data are fitted and all three energy functions ψB , ψH and ψM are computed. The obtained material model proves to have excellent predictive capabilities when compared to other experiments not used in the fitting process. The use of numerical tools is necessary to make sure that the calculated material parameters are not influenced by the shape of experimental specimens.The last part of this work details the numerical implementation of the different variational formulations. For each one of them, it is found that isoparametric elements are well suited to simulate coupled magneto-mechanical boundary value problems. We show that special care is needed when implementing variational formulations using the displacement vector and the magnetic vector potential as independent variables. Indeed, ensuring the uniqueness of the vector potential requires to numerically enforce the Coulomb gauge, which leads to numerical complications that are addressed in this thesis. Before describing the different patch tests that have been considered to validate the numerical codes, we show which are the valid boundary conditions for the magnetic vector potential and how to use the symmetry properties of a given boundary value problem to reduce its complexity and the computational resources needed to solve it.
• Importance of electric resistance monitoring in shear test
  
  • Zanella S.
  • Lecavelier Des Etangs-Levallois A.
  • Charkaluk E.
  • Maia Filho W.C.
  • Constantinescu A.
  Microelectronics Reliability, Elsevier, 2018, 88-90, pp.733-737. (10.1016/j.microrel.2018.06.063)
  DOI : 10.1016/j.microrel.2018.06.063
• Nonlinear and multiphysics evaluation of residual stresses in coils
  
  • Weisz-Patrault Daniel
  Applied Mathematical Modelling, Elsevier, 2018. This article deals with residual stress evaluation within the framework of numerical simulation of the coiling process of steel. Plastic deformations along with multiphase transitions are responsible for large irreversible strain leading to major residual stress issues. A nonlinear mixed analytical/numerical approach is developed to compute residual stresses generated by different contributions of inelastic strain occurring during the coiling process (including both winding and cooling). In particular, transformation-induced plasticity is taken into account. Contact stresses at the interfaces between layers are updated at each time step by a minimization procedure. The analytical part of the proposed strategy is validated by comparison with a finite element computation. Then a coil composed of 196 layers is considered under typical industrial conditions. Numerical results are discussed to present the industrial application of the proposed approach.
• Random distribution of polydisperse ellipsoidal inclusions and homogenization estimates for porous elastic materials
  
  • Anoukou K.
  • Brenner R.
  • Hong F.
  • Pellerin M.
  • Danas K.
  Computers & Structures, Elsevier, 2018. This work proposes an extension of the well-known random sequential adsorption (RSA) method in the context of non-overlapping random mono-and polydisperse ellipsoidal inclusions. The algorithm is general and can deal with inclusions of different size, shape and orientation with or without periodic geometrical constraints. Specifically, polydisperse inclusions, which can be in terms of different size, shape, orientation or even material properties, allow for larger volume fractions without the need of additional changes in the main algorithm. Unit-cell computations are performed by using either the fast Fourier transformed-based numerical scheme (FFT) or the finite element method (FEM) to estimate the effective elastic properties of voided particulate microstructures. We observe that an isotropic overall response is very difficult to obtain for random distributions of spheroidal inclusions with high aspect ratio. In particular, a substantial increase (or decrease) of the aspect ratio of the voids leads to a markedly anisotropic response of the porous material, which is intrinsic of the RSA construction. The numerical estimates are probed by analytical Hashin-Shtrikman-Willis (HSW) estimates and bounds. (10.1016/j.compstruc.2018.08.006)
  DOI : 10.1016/j.compstruc.2018.08.006
• Bifurcation analysis of twisted liquid crystal bilayers
  
  • Danas K.
  • Mukherjee D.
  • Haldar K.
  • Triantafyllidis N.
  Journal of the Mechanics and Physics of Solids, Elsevier, 2018. This work presents a general methodology to analyze three-dimensional Freedericksz transitions in twisted-nematic liquid crystal (LC) bilayers. Using two equivalent coupled electromechanical variational formulations, the problem is treated as a bifurcation instability triggered by an externally applied electric field. Specifically, we consider LC bilayer materials anchored between two bounding plates and subjected to an electric field across the bilayer thickness. The plates are also twisted by an overall angle leading to different orientations of the directors in each layer. We first evaluate the corresponding ground state of the director field, and subsequently, we analyze the bifurcation problem by using a combined analytical-numerical method leading to a one-dimensional finite element discretization of the resulting stiffness matrix of the system. An analytical solution for the zero-twist bilayer is also obtained. The developed methodology is used to study the effect of the volume fraction of the constituents forming the bilayer upon the resulting critical electric field and corresponding eigenmodes. We find that by assembling a relatively thin 5CB layer with a thicker 7E layer, one can obtain periodic Freedericksz transitions (i.e. local modes) even for a zero-twist LC bilayer. We also show that when a 5CB material is assembled together with another electrically similar LC, such as a PCH12, the combined LC bilayer can exhibit an even lower Freedericksz transition than a LC of the same thickness consisting of any of the two constituents alone. (10.1016/j.jmps.2018.09.008)
  DOI : 10.1016/j.jmps.2018.09.008
• Controlled, Low-Temperature Nanogap Propagation in Graphene Using Femtosecond Laser Patterning
  
  • Maurice Ange
  • Bodelot Laurence
  • Tay Beng Kang
  • Lebental Bérengère
  Small, Wiley-VCH Verlag, 2018, 14 (31), pp.1801348. Graphene nanogap systems are promising research tools for molecular electronics, memories, and nanodevices. Here, a way to control the propagation of nanogaps in monolayer graphene during electroburning is demonstrated. A tightly focused femtosecond laser beam is used to induce defects in graphene according to selected patterns. It is shown that, contrary to the pristine graphene devices where nanogap position and shape are uncontrolled, the nanogaps in prepatterned devices propagate along the defect line created by the femtosecond laser. Using passive voltage contrast combined with atomic force microscopy, the reproducibility of the process with a 92% success rate over 26 devices is confirmed. Coupling in situ infrared thermography and finite element analysis yields a real-time estimation of the device temperature during electrical loading. The controlled nanogap formation occurs well below 50 °C when the defect density is high enough. In the perspective of graphene-based circuit fabrication, the availability of a cold electroburning process is critical to preserve the full circuit from thermal damage. (10.1002/smll.201801348)
  DOI : 10.1002/smll.201801348
• Buckling of a spinning elastic cylinder: linear, weakly nonlinear and post-buckling analyses
  
  • Richard Franck
  • Chakrabarti Aditi
  • Audoly Basile
  • Pomeau Yves
  • Mora Serge
  Proceedings of the Royal Society A: Mathematical, Physical and Engineering Sciences, Royal Society, The, 2018, 474 (2216), pp.20180242. An elastic cylinder spinning about a rigid axis buckles beyond a critical angular velocity, by an instability driven by the centrifugal force. This instability and the competition between the different buckling modes are investigated using analytical calculations in the linear and weakly nonlinear regimes, complemented by numerical simulations in the fully post-buckled regime. The weakly nonlinear analysis is carried out for a generic incompressible hyperelastic material. The key role played by the quadratic term in the expansion of the strain energy density is pointed out: this term has a strong effect on both the nature of the bifurcation, which can switch from supercritical to subcritical, and on the buckling amplitude. Given an arbitrary hyperelastic material, an equivalent shear modulus is proposed, allowing the main features of the instability to be captured by an equivalent neo-Hookean model. (10.1098/rspa.2018.0242)
  DOI : 10.1098/rspa.2018.0242
• Delayed feedback control method for computing the cyclic steady states of evolution problems
  
  • Khristenko Ustim
  • Le Tallec Patrick
  Computer Methods in Applied Mechanics and Engineering, Elsevier, 2018, 338, pp.241 - 274. (10.1016/j.cma.2018.04.015)
  DOI : 10.1016/j.cma.2018.04.015
• Perioperative management of patients with coronary artery disease undergoing non-cardiac surgery: Summary from the French Society of Anaesthesia and Intensive Care Medicine 2017 convention
  
  • Fellahi Jean-Luc
  • Godier Anne
  • Benchetrit Deborah
  • Berthier Francis
  • Besch Guillaume
  • Bochaton Thomas
  • Bonnefoy-Cudraz Eric
  • Coriat Pierre
  • Gayat Etienne
  • Hong Alex
  • Jenck Sophie
  • Le Gall Arthur
  • Longrois Dan
  • Martin Anne-Céline
  • Pili-Floury Sébastien
  • Piriou Vincent
  • Provenchère Sophie
  • Rozec Bertrand
  • Samain Emmanuel
  • Schweizer Rémi
  • Billard Valérie
  Anaesthesia Critical Care & Pain Medicine, Elsevier Masson, 2018, 37 (4), pp.367 - 374. This review summarises the specific stakes of preoperative, intraoperative, and postoperative periods of patients with coronary artery disease undergoing non-cardiac surgery. All practitioners involved in the perioperative management of such high cardiac risk patients should be aware of the modern concepts expected to decrease major adverse cardiac events and improve short- and long-term outcomes. A multidisciplinary approach via a functional heart team including anaesthesiologists, cardiologists and surgeons must be encouraged. Rational and algorithm-guided management of those patients should be known and implemented from preoperative to postoperative period. (10.1016/j.accpm.2018.02.021)
  DOI : 10.1016/j.accpm.2018.02.021
• Residual stress on the run out table accounting for multiphase transitions and transformation induced plasticity
  
  • Weisz-Patrault Daniel
  • Koedinger Thomas
  Applied Mathematical Modelling, Elsevier, 2018. The development of harder and thinner new steel grades necessitates reasonably fast numerical simulations of forming processes in order to optimize industrial conditions through parametric studies. This paper focuses on the evolution of residual stresses of thin strips during cooling on the run out table. Since the problem involves multiphysics and non-linear processes, comprehensive and fully coupled numerical approaches may be difficult to use to design or optimize industrial conditions because of extensive computation times. Therefore, a simplified numerical simulation has been developed and consists in solving first the thermal problem coupled with multiphase transitions and then the mechanical problem accounting for thermal expansion, metallurgical deformation and transformation induced plasticity. Residual stress profiles through the strip thickness are also computed in order to evaluate classic flatness defects such as crossbow and longbow. A post-processing is also included in order to quantify out of plane displacements that would take place if the strip were cut off the production line. It consists in computing at finite strain the relaxation of residual stresses when the tension applied by the coiler is released. The proposed numerical strategy has been tested on common industrial conditions.
• A critical comparison of shear tests for adhesive joints
  
  • Jouan Alexandre
  • Constantinescu Andrei
  International Journal of Adhesion and Adhesives, Elsevier, 2018, 84, pp.63-79. Adhesive joints get a growing part of assembly solutions in various industrial applications and are considered as an alternative to soldering and welding. Their small thickness to length or aspect ratio and the importance of the interface with the assembled parts increase the difficulties for measuring their characteristic mechanical properties such as constitutive law, endurance limit, etc… Several testing configurations and methods have been proposed in the literature without a clear emergence of an optimal configuration. This paper proposes a critical review of four different methods taken from the literature and industrial standards: the single lap joint shear test, the thick adherent shear test, the ARCAN test and napkin ring test. In order to contribute to the emergence and to help the experimentalist find an optimal specimen design, the heterogeneities of stress and strain field distributions are here discussed. The test specimens and configurations under scrutiny are compared using both closed-form expressions and Finite-Element computations and considering two different criteria: the spatial distribution of shear stress and the triaxiality ratio between normal and shear stresses in the joint. This study highlights both advantages and limits of each method for mechanical behavior and fatigue characterization. As a final consequence of the remarks an optimal specimen configuration is proposed. (10.1016/j.ijadhadh.2018.02.035)
  DOI : 10.1016/j.ijadhadh.2018.02.035
• Continuum Electromechanical Theory for Nematic Continua with Application to Freedericksz Instability
  
  • Pampolini G.
  • Triantafyllidis N.
  Journal of Elasticity, Springer Verlag, 2018, 132 (2), pp.219 - 242. (10.1007/s10659-017-9665-y)
  DOI : 10.1007/s10659-017-9665-y
• Selection of hexagonal buckling patterns by the elastic Rayleigh-Taylor instability
  
  • Chakrabarti Aditi
  • Mora Serge
  • Richard Franck
  • Phou Ty
  • Fromental Jean-Marc
  • Pomeau Yves
  • Audoly Basile
  Journal of the Mechanics and Physics of Solids, Elsevier, 2018, 121, pp.234 - 257. We investigate the non-linear buckling patterns produced by the elastic Rayleigh-Taylor instability in a hyper-elastic slab hanging below a rigid horizontal plane, using a combination of experiments, weakly non-linear expansions and numerical simulations. Our experiments reveal the formation of hexagonal patterns through a discontinuous transition. As the unbuckled state is transversely isotropic, a continuum of linear modes become critical at the first bifurcation load: the critical wavevectors form a circle contained in a horizontal plane. Using a weakly non-linear post-bifurcation expansion, we investigate how these linear modes cooperate to produce buckling patterns: by a mechanism documented in other transversely isotropic structures, three-modes coupling make the unbuckled configuration unstable with respect to hexagonal patterns by a transcritical bifurcation. Stripe and square patterns are solutions of the post-bifurcation expansion as well but they are unstable near the threshold. These analytical results are confirmed and complemented by numerical simulations. (10.1016/j.jmps.2018.07.024)
  DOI : 10.1016/j.jmps.2018.07.024
• Modeling (some aspects of) the female reproductive system : Gonadotropin and follicular dynamics
  
  • Clement Frederique
  • Monniaux Danielle
  • Postel Marie
  • Robin Frédérique
  • Yvinec Romain
  , 2018.
• Quasi-static and dynamic characterization of the anisotropic viscoplastic behaviour of a titanium alloy for leading edge of aircraft engine fan blade
  
  • Ruiz de Sotto Miguel
  • Longère Patrice
  • Doquet Véronique
  • Papasidero Jessica
  , 2018.
• A continuum relaxed growth model for controlling growth-induced residual stresses in living tissues
  
  • Genet Martin
  , 2018. Living tissues are naturally prestrained (or, equivalently, residually stressed), as clearly illustrated by Y.C. Fung's famous opening angle experiment on arteries [1]. If the biomechanical role of residual stresses has been well described for many tissues, their origin remains an open question; growth, however, is a likely candidate. Indeed, differential (i.e., heterogeneous) growth induces residual stresses, in order to accommodate for the non-compatible growth [2]. In continuum models of growth, growth-induced residual stresses have been either kept [2] or dismissed [3], depending on the modeling objective: growth-induced residual stresses allow to quantitatively reproduce the opening angle experiments on the left ventricle [2], on the other hand they can lead to inconsistencies in the mechanical response of hypertrophied ventricles [3]. Moreover, only a partial organ growth is required to induce residual stresses that are compatible with measured opening angle [2], suggesting that not all residual stresses induced by the complete growth process are present in the mature organ. In this work I propose a new model of soft tissue biomechanics, including both strain-or stress-driven growth as well as relaxation of residual stresses. It is written in the general framework of finite deformation continuum mechanics, and based on the multiplicative decomposition of the deformation gradient as illustrated on the figure: the reference configuration, denoted \Omega_0, with prestrain field \F_0, deforms into the current configuration \Omega through mapping \Phi with gradient \F = \nabla\Phi; now the total transformation is composed into growth, prestrain, relaxation and loading parts such that the elastic deformation gradient is \F_e = \F \F'^-1 \F_r \F_p \F_0. In addition to the constitutive relation that relates this elastic deformation gradient to stress, two additional evolution laws are needed to close the model: for growth, and relaxation. Depending on the ratio between the time constants of both laws, the models allows controlling the part of growth-induced residual stresses that is kept in the tissue vs. the part that is relaxed away. From the computational perspective, in order to compute all terms in \F_e, two computations must be performed in parallel: the deformed configuration \Omega, obviously, but also the unloaded configuration \Omega_r.
• Regulation of initiation of follicle growth and dynamics of early follicular development in the sheep
  
  • Monniaux Danielle
  • Cadoret Véronique
  • Clement Frederique
  • Fabre Stéphane
  • Locatelli Yann
  • Monget Philippe
  • Dalbies-Tran Rozenn
  , 2018, 96 (S3), pp.Abstract 126. Primordial follicles embedded in the ovarian cortex are the source of developing follicles. Follicle growth activa- tion and development up to the small antral follicle stage are controlled by cell interactions. The sheep ovary offers an appropriate non-rodent model to study these interac- tions, thanks to the development of in vitro and in vivo experimental approaches, ex vivo molecular analyses and in silico mathematical modeling. Each primordial follicle, composed of an oocyte surrounded by a single layer of quiescent granulosa cells, relies on nutrients and growth factors supplied by the surrounding stroma of connective tissue. In vitro cultures of ovarian cortex have shown that primordial follicles are activated by the lifting of mech- anisms maintaining quiescence, some of them involving AMH secreted by already growing follicles. Afterwards, follicle development is supported by a finely tuned molec- ular dialog between the growing oocyte and proliferating granulosa cells. The isolation of preantral follicles and their development in vitro as individual follicles perturb this dialog, leading to an acceleration of follicular matu- ration. In vivo, mutations in the oocyte factors BMP15, GDF9 or their receptor BMPR1B also impair this dia- log, leading to an imbalance between oocyte growth and cell proliferation, which can be reproduced by models for cell dynamics. During the growth of preantral follicles, the recruitment and differentiation of theca cells from the ovarian stroma provide them with a structural and vascularized support. In vivo exposure of sheep fetal ovaries to testosterone imprints the stroma cells so that the expression of genes involved in extracellular matrix organization and cell-cell adhesion is affected in theca at adulthood; this leads to a lower ovarian tissue rigid- ity that can account for the accelerated follicle growth observed in androgenized ewes. A better knowledge of cell interactions during early follicular development will help to improve the biotechnology methods of fertility preservation.
• Equilibrated Warping: Finite Element Image Registration with Equilibrium Gap Regularization
  
  • Genet Martin
  • Stoeck Christian T
  • Berberoglu Ezgi
  • Kozerke Sebastian
  , 2018. Image processing, in particular motion tracking, is playing an important role in biomedical engineering and in other domains such as materials and mechanical engineering. However, despite important progress made in the past decades, robustness, efficiency and precision of the existing methods must still be improved to translate them into medical and engineering applications [1]. Equilibrated Warping is a novel image registration approach, based on the finite element method and the equilibrium gap regularization [2]. The finite element method is used to formulate the image registration problem, i.e., to find the displacement field that best match the source and target images, allowing to ensure some regularity to the solution. However, because of image limited resolution and noise, this problem is ill-posed, and require regularization. The equilibrium gap regularization essentially penalizes any deviation from the solution of a hyperelastic body in equilibrium with arbitrary loads prescribed at the boundary [3]. It thus represents a regularization with strong mechanical basis. In the presentation, we will first describe the consistent linearization and discretization of the regularized image correlation problem. On simple synthetic images examples, we will show that the equilibrated warping method is effective and robust: regularization strength and image noise have minimal impact on motion tracking, especially when compared to strain-based regularization methods such as hyperelastic warping, or methods based on incompressibility constraint. We will also show results of the equilibrated warping method applied to in vivo tagged (3D CSPAMM, see Figure) and untagged (CINE) cardiac magnetic resonance images of a healthy volunteer: the method allows to extract main deformation features of the left ventricle, including radial thickening, circumferential and longitudinal shortening, as well as ventricular twist. It is also able to extract finer features of deformation, such as (i) larger radial strains in the free wall compared to the septum (because of the right ventricular pressure, this is only seen in untagged images); (ii) longitudinal variations of the radial-circumferential shear strain from apex to base. Finally, we will show that equilibrated warping compares very well with other image registration methods on a public cardiac motion tracking challenge data [1].
• A micromechanical model for polycrystalline shape memory alloy wires integrated into smart structures
  
  • Hannequart Philippe
  • Peigney Michael
  • Caron Jean-François
  • Viglino Emmanuel
  , 2018, pp.1p.
• Identification of constitutive equations, deformation and damage micro-mechanisms of Ti-6Al-4V for aircraft engine fan blades
  
  • Ruiz de Sotto Miguel
  • Doquet Véronique
  • Longère Patrice
  • Papasidero Jessica
  , 2018.
• Convergence of discrete-time Kalman filter estimate to continuous-time estimate for systems with unbounded observation
  
  • Aalto Atte
  Mathematics of Control, Signals, and Systems, Springer Verlag, 2018, 30 (3), pp.9. In this article, we complement recent results on the convergence of the state estimate obtained by applying the discrete-time Kalman filter on a time-sampled continuous-time system. As the temporal discretization is refined, the estimate converges to the continuous-time estimate given by the Kalman–Bucy filter. We shall give bounds for the convergence rates for the variance of the discrepancy between these two estimates. The contribution of this article is to generalize the convergence results to systems with unbounded observation operators under different sets of assumptions, including systems with diagonaliz-able generators, systems with admissible observation operators, and systems with analytic semigroups. The proofs are based on applying the discrete-time Kalman filter on a dense, numerable subset on the time interval [0,T] and bounding the increments obtained. These bounds are obtained by studying the regularity of the underlying semigroup and the noise-free output. (10.1007/s0049)
  DOI : 10.1007/s0049