Publications

2020

• Two families of explicit models constructed from a homogenization solution for the magnetoelastic response of MREs containing iron and ferrofluid particles
  
  • Lefèvre Victor
  • Danas Kostas
  • Lopez-Pamis Oscar
  International Journal of Non-Linear Mechanics, Elsevier, 2020, 119, pp.103362. This work puts forth two families of fully explicit continuum or phenomenological models that are constructed by approximating an analytical (but implicit) homogenization solution recently derived for the free-energy function describing the macroscopic magnetoelastic response of two classes of MREs comprised of an isotropic incompressible elastomer filled with a random isotropic distribution of: i) spherical iron particles and ii) spherical ferrofluid particles. Both families are given in terms of free-energy functions W H = W H (F, H) that depend on the deformation gradient F and the Lagrangian magnetic field H and are constructed so as to agree identically with the homogenization solution for small and large applied magnetic fields, this for arbitrary finite deformations and arbitrary volume fractions c of particles in the entire physical range c ∈ [0, 1]. The accuracy of the proposed phenomenological models is assessed inter alia via the direct comparison of their predictions with that of the homogenization solution for a boundary-value problem of both fundamental and practical significance: the magnetostriction response of a spherical MRE specimen subject to a remotely applied uniform magnetic field. (10.1016/j.ijnonlinmec.2019.103362)
  DOI : 10.1016/j.ijnonlinmec.2019.103362
• Experimental investigation and modelling of the curing behaviour of photopolymers
  
  • Rehbein T.
  • Lion A.
  • Johlitz M.
  • Constantinescu Andrei
  Polymer Testing, Elsevier, 2020, 83, pp.106356. This paper focuses on the experimental investigation and modelling of the crosslinking reaction of photopolymers used in additive manufacturing processes such as digital light processing and stereolithography. Starting with general mathematical concepts for the description of the material behaviour of polymeric materials, the importance of modelling the crosslinking reaction is emphasized. In order to characterise the crosslinking reaction experimentally, photocalorimetric measurements with varying isothermal temperature and light intensity are shown. From the exothermic heat flows measured during the crosslinking reaction, the degree of cure can be determined for each experimental scenario as a function of time. It is shown that the test temperature and light intensity have a significant influence on the crosslinking reaction. A modelling approach for the description of the crosslinking reaction incorporating temperature and light intensity is presented. Moreover, parameter identification and a comparison of the proposed model with the experiments are conducted. The indentified model has an excellent match with experimental data, resulting in a least square error smaller than 3 %. The proposed model and identification method opens up several extensions for the modelling of the material behaviour. (10.1016/j.polymertesting.2020.106356)
  DOI : 10.1016/j.polymertesting.2020.106356
• Asymptotically exact strain-gradient models for nonlinear slender elastic structures: a systematic derivation method
  
  • Lestringant Claire
  • Audoly Basile
  Journal of the Mechanics and Physics of Solids, Elsevier, 2020, pp.103730. We propose a general method for deriving one-dimensional models for nonlinear structures. It captures the contribution to the strain energy arising not only from the macroscopic elastic strain as in classical structural models, but also from the strain gradient. As an illustration, we derive one-dimensional strain-gradient models for a hyper-elastic cylinder that necks, an axisymmetric membrane that produces bulges, and a two-dimensional block of elastic material subject to bending and stretching. The method o↵ers three key advantages. First, it is nonlinear and accounts for large deformations of the cross-section, which makes it well suited for the analysis of localization in slender structures. Second, it does not require any a priori assumption on the form of the elastic solution in the cross-section, i.e., it is Ansatz-free. Thirdly, it produces one-dimensional models that are asymptotically exact when the macroscopic strain varies on a much larger length scale than the cross-section diameter. (10.1016/j.jmps.2019.103730)
  DOI : 10.1016/j.jmps.2019.103730
• Contributions à la réalisation d’une touche de piano numérique : modélisation, simulation, réalisation de prototype
  
  • Chichignoud Jérémie
  , 2020. Le travail présenté dans cette thèse représente une étape dans le projet de longs termes visant à obtenir une touche de synthétiseur muni d'un actuateur dont la fonction est de reproduire le toucher d'un piano à queue.Après une introduction présentant les problématiques du projet et l’état de l’art, nous présentons le modèle mécanique de la touche proposé par J. Lozada, puis A. Thorin, dont certains éléments ont dû être précisés et auquel plusieurs approximations et simplifications ont dû être ajoutées explicitement. Une description détaillée de la géométrie des contacts est donnée.La simulation proposée jusqu’ici était loin du temps réel en raison du choix logiciel effectué. Nous avons écrit deux programmes de simulation du modèle : l’un en matlab, à fins de prototypage et l’autre en C qui tourne en temps réel jusqu’à 6 us sur un processeur i7. Cette simulation respecte le caractère non-régulier des frottements secs dans les articulations et certains contacts unilatéraux.Les points critiques susceptibles de déstabiliser la simulation en temps réel sont discutés.L’actuateur retenu depuis longtemps dans la coopération LMS-LISA – frein à FMR – ne permettait pas de faire remonter la touche après l’action du pianiste. Nous avons donc complété le dispositif par un ensemble d’aimants permanents dont nous présentons le dimensionnement.La troisième partie du mémoire est consacrée à l’implémentation du dispositif « touche numérique ». Le choix d'un ordinateur standard comme organe de calcul et les contraintes du temps réel à une fréquence d’au moins 2 kHz ont nécessité le développement d'une communication USB spécifique pour échanger les données (mesures et commande) entre le micro-ordinateur et l’ensemble capteurs-actionneur.Nous donnons in fine les tout premiers résultats obtenus sur cet ensemble intégré. Leur analyse permet de tirer quelques conclusions relatives au développement futur du projet. (10.70675/a3a5e15az34e1z4488z9c01zca56bcecb93a)
  DOI : 10.70675/a3a5e15az34e1z4488z9c01zca56bcecb93a
• Investigation of phase-contrast magnetic resonance imaging underestimation of turbulent flow through the aortic valve phantom: Experimental and computational study using lattice Boltzmann method
  
  • Fučík Radek
  • Galabov Radek
  • Pauš Petr
  • Eichler Pavel
  • Klinkovský Jakub
  • Straka Robert
  • Tintěra Jaroslav
  • Chabiniok Radomir
  Magnetic Resonance Materials in Physics, Biology and Medicine, Springer Verlag, 2020, 33 (5), pp.649-662. Objective: The accuracy of phase-contrast magnetic resonance imaging (PC-MRI) measurement is investigated using a computational fluid dynamics (CFD) model with the objective to determine the magnitude of the flow underestimation due to turbulence behind a narrowed valve in a phantom experiment. Materials and Methods: An acrylic stationary flow phantom is used with three insertable plates mimicking aortic valvular stenoses of varying degrees. Positive and negative horizontal fluxes are measured at equidistant slices using standard PC-MRI sequences by 1.5T and 3T systems. The CFD model is based on the 3D lattice Boltzmann method (LBM). The experimental and simulated data are compared using the Bland-Altman-derived limits of agreement. Based on the LBM results, the turbulence is quantified and confronted with the level of flow underestimation. Results: LBM gives comparable results to PC-MRI for valves up to moderate stenosis on both field strengths. The flow magnitude through a severely stenotic valve was underestimated due to signal void in the regions of turbulent flow behind the valve, consistently with the level of quantified turbulence intensity. Discussion: Flow measured by PC-MRI is affected by noise and turbulence. LBM can simulate turbulent flow efficiently and accurately, it has therefore the potential to improve clinical interpretation of PC-MRI.
• Dobutamine stress testing in patients with Fontan circulation augmented by biomechanical modeling
  
  • Ruijsink Bram
  • Zugaj Konrad
  • Wong James
  • Pushparajah Kuberan
  • Hussain Tarique
  • Moireau Philippe
  • Razavi Reza
  • Chapelle Dominique
  • Chabiniok Radomir
  PLoS ONE, Public Library of Science, 2020. Understanding (patho)physiological phenomena and mechanisms of failure in patients with Fontan circulation-a surgically established circulation for patients born with a functionally single ventricle-remains challenging due to the complex hemodynamics and high inter-patient variations in anatomy and function. In this work, we present a biomechanical model of the heart and circulation to augment the diagnostic evaluation of Fontan patients with early-stage heart failure. The proposed framework employs a reduced-order model of heart coupled with a simplified circulation including venous return, creating a closed-loop system. We deploy this framework to augment the information from data obtained during combined cardiac catheterization and magnetic resonance exams (XMR), performed at rest and during dobutamine stress in 9 children with Fontan circulation and 2 biventricular controls. We demonstrate that our modeling framework enables patient-specific investigation of myocardial stiffness, contractility at rest, contractile reserve during stress and changes in vascular resistance. Hereby, the model allows to identify key factors underlying the pathophysiological response to stress in these patients. In addition, the rapid personalization of the model to patient data and fast simulation of cardiac cycles make our framework directly applicable in a clinical workflow. We conclude that the proposed modeling framework is a valuable addition to the current clinical diagnostic XMR exam that helps to explain patient-specific stress hemodynamics and can identify potential mechanisms of failure in patients with Fontan circulation. (10.1371/journal.pone.0229015)
  DOI : 10.1371/journal.pone.0229015
• Characterization and modeling of the thermo-mechanical behavior of a Ti-6Al-4V alloy under dynamic complex loading
  
  • Ruiz de Sotto Miguel
  , 2020. During the aircraft engine certification, various components are tested against ballistic phenomena. The engine fan must accordingly resist bird strike and blade loss without compromising the whole engine thrust performance. Fan blades, and particularly their leading edge, undergo large deformation under high strain rate, non-proportional loading paths and plastic dissipation induced self-heating. Due to their high specific mechanical properties, Ti-6Al-4V titanium alloys are promising candidates for fan multi-component blade leading edge. In this work, an experimental campaign has been carried out on a cold rolled Ti-6Al-4V alloy comprising tension, compression and shear tests performed at various temperatures and (low and high) strain rates, under monotonic and alternated loading paths. Based on these results, a constitutive model has been developed accounting for the combined effects of orthotropy, strength differential, nonlinear kinematic and isotropic hardenings, strain rate hardening as well as thermal softening. Material constants have been identified using Zset software. The model has been implemented as user material (Fortran) subroutine into the commercial finite element computation code LS-DYNA. The performances of the numerical model have then been estimated by conducting numerical simulations considering a volume element under various loading paths as well as the specimens used for the experimental campaign.
• Determination of effective stress intensity factors under mixed‐mode from digital image correlation fields in presence of contact stresses and plasticity
  
  • Bonniot Thomas
  • Doquet Véronique
  • Mai Si Hai
  Strain, Wiley-Blackwell, 2020. Digital image correlation (DIC) is more and more popular to monitor fatigue crack growth and to determine the stress intensity factors. However, the posttreatment of the recorded displacement fields becomes tricky when the crack faces are not stress‐free and when crack tip plasticity becomes significant. Several posttreatment methods to locate the crack tip and measure the effective stress intensity factors in such cases are compared, using finite element method‐computed displacement fields, and then used on real DIC fields. An approach coupling DIC and finite element method is proposed to estimate the contact stresses along the crack. (10.1111/str.12332)
  DOI : 10.1111/str.12332
• Intrinsic formulations of the nonlinear Kirchhoff-Love-von Kármán plate theory
  
  • Geymonat Giuseppe
  • Krasucki Françoise
  , 2020. We use a special duality by perturbation in optimization to find two different bi-duals problems of the non-linear Kirchhoff-Love-von Kármán plate theory. The first gives exactly the intrinsic approach developed by P. G. Ciarlet, the second gives an intrinsic approach implied by a J.J. Telega complementary energy.
• Stability of Vicinal Surfaces: Beyond the Quasistatic Approximation
  
  • Guin Laurent
  • Jabbour Michel E
  • Shaabani Ardali Léopold
  • Benoit-Marechal Lucas
  • Triantafyllidis Nicolas
  Physical Review Letters, American Physical Society, 2020. We revisit the step bunching instability without recourse to the quasistatic approximation and show that the stability diagrams are significantly altered, even in the low-deposition regime where it was thought sufficient. In particular, steps are unstable against bunching for attachment-detachment limited growth. By accounting for the dynamics and chemical effects, we can explain the onset of step bunching in Si(111)−(7×7) and GaAs(001) without resort to the inverse Schwoebel barrier or step-edge diffusion. Further, the size-scaling analysis of step-bunch growth, as induced by these two combined effects, agrees with the bunching regime observed at 750 °C in Si(111)−(7×7). (10.1103/PhysRevLett.124.036101)
  DOI : 10.1103/PhysRevLett.124.036101
• Surface waves from flexural and compressional resonances of beams
  
  • Marigo Jean-Jacques
  • Pham Kim
  • Maurel Agnes
  • Guenneau Sébastien
  , 2020.
• Effective model for elastic waves propagating in a substrate supporting a dense array of plates/beams with flexural resonances
  
  • Marigo Jean-Jacques
  • Pham Kim
  • Maurel Agnès
  • Guenneau Sebastien
  , 2020. We consider the effect of an array of plates or beams over a semi-infinite elastic ground on the propagation of elastic waves hitting the interface. The plates/beams are slender bodies with fle-xural resonances at low frequencies able to perturb significantly the propagation of waves in the ground. An effective model is obtained using asymptotic analysis and homogenization techniques , which can be expressed in terms of the ground alone with effective dynamic (frequency-dependent) boundary conditions of the Robin's type. For an incident plane wave at oblique incidence, the displacement fields and the reflection coefficients are obtained in closed forms and their validity is inspected by comparison with direct numerics in a two-dimensional setting.
• Personalized Pulmonary Poromechanics
  
  • Genet Martin
  • Patte Cécile
  • Chapelle Dominique
  , 2020. Lung biomechanics has been extensively studied by physiologists, experimentally as well as theoretically , laying the ground for our current fundamental understanding of the relationship between function and mechanical behavior. However, many questions remain, notably in the intricate coupling between the multiple constituents. These fundamental questions represent real clinical challenges, as pulmonary diseases are an important health burden. Interstitial lung diseases, for instance, affect several million people globally. Idiopathic Pulmonary Fibrosis (IPF), notably, a progressive form of interstitial lung diseases where some alveolar septa get thicker and stiffer while others get completely damaged, remains poorly understood, poorly diagnosed, and poorly treated. In this presentation, I will first describe our recently developed lung poromechanical model. It lies at the organ space scale and breathing time scale, and is written in a general poromechanical mixture framework. I will also detail the specific boundary conditions imposed on the lungs themselves, modeling the effect of diaphragm-induced loading and rib cage. The second part of the presentation will deal with the personalization procedure we developed alongside the model. It allows to personalize parts of the boundary conditions and material model from biomedical images, after processing. I will notably insist on the inverse problem of finding the unloaded configuration associated to the loaded configuration observed in vivo, and associated issues. Then I will show how regional mechanical parameters can be estimated in diseased lungs, illustrating how this model could be used as a diagnosis tool in the clinic.
• A constitutive model for a rate and temperature dependent, plastically anisotropic titanium alloy
  
  • Ruiz de Sotto Miguel
  • Longère Patrice
  • Doquet Véronique
  • Papasidero Jessica
  , 2020. This study focuses on the design of fan blades regarding impact loading resulting mostly from bird strike or engine fan blade loss and involving large deformation, high rate of deformation, non-proportional loading paths, plastic dissipation induced heating and potential damage and fracture. Due to their high strength-to-weight ratio and good toughness, Ti-6Al-4V titanium alloys are promising candidates for the leading edge of multi-component fan blades. To get a reliable prediction of the resistance of the whole engine structure, an extensive experimental campaign has been carried out and a constitutive model has been developed for a grade of Ti-6Al-4V titanium alloy provided in the form of cold rolled plates. The thermo-mechanical characterization, consisted of tension, compression and shear tests at various temperatures, (quasi static and dynamic) strain rates and (monotonic and alternate) loading paths, has evidenced a strong temperature and rate dependence as well as an orthotropic behavior with a significant tension/compression dissymmetry and a combination of isotropic and kinematic strain hardening. A constitutive model has been accordingly developed accounting for the combined effect of nonlinear, isotropic and kinematic strain hardening, strain rate hardening, tension/compression dissymmetry and machining direction. For that purpose, orthotropic plasticity models have been extended within a rate-dependent and isotropic vs. kinematic hardening formulation. The identification of the constitutive model constants has been conducted by means of the commercial software Zset. The constitutive model has then been implemented as a user material subroutine into the commercial finite element computation code LS-DYNA. Numerical simulations have been conducted considering some basic cases involving representative volume elements as well as structures such as the specimens used for the experimental campaign.
• Effect of Strains on the Dark Current-Voltage Characteristic of Silicon Heterojunction Solar Cells
  
  • Guin Laurent
  • Roca I Cabarrocas Pere
  • Jabbour Michel E
  • Triantafyllidis Nicolas
  Solar Energy, Elsevier, 2020. Anisotropic mechanical strain as low as 0.1% modifies the electronic response of crystalline semiconductor- based devices and in particular affects the performance of solar cells. We measure the dark current-voltage characteristic of silicon heterojunction solar cells under different levels of tensile uniaxial stress and observe a reversible change of the j-V curve with applied strain. Using a two-exponential description of the j-V char- acteristic to fit our experimental data, we obtain the strain dependence of the diffusion saturation current and find a decrease of about 3% for a tensile strain level of 6.7 × 10 4 . We compare these experiments to a theoretical model that accounts for the effect of strain on the band energy levels, densities of states and mobilities of carriers. The theoretical estimation of the change in saturation current is found to be in reasonable agreement with experimental results. (10.1016/j.solener.2019.12.037)
  DOI : 10.1016/j.solener.2019.12.037
• Foreword
  
  • Diani J.
  • Castelnau O
  • Chinesta Francisco
  Comptes Rendus. Mécanique, Académie des sciences (Paris), 2020, 348, pp.781-783. La mécanique des matériaux, qu’il s’agisse d’alliages métalliques, de polymères, de composites, ou encore de minéraux, est un domaine de recherche vaste s’appuyant aussi bien sur la physique, la chimie, les mathématiques, les techniques numériques, que les sciences expérimentales. Elle a la particularité de traverser les échelles de l’atome au milieu continu macroscopique.En particulier, la prise en compte des mécanismes élémentaires de déformation aux échelles pertinentes permet la construction de modèles de comportement robustes, i.e. qui soient capables non seulement de reproduire fidèlement les observations mais aussi de prédire le comportement mécanique dans des conditions inexplorées ou inexplorables expérimentalement. (10.5802/crmeca.65)
  DOI : 10.5802/crmeca.65
• Analysis of boundary layer effects due to usual boundary conditions or geometrical defects in elastic plates under bending: an improvement of the Love-Kirchhoff model
  
  • León Baldelli Andrés Alessandro
  • Marigo Jean-Jacques
  • Pideri Catherine
  Journal of Elasticity, Springer Verlag, 2020. We propose a model of flexural elastic plates accounting for boundary layer effects due to the most usual boundary conditions or to geometrical defects, constructed via matched asymptotic expansions. In particular, considering a rectangular plate clamped at two opposite edges while the other two are free, we derive the effective boundary conditions or effective transmission conditions that the two first terms of the outer expansion must satisfy. The new boundary value problems thus obtained are studied and compared with the classical Love-Kirchhoff plate model. Two examples of application illustrate the results. (10.1007/s10659-020-09804-6)
  DOI : 10.1007/s10659-020-09804-6
• Maximum admissible pressure in salt caverns used for brine production and hydrocarbon storage
  
  • Berest Pierre
  • Brouard Benoît
  • Karimi-Jafari Mehdi
  • Réveillère Arnaud
  Oil & Gas Science and Technology - Revue d'IFP Energies nouvelles, Institut Français du Pétrole (IFP), 2020, 75, pp.76. Tightness is a fundamental prerequisite to any underground storage. In storage salt caverns, a safe maximum admissible pressure must be selected to avoid product loss. The tensile strength of salt is small, and cavern pressure must be kept lower than geostatic pressure or, more precisely, lower than the least compressive stress at the cavern wall. The vertical stress can be assessed through density logs. The redistribution of stresses in the rock mass, due to the visco-plastic nature of rock salt, must be taken into account. A couple of cases in which a hydraulic connection between one cavern and another cavern, or between a cavern and the edge of a salt dome, are known. These connections originated in geological anomalies rather than in the creation of a fracture. There exists a pressure threshold, lower than the geostatic pressure, for which micro-fracturing and an increase in salt permeability occur, vindicating the position that a safety margin is needed when selecting the maximum pressure. Well tightness is important as well; it depends on several factors, among which are the quality of the cement, and the maximum fluid pressure in the cavern and along the access well. A tightness test is mandatory. The Nitrogen Leak Test is the most common such test. A review of selected gas-storage sites shows that, in most cases, the maximum admissible gradient at the casing shoe is 0.018 MPa/m (0.8 psi/ft), and up to 0.019 MPa/m (0.85 psi/ft) in some American states, values that are consistent with the considerations listed above. (10.2516/ogst/2020068)
  DOI : 10.2516/ogst/2020068
• Digital image correlation for microstructural analysis of deformation pattern in additively manufactured 316L thin walls
  
  • Balit Yanis
  • Charkaluk Eric
  • Constantinescu Andrei
  Additive Manufacturing, Elsevier, 2020, 31, pp.100862. In additive manufacturing, the process parameters have a direct impact on the microstructure of the material and consequently on the mechanical properties of the manufactured parts. The purpose of this paper is to explore this relation by characterizing the local microstructural response via in situ tensile test under a scanning electron microscope (SEM) combined with high resolution digital image correlation (HR-DIC) and electron backscat-ter diffraction (EBSD) maps. The specimens under scrutiny were extracted from bidirectionally-printed single-track thickness 316L stainless steel walls built by directed energy deposition. The morphologic and crystallographic textures of the grains were characterized by statistical analysis and associated with the particular heat flow pattern of the process. Grains were sorted according to their size into large columnar grains located within the printed layer and small equiaxed grains located at the interfaces between successive layers. In situ tensile experiments were performed with a loading direction either perpendicular or along the printing direction and exhibit different mechanisms of deformation. A statistical analysis of the average deformation per grain indicates that for a tensile loading along the building direction, small grains deform less than the large ones. In addition, HR-DIC combined with EBSD maps showed strain localization situated at the interface between layers in the absence of small grains either individual or in clusters. For tensile loads along the printing direction, the strain localization was present 1 Additive Manufacturing 31 (2020) 100862 https://doi. in several particular large grains. These observations permit to justify the differences in yield and ultimate strength noticed during macroscopic tensile tests for both configurations. Moreover, they indicate that an optimization of the process parameters could trigger the control of microstructure and consequently the macroscopic mechanical behavior. (10.1016/j.addma.2019.100862)
  DOI : 10.1016/j.addma.2019.100862
• Uncertainty estimation and hierarchical Bayesian analysis of mechanical dynamic tests
  
  • Weisz-Patrault Daniel
  • Francart Charles
  • Seisson Gabriel
  Journal of Dynamic Behavior of Materials, Springer Verlag, 2020, 7, pp.447-468. A methodology is presented to quantify uncertainties resulting from the analysis of dynamic tests performed on classic split Hopkinson pressure bar system in order to improve material parameter estimation within the framework of Bayesian inference. Since the experimental setup is imperfectly known, the proposed methodology consists in modeling experimental parameters as random variables. Then, cumulative effects of all experimental uncertainties are estimated by a statistical analysis based on one-dimensional wave interpretation. For each test, results consist in stress and strain-rate given as normal random variables. In addition, an experimental campaign is performed on the aluminum alloy AA7075-O, in order to identify material variability and repeatability of tests. Additional tests in the quasi-static regime are performed at two different temperatures to characterize temperature dependence of behavior. Material parameters of a simple Steinberg-Cochran-Guinan model are then estimated by (i) standard Bayesian inference exploiting data in the quasi-static regime, and (ii) a hierarchical Bayesian model exploiting data in the dynamic regime. The fitted model agrees well with the measurements and model uncertainties are easily quantified. Results are presented in the form of posterior probability density functions. The systematic quantification of uncertainties in dynamic tests opens interesting perspectives to analyze the response of structures and materials to impact.
• Three-dimensional biventricular strains in pulmonary arterial hypertension patients using hyperelastic warping
  
  • Zou Hua
  • Leng Shuang
  • Xi Ce
  • Zhao Xiaodan
  • Koh Angela S
  • Gao Fei
  • Le Tan Ju
  • Tan Ru-San y
  • Allen John C
  • Lee Lik Chuan
  • Genet Martin
  • Zhong Liang
  Computer Methods and Programs in Biomedicine, Elsevier, 2020, 189. Background and Objective: Evaluation of biventricular function is an essential component of clinical management in pulmonary arterial hypertension (PAH). This study aims to examine the utility of biventricular strains derived from a model-to-image registration technique in PAH patients in comparison to age-and gender-matched normal controls. Methods: A three-dimensional (3D) model was reconstructed from cine short-and long-axis cardiac magnetic resonance (CMR) images and subsequently partitioned into right ventricle (RV), left ventricle (LV) and septum. The hyperelastic warping method was used to register the meshed biventricular finite element model throughout the cardiac cycle and obtain the corresponding biventricular circumferential, longitudinal and radial strains. Results: Intra-and inter-observer reproducibility of biventricular strains was excellent with all intra-class correlation coefficients > 0.84. 3D biventricular longitudinal, circumferential and radial strains for RV, LV and septum were significantly decreased in PAH patients compared with controls. Receiver operating characteristic (ROC) analysis showed that the 3D biventricular strains were better early markers (Area under the ROC curve = 0.96 for RV longitudinal strain) of ventricular dysfunction than conventional parameters such as two-dimensional strains and ejection fraction. Conclusions: Our highly reproducible methodology holds potential for extending CMR imaging to characterize 3D biventricular strains, eventually leading to deeper understanding of biventricular mechanics in PAH. (10.1016/j.cmpb.2020.105345)
  DOI : 10.1016/j.cmpb.2020.105345
• Aspects of the thermodynamic behavior of salt caverns used for gas storage
  
  • Bérest Pierre
  • Louvet Floriane
  Oil & Gas Science and Technology - Revue d'IFP Energies nouvelles, Institut Français du Pétrole (IFP), 2020, 75, pp.57. New evidence supporting views previously expressed in a paper dedicated to the thermodynamic behavior of gas storage caverns (Bérest, 2019, Heat transfer in salt caverns, Int. J. Rock Mech. Rock Eng. Sci. 120, 82–95. https://doi.org/10.1016/j.ijrmms.2019.06.009) is provided. In a fluid-filled cavern, conditions for the onset of natural convection are always met, at least in principle. In fact, for gas storage caverns, convection is present in the upper part of a cavern, where gas temperature and moisture content tend to homogenize. In the lower part of a cavern, below the temperature-gradient inversion depth, temperature is a decreasing function of depth, and no convection is observed. The reason is that brine trapped in the cavern sump remains consistently colder than the rock mass, even decades after cavern creation. Sump brine is cooled during each gas pressure cycle: most often, during depressurization, condensation of water vapor occurs; it is spread in the entire gas body from which latent heat is withdrawn, resulting in “raining” in the cavern. During re-pressurization, vaporization takes place at the gas-brine interface, where latent heat is provided by the brine sump, resulting, during a cycle, in net cooling of the brine sump. This process is more effective when a larger number of cycles per year is run. The buffer created above the brine-gas interface hinders vapor diffusion to the cavern main body. It is hypothetized that, when cavern pressure is cycled more frequently, water content in the withdrawn gas is smaller. This might be an advantage in natural gas caverns, as hydrate formation is lessened, and in hydrogen or compressed air caverns, as less dehydration efforts are required. (10.2516/ogst/2020040)
  DOI : 10.2516/ogst/2020040
• A Statistical Framework for Generating Microstructures of Two-Phase Random Materials: Application to Fatigue Analysis
  
  • Khristenko Ustim
  • Constantinescu Andrei
  • Le Tallec Patrick
  • Oden J. Tinsley
  • Wohlmuth Barbara I
  Multiscale Modeling and Simulation: A SIAM Interdisciplinary Journal, Society for Industrial and Applied Mathematics, 2020, 18 (1), pp.21-43. Random microstructures of heterogeneous materials play a crucial role in the material macroscopic behavior and in predictions of its effective properties. A common approach to modeling random multiphase materials is to develop so-called surrogate models approximating statistical features of the material. However , the surrogate models used in fatigue analysis usually employ simple mi-crostructure, consisting of ideal geometries such as ellipsoidal inclusions, which generally does not capture complex geometries. In this paper, we introduce a simple but flexible surrogate microstructure model for two-phase materials through a level-cut of a Gaussian random field with covariance of Matérn class. Such parametrization of the covariance function allows for the representation of a few key design parameters while representing the geometry of inclusions in a more general setting for a large class of random heterogeneous two-phase media. In addition to the traditional morphology descriptors such as porosity, size and aspect ratio, it provides control of the regularity of the inclusions interface and sphericity. These parameters are estimated from a small number of real material images using Bayesian inversion. An efficient process of evaluating the samples, based on the Fast Fourier Transform, makes possible the use of Monte-Carlo methods to estimate statistical properties for the quantities of interest in a given material class. We demonstrate the overall framework of the use of the surrogate material model in application to the uncertainty quantification in fatigue analysis, its feasibility and efficiency, and its role in the microstructure design. (10.1137/19M1259286)
  DOI : 10.1137/19M1259286
• Effective resonant model and simulations in the time-domain of wave scattering from a periodic row of highly-contrasted inclusions
  
  • Touboul Marie
  • Pham Kim
  • Maurel Agnès
  • Marigo Jean-Jacques
  • Lombard Bruno
  • Bellis Cédric
  Journal of Elasticity, Springer Verlag, 2020, 142 (1), pp.53-82. The time-domain propagation of scalar waves across a periodic row of inclusions is considered in 2D. As the typical wavelength within the background medium is assumed to be much larger than the spacing between inclusions and the row width, the physical configuration considered is in the low-frequency homogenization regime. Furthermore, a high contrast between one of the constitutive moduli of the inclusions and of the background medium is also assumed. So the wavelength within the inclusions is of the order of their typical size, which can further induce local resonances within the microstructure. In Pham et al. (2017), two-scale homogenization techniques and matched-asymptotic expansions have been employed to derive, in the harmonic regime, effective jump conditions on an equivalent interface. This homogenized model is frequency-dependent due to the resonant behavior of the inclusions. In this context, the present article aims at investigating, directly in the time-domain, the scattering of waves by such a periodic row of resonant scatterers. Its effective behavior is first derived in the time-domain and some energy properties of the resulting homogenized model are analyzed. Time-domain numerical simulations are then performed to illustrate the main features of the effective interface model obtained and to assess its relevance in comparison with full-field simulations.
• High resolution digital image correlation for microstructural strain analysis of a stainless steel repaired by Directed Energy Deposition
  
  • Balit Yanis
  • Guévenoux Camille
  • Tanguy Alexandre
  • Upadhyay Manas V
  • Charkaluk Eric
  • Constantinescu Andrei
  Materials Letters, Elsevier, 2020, 270, pp.127632. Deformations within a microstructural gradient zone of stainless steel repaired specimens are investigated. The repair, added material by Directed Energy Deposition over a hot rolled sheet substrate, was tested in monotonic tensile experiments. In situ tests, scanning electron microscope images combined with high resolution digital image correlation and electron backscatter diffraction maps, permitted to monitor the local strain distribution. The strain distribution is homogeneous in the substrate and exhibits a heterogeneous pattern in the printed part with localization correlating spatially with the position of interlayers. The vicinity of the interface has smaller strains and exhibits larger hardness. (10.1016/j.matlet.2020.127632)
  DOI : 10.1016/j.matlet.2020.127632