Publications

2019

• Modélisation de l'endurance en fatigue sous chargement complexe, statique et vibratoire
  
  • de Moura Pinho Raùl
  , 2019. Les matériaux dans les machines tournantes sont soumis à des chargementscomplexes consistant en la superposition d’un chargement de forte amplitude (LCF) et d’un chargement vibratoire de faible amplitude (HCF). En pratique, le nombre de cycles LCF autorisé est limité, mais en aucun cas la contribution HCF doit conduire à de l’endommagement. Ce travail de recherche vise donc à proposer une approche permettant de rendre compte de l’impact du chargement LCFsur la limite d’endurance HCF.Pour cela, une approche fondée sur la mécanique linéaire élastique de la rupture a été proposée. Un critère non-local de plasticité en pointe de fissure a été développé. Le fait qu’il nécessite la connaissance des contraintes Tpermet de rendre compte de l’effet de fissure courte. Son extension au cas non-linéaire a conduit à une meilleure prise en compte des effets de rapport de charge dans la construction de diagrammes de Haigh.Enfin, un nouveau moyen d’essai, ainsi qu’une nouvelle géométrie d’éprouvette ont été développés pour la recherche du seuil de non-propagation de fissure à hautes fréquences. L’absence d’effet de la fréquence sur le Ti-6Al-4V d’étude a été démontrée entre 30 Hz et 800 Hz.
• Multiscale mechanics of soft tissues
  
  • Wijanto Florent
  , 2019. Fibre networks are ubiquitous structures in biological tissues, both at the macroscopic level being the main ingredient in soft tissues and at the microscopic level, as constituents of collagen structures or the cytoskeleton. The goal of this work is to propose a model based on the physical microstructure of fibre networks in order to provide an understanding of the mechanical behaviour of biological fibre networks. The current model starts from fibres sliding with respect to one another and interacting via spring-like cross-bridges. These cross-bridges can attach and detach stochastically with a load-dependent detachment rate. Compared to existing modelling approaches, this work features a dynamic sliding configuration for the interacting fibres and discrete binding sites which permit attachment on localised spaces of the fibre. The detachment of cross-bridges is based on thermal diffusion out of an energy well, following the Kramers rate theory. This theory provides a physical background to the detachment dynamics as well as a natural load dependency in the tilting of the energy landscape by the load force. The model provides two modes by which the depicted system may be driven: an imposed velocity driving, called a hard device and an imposed load driving, called a soft device. The work also provides a way of visualising the behaviour of the model by performing a stochastic simulation. The simulations provided present two algorithms, each tailored to represent the driving of the system, whether in hard or soft device, respecting the causality in each of the driving mode. Simulation results are explored via data visualisation of simulation output. These visualisation serve as an entry point into parametric investigation of the model behaviour and anchor the interpretation of the results into physical systems. In particular, the influence of binding site spacing, one of the key features of the model, is investigated. We also investigate the effects of complex loading paths (transitory, cyclic, etc.) which can be associated to the physiological loadings fibrous tissues.
• Apprehending the effects of mechanical deformations in cardiac electrophysiology: A homogenization approach
  
  • Collin Annabelle
  • Imperiale Sébastien
  • Moireau Philippe
  • Gerbeau Jean-Frédéric
  • Chapelle Dominique
  Mathematical Models and Methods in Applied Sciences, World Scientific Publishing, 2019, 29 (13), pp.2377-2417. We follow a formal homogenization approach to investigate the effects of mechanical deformations in electrophysiology models relying on a bidomain description of ionic motion at the microscopic level. To that purpose, we extend these microscopic equations to take into account the mechanical deformations, and proceed by recasting the problem in the framework of classical two-scale homogenization in periodic media, and identifying the equations satisfied by the first coefficients in the formal expansions. The homogenized equations reveal some interesting effects related to the microstructure - and associated with a specific cell problem to be solved to obtain the macroscopic conductivity tensors - in which mechanical deformations play a non-trivial role, i.e. do not simply lead to a standard bidomain problem posed in the deformed configuration. We then present detailed numerical illustrations of the homogenized model with coupled cardiac electrical-mechanical simulations - all the way to ECG simulations - albeit without taking into account the abundantly-investigated effect of mechanical deformations in ionic models, in order to focus here on other effects. And in fact our numerical results indicate that these other effects are numerically of a comparable order, and therefore cannot be disregarded. (10.1142/S0218202519500490)
  DOI : 10.1142/S0218202519500490
• Grain Boundary Sliding and Strain Rate Sensitivity of Coarse and Fine/Ultrafine Grained 5083 Aluminum Alloys
  
  • Goyal A.
  • Doquet V.
  • Pouya Amade
  Metallurgical and Materials Transactions A, Springer Verlag/ASM International, 2019, 51 (3), pp.1109-1122. The viscoplastic behavior of coarse and bimodal fine/ultrafine grained (F/UFG) Al5083 alloy was investigated between 20 °C and 200 °C through tensile tests at various strain rates, and stress relaxation tests to deduce the strain rate sensitivity (SRS). The plastic strain fields were measured by correlation of SEM images. In the F/UFG material at high temperature, very high strains were measured in shear bands which sometimes crossed the whole gage width and exhibited intensive grain boundary sliding (GBS). Both the SRS and ductility rose with the temperature, and as the strain rate decreased, mainly due to a rising contribution of GBS, which accommodated a much larger fraction of the global strain in the F/UFG material. The boundary between the temperature–strain rate domains where grain refinement led either to strengthening or to softening was determined. Finite element simulations of tension and relaxation tests with viscoplastic grains and sliding grain boundaries captured the macro-scale behavior of the F/UFG material. It also provided some insight into the mechanisms of correlated and cooperative GBS and grain rotation along percolation paths (both inter and intragranular), probably, responsible for macro shear banding. (10.1007/s11661-019-05583-5)
  DOI : 10.1007/s11661-019-05583-5
• Mechanical properties of additively manufactured or repaired single-track thickness structures by Directed Energy Deposition.
  
  • Balit Yanis
  , 2019. This thesis was dedicated to the study of 316L stainless steel additively manufactured or repaired specimens by Directed Energy Deposition (DED). Different configurations were manufactured under optimal process parameters. The novelty of this work is the observation of the microstructural strain localization. This experiment combined an in situ tensile test inside a scanning electron microscope with high resolution digital image correlation and an electron backscatter diffraction map. These results allowed for a fresh interpretation of monotonic tensile tests as well as of self-heating experiments under cyclic loading and the failure patterns observed at the surface of specimens. The first objective was to understand the deformation mechanisms at the grain scale which could explain the observed macroscopic anisotropy of the tensile properties as reported in literature. Two loading directions, along and perpendicular, were considered with respect to the printing direction for fully printed specimens. We observed that for a tensile load perpendicular to the printing direction, the strain localization is mainly situated at some interlayers. For a tensile load along the printing direction, the strain localization was observed in some particular regions of large grains. The second objective was the assessment of DED as a repair technology. Dog bone shaped repaired specimens (half hot rolled sheet and half printed) were designed and they exhibited an important hierarchical microstructural gradient. We noticed that the interface is not a weak area during a monotonic tensile test. Moreover, while homogeneous strain was observed in the substrate half, the printed half showed a strain heterogeneity, with the highest localization found at some interlayers. An unstrained zone was observed at both sides of the interface and was associated with higher hardness. The last objective was to evaluate the fatigue properties by self-heating tests. The experiment has proven that the difficulties due to the small dimensions of the single-track thickness specimens can be overcome by careful construction of the experimental set-up. The results revealed a certain correlation between the pattern of the microstructure, the deformation pattern at this scale and the self-heating results. Anisotropy was highlighted during these cyclic tests where specimens tested perpendicularly to the printing direction showed higher fatigue limits in comparison to the ones tested along the printing direction. Post mortem analysis revealed a multitude of cracks at interlayers for the specimens tested perpendicularly to the printing direction creating several sites of heat diffusion. For the specimens tested along the printing direction, a more classical fatigue scenario was observed with one dominating crack and thus a localized heat dissipation.
• Modélisation multi-échelles de la contraction musculaire : De la dynamique stochastique des moteurs moléculaires à la mécanique des milieux continus
  
  • Kimmig François
  , 2019. This PhD thesis deals with the mathematical description of the micro-scale muscle contraction mechanisms with the aim of proposing and integrating our models into a multiscale heart simulation framework.This research effort is made in the context of digital medicine, which proposes to improve the treatment of patients with the use of numerical tools.The first contribution of this thesis is a literature review of the experimental works characterizing the actin-myosin interaction and its regulations to compile information in a useable form for the development of models.This stage is an essential prerequisite to modeling.We then propose a hierarchy of muscle contraction models starting from a previously proposed refined stochastic model, which was only validated for skeletal muscles, and applying successive simplification assumptions.The simplification stages transform the initial stochastic differential equation into a partial differential equation with a model that is part of the Huxley'57 model family.A further simplification then leads to a description governed by a set of ordinary differential equations.The relevance of these models, targeting different time scales, is demonstrated by comparing them with experimental data obtained with cardiac muscles and their range of validity is investigated.To integrate these microscopic descriptions into a heart simulation framework, we extend the models to take into account the force regulation mechanisms that take place in vivo, leading to the derivation of new partial differential equations.Then, we link the microscopic contraction models to the macroscopic organ model.We follow for that an approach based on the thermodynamical principles to deal with the multi-scale nature in time and space of the muscle tissue at the continuous and at the discrete levels.The validity of this simulation framework is demonstrated by showing its ability to reproduce the heart behavior and in particular to capture the essential features of the Frank-Starling effect.
• Etude micromécanique du lien entre endommagement local et comportement macroscopique de propergols solides
  
  • de Francqueville Foucault
  , 2019. L'objectif de ce projet de recherche est le développement de simulations numériques de l'endommagement de propergols solides, qui sont dédiés à la propulsion anaérobique, afin d'identifier quelles propriétés affectent leur comportement. Pour étudier l'effet de la géométrie des différentes particules énergétiques, des microstructures 3D sont générées avec une dispersion aléatoire d'inclusions monomodales, sphériques ou polyédriques, à très forte fraction volumique (55%). Dans le cas des sphères, les propriétés élastiques des volumes élémentaires représentatifs (VER) sont confrontées à un modèle analytique et à des caractérisations expérimentales de composites modèles, démontrant une cohérence remarquable entre les trois approches. Par la suite, la réponse linéaire des VER contenant des polyèdres est comparée à celle des particules sphériques, démontrant un effet très limité de la géométrie des charges. L'endommagement de ces matériaux étant majoritairement dû aux décohésions matrice/particules, une loi de zone cohésive bilinéaire est implémentée avec une régularisation visqueuse et un affichage de l'état d'endommagement des interfaces. L'influence du premier ordre des paramètres de zone cohésive, tant sur le comportement global que l'endommagement local, peut alors être démontrée. Si les difficultés de convergence numérique ne permettent pas d'envisager une confrontation quantitative avec les données expérimentales, les tendances spécifiques de ces dernières sont remarquablement reproduites, que ce soit à l'échelle des particules ou du matériau. Une étude paramétrique permet de mettre en évidence l'influence des paramètres de zone cohésive sur la réponse globale. L'analyse de l'impact de la géométrie des particules sur le comportement endommagé conduit au même effet du second ordre observé précédemment. Enfin, une étude des propriétés de quasi-propergols, représentatifs des propergols les plus communs, est proposée. En suivant un processus industriel de caractérisation, leurs propriétés d’interface sont identifiées qualitativement, en s'appuyant sur les tendances suggérées par les simulations. Afin de compléter cette analyse, des caractérisations non conventionnelles sont proposées, permettant de valider sa cohérence et de fournir de nouveaux éléments d'identification des propriétés d'adhésions.
• Fast simulation of temperature and phase transitions in directed energy deposition additive manufacturing
  
  • Weisz-Patrault Daniel
  Additive Manufacturing, Elsevier, 2019. In this contribution, a simplified macroscopic and semi-analytical thermal analysis of directed energy deposition (DED) is presented to obtain computationally efficient simulations of the entire process. Solidification and solid-state phase transitions are taken into account. The model is derived for laser metal powder directed energy deposition, although it can be simply adapted for other focused thermal energy (e.g., electron beam, or plasma arc). The gas flow used for carrying the powder significantly influences cooling conditions, which is included in the model. The proposed simulation strategy applies to multilayer composites with a wide range of shapes in the horizontal plane and arbitrary laser scanning strategies (continuous way, back and forth, etc.). The proposed work provides a simple tool to study the influence of most process parameters, design in-situ experiments and in turn develop optimization loops to reach material requirements and specific microstructures. In-situ pyrometer measurements have been compared to the model, and good agreement has been observed with 2.6% error in average. The model is used to demonstrate the effect of various process parameters for a simple cylindrical geometry and a more complex auxetic cell.
• Computational fatigue assessment of mooring chains under tension loading
  
  • Martinez Perez Imanol
  • Constantinescu Andrei
  • Bastid Philippe
  • Zhang Yan-Hui
  • Venugopal Vengatesan
  Engineering Failure Analysis, Elsevier, 2019, 106. This paper presents a computational fatigue assessment method of mooring chains under tensile loading, it is composed of a mechanical analysis followed by a fatigue analysis. The mechanical analysis is performed in two steps: residual stress prediction and service loading. From this analysis, the shakedown cycle is extracted at the critical points, ie: the asymptotically stabilized stress-strain cycles. As the mooring chain under service loading is under elastic shakedown, the Dang Van fatigue criterion is applied for the fatigue analysis. The accuracy of the proposed fatigue assessment method is proved by comparing with the experimental results from full-scale fatigue testing of mooring chain in seawater. The numerical results match both the experimental observations with respect to the localization of the damage zone and the lifetime. (10.1016/j.engfailanal.2019.06.073)
  DOI : 10.1016/j.engfailanal.2019.06.073
• Active gel segment behaving as an active particle
  
  • Recho Pierre
  • Putelat T.
  • Truskinovsky L.
  Physical Review E, American Physical Society (APS), 2019, 100 (6). We reduce a one-dimensional model of an active segment (AS), which is used, for instance, in the description of contraction driven cell motility on tracks, to a zero-dimensional model of an active particle (AP) characterized by two internal degrees of freedom: position and polarity. Both models give rise to hysteretic force-velocity relations showing that an active agent can support two opposite polarities under the same external force and that it can maintain the same polarity while being dragged by external forces with opposite orientations. This double bi-stability results in a rich dynamic repertoire which we illustrate by studying static, stalled, motile and periodically repolarizing regimes displayed by an active agent confined in a visco-elastic environment. We show that the AS and AP models can be calibrated to generate quantitatively similar dynamic responses. (10.1103/physreve.100.062403)
  DOI : 10.1103/physreve.100.062403
• A multiscale mathematical model of cell dynamics during neurogenesis in the mouse cerebral cortex
  
  • Postel Marie
  • Karam Alice
  • Pézeron Guillaume
  • Schneider-Maunoury Sylvie
  • Clément Frédérique
  BMC Bioinformatics, BioMed Central, 2019, 20 (1), pp.470. Neurogenesis in the murine cerebral cortex involves the coordinated divisions of two main types of progenitor cells, whose numbers, division modes and cell cycle durations set up the final neuronal output. To understand the respective roles of these factors in the neurogenesis process, we combine experimental in vivo studies with mathematical modeling and numerical simulations of the dynamics of neural progenitor cells. A special focus is put on the population of intermediate progenitors (IPs), a transit amplifying progenitor type critically involved in the size of the final neuron pool. A multiscale formalism describing IP dynamics allows one to track the progression of cells along the subsequent phases of the cell cycle, as well as the temporal evolution of the different cell numbers. Our model takes into account the dividing apical progenitors (AP) engaged into neurogenesis, both neurogenic and proliferative IPs, and the newborn neurons. The transfer rates from one population to another are subject to the mode of division (symmetric, asymmetric, neurogenic) and may be time-varying. The model outputs are successfully fitted to experimental cell numbers from mouse embryos at different stages of cortical development, taking into account IPs and neurons, in order to adjust the numerical parameters. Applying the model to a mouse mutant for Ftm/Rpgrip1l, a gene involved in human ciliopathies with severe brain abnormalities, reveals a shortening of the neurogenic period associated with an increased influx of newborn IPs from apical progenitors at mid-neurogenesis. Additional information is provided on cell kinetics, such as the mitotic and S phase indexes, and neurogenic fraction. Our model can be used to study other mouse mutants with cortical neurogenesis defects and can be adapted to study the importance of progenitor dynamics in cortical evolution and human diseases. 2 Introduction The multiple functions of the mammalian cerebral cortex in integrating sensory stimuli, controlling motor output and mediating cognitive functions are supported by an extraordinary diversity of neuronal sub-types mutually connected through complex neuronal circuitry. The formation of this structure requires producing the correct numbers and subtypes of neurons at the proper position during a specific period (10.1186/s12859-019-3018-8)
  DOI : 10.1186/s12859-019-3018-8
• Wrinkling to crinkling transitions and curvature localization in a magnetoelastic film bonded to a non-magnetic substrate
  
  • Psarra E.
  • Bodelot L.
  • Danas Kostas
  Journal of the Mechanics and Physics of Solids, Elsevier, 2019, 133, pp.103734. This work studies experimentally and numerically the post-bifurcation response of a magnetorheological elastomer (MRE) film bonded to a soft non-magnetic (passive) substrate. The film-substrate system is subjected to a combination of an axial mechanical pre-compression and a transverse magnetic field. The non-trivial interaction of the two fields leads to a decrease of the critical magnetic field with applied pre-compression, while the observed wrinkling patterns evolve into crinkles, a bifurcation mode that is defined by the accompanied curvature localization and strong shearing of the side faces of the wrinkled geometry. Using a magneto-elastic variational formulation in a two-dimensional finite element numerical setting, we find that the crinkling is an intrinsic feature of magnetoelasticity and its presence is directly associated with the repulsive magnetic forces of the neighboring wrinkled-crinkled faces. As a result, the presence of the magnetic field prohibits the formation of creases and folds. In an effort to obtain a good quantitative agreement between the numerical and the experimental results, we also introduce an approximate way to model the friction of the lateral film-substrate faces. This analysis reveals the strong effects of friction upon the magneto-mechanical wrinkling modes. (10.1016/j.jmps.2019.103734)
  DOI : 10.1016/j.jmps.2019.103734
• Root Hair Sizer: an algorithm for high throughput recovery of different root hair and root developmental parameters
  
  • Guichard Marjorie
  • Allain Jean-Marc
  • Bianchi Michele Wolfe
  • Frachisse Jean-Marie
  Plant Methods, BioMed Central, 2019, 15, pp.104. Background: The root is an important organ for water and nutrient uptake, and soil anchorage. It is equipped with root hairs (RHs) which are elongated structures increasing the exchange surface with the soil. RHs are also studied as a model for plant cellular development, as they represent a single cell with specific and highly regulated polarized elon-gation. For these reasons, it is useful to be able to accurately quantify RH length employing standardized procedures. Methods commonly employed rely on manual steps and are therefore time consuming and prone to errors, restricting analysis to a short segment of the root tip. Few partially automated methods have been reported to increase measurement efficiency. However, none of the reported methods allow an accurate and standardized definition of the position along the root for RH length measurement, making data comparison difficult. Results: We developed an image analysis algorithm that semi-automatically detects RHs and measures their length along the whole differentiation zone of roots. This method, implemented as a simple automated script in ImageJ/Fiji software that we termed Root Hair Sizer, slides a rectangular window along a binarized and straightened image of root tips to estimate the maximal RH length in a given measuring interval. This measure is not affected by heavily bent RHs and any bald spots. RH length data along the root are then modelled with a sigmoidal curve, generating several biologically significant parameters such as RH length, positioning of the root differentiation zone and, under certain conditions, RH growth rate. Conclusions: Image analysis with Root Hair Sizer and subsequent sigmoidal modelling of RH length data provide a simple and efficient way to characterize RH growth in different conditions, equally suitable to small and large scale phenotyping experiments. (10.1186/s13007-019-0483-z)
  DOI : 10.1186/s13007-019-0483-z
• Eczéma ou mycosis fongoïde ? Analyses conjointes par spectroscopies Raman et infrarouge FTIR
  
  • Luong Minh Son
  • Jdaini Jihane
  • Maurin David
  • Michel Thierry
  • Szablewski Vanessa
  • Durand Luc
  • Luong Minh-Phong
  • Costes-Martineau Valérie
  • Bantignies Jean-Louis
  • Dereure Olivier
  Annales de Dermatologie et de Vénéréologie, Elsevier Masson, 2019, 146 (12), pp.A262-A263. L’eczéma, dermatose très courante, peut, dans certains cas, être difficile à différencier du mycosis fongoïde (MF) par la clinique, la dermatoscopie et l’histologie. Une approche moléculaire par 2 spectrométries vibrationnelles complémentaires, Raman (diffusion inélastique) et infrarouge (absorption) a été évaluée. (10.1016/j.annder.2019.09.418)
  DOI : 10.1016/j.annder.2019.09.418
• Supersonic kinks and solitons in active solids
  
  • Gorbushin N.
  • Truskinovsky L.
  Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences, Royal Society, The, 2019, 378 (2162), pp.20190115. To show that steadily propagating nonlinear waves in active matter can be driven internally, we develop a prototypical model of a topological kink moving with a constant supersonic speed. We use a model of a bi-stable mass-spring (Fermi–Pasta–Ulam) chain capable of generating active stress. In contrast to subsonic kinks in passive bi-stable chains that are necessarily dissipative, the obtained supersonic solutions are purely anti-dissipative. Our numerical experiments point towards the stability of the obtained kink-type solutions and the possibility of propagating kink-anti-kink bundles reminiscent of solitons. We show that even the simplest quasi-continuum approximation of the discrete model captures the most important features of the predicted active phenomena. (10.1098/rsta.2019.0115)
  DOI : 10.1098/rsta.2019.0115
• A study on the effective elastic properties of random porous materials : 3D Printing, Experiments and Numerics.
  
  • Zerhouni Othmane
  , 2019. This thesis deals with the 3D-printing, numerical simulation and experimental testing of porous materials with random isotropic microstructures. In particular, we attempt to assess by means of well-chosen examples the effect of partial statistical descriptors (i.e., porous volume fraction or porosity, two-point correlation functions and chord-length distribution) upon the linear effective elastic response of random porous materials and propose (nearly) optimal microstructures by direct comparison with available theoretical mathematical bounds. To achieve this, in the first part of this work, we design ab initio porous materials comprising single-size (i.e. monodisperse) and multiple-size (polydisperse) spherical and ellipsoidal non-overlapping voids. The microstructures are generated using a random sequential adsorption (RSA) algorithm that allows to reach very high porosities (e.g. greater than 80%). The created microstructures are then numerically simulated using finite element (FE) and Fast Fourier Tranform (FFT) methods to obtain representative isotropic volume elements in terms of both periodic and kinematic boundary conditions. This then allows for the 3D-printing of the porous microstructures in appropriately designed dog-bone specimens. An experimental setup for uniaxial tension loading conditions is then developed and the 3D-printed porous specimens are tested to retrieve their purely linear elastic properties. This process allows, for the first time experimentally, to show that such polydisperse (multiscale) microstructures can lead to nearly optimal effective elastic properties when compared with the theoretical Hashin-Shtrikman upper bounds for a very large range of porosities spanning values between 0-82%. To understand further the underlying mechanisms that lead to such a nearly optimal response, we assess the influence of several statistical descriptors (such as the one- and two-point correlation functions, the chord-length distribution function) of the microstructure upon the effective elastic properties of the porous material. We first investigate the ability of the two-point correlation function to predict accurately the effective response of random porous materials by choosing two different types of microstructures, which have exactly the same first (i.e., porosity) and second-order statistics. The first type consists of non-overlapping spherical and ellipsoidal pores generated by the RSA process. The second type, which uses the thresholded Gaussian Random Field (GRF) method, is directly reconstructed by matching the one- and two-point correlation functions from the corresponding RSA microstructure. The FFT-simulated effective elastic properties of these two microstructures reveal very significant differences that are in the order of 100% in the computed bulk and shear moduli. This analysis by example directly implies that the two-point statistics can be highly insufficient to predict the effective elastic properties of random porous materials. We seek to rationalize further this observation by introducing controlled connectivity in the original non-overlapping RSA microstructures. The computed effective elastic properties of these microstructures show that the pore connectivity does not change neither the two-point correlation functions nor the chord-length distribution but leads to a significant decrease in the effective elastic properties. In order to quantify better the differences between those three microstructures, we analyze the link between the local geometry of the porous phase and the corresponding computed elastic fields by computing the first (average) and second moments of the elastic strain fluctuations. This last analysis suggests that partial statistical information of the microstructure (without any input from the corresponding elasticity problem) might be highly insufficient even for the qualitative analysis of a porous material and by extension of any random composite material.
• Fissuration par fatigue en mode mixte non proportionnel des rails de chemins de fer : De l’étude expérimentale à la mise en œuvre d’un modèle
  
  • Bonniot Thomas
  , 2019. Rails are submitted to Rolling Contact Fatigue due to repeated passages of train wheels, which induces several types of cracks, such as Squat-type cracks. Those cracks undergo non-proportional mixed-mode I + II + III loading, including compression phases, in variable proportions along the crack front, making the prediction of their paths and growth rates a challenge.Mode I crack growth kinetics, for positive and negative R ratios, were first determined in R260 steel, as well as friction-corrected crack growth kinetics for fully-reversed combined mode II and III. The effective Stress Intensity Factors (SIFs) were deduced from the measured in-plane and out-of-plane crack face sliding displacements. From those kinetic laws, it was deduced that neither pure mode I, nor pure shear mode loadings can explain the crack growth rates observed in rails. A combination of those three loading modes, according to complex loading paths had thus to be prospected.Non-proportional mixed-mode I + II fatigue crack growth tests were then performed, following representative loading paths. Stereo digital image correlation was used to measure the near-tip displacement field. Post-treatment methods generally used to deduce the effective SIFs from these fields were inappropriate because of contact and friction stresses along the crack face. New methods were thus developed. The crack paths and growth rates were analyzed, using the effective SIFs. Crack path prediction by the maximum tangential stress criterion was found not to be very reliable, but substantially improved when crack tip plasticity and the presence of contact and friction stresses along the crack faces were taken into account. The measured crack growth rates correlated well with a combination of the three effective SIFs in a Paris-type law.From these experiments, it appears that due to crack face roughness, asperities interlocking and friction substantially reduce the effective SIFs, even without any normal compression, which cannot be captured by a simple Coulomb’s law. Besides, crack faces wear also has a large influence on the effective SIFs. The challenge for structural applications is thus not only to choose the most appropriate bifurcation criterion and crack growth law, but also to take crack face roughness and wear into account, in order to estimate the correct effective SIFS to use in these models.For industrial applications, a simple engineering approach was proposed to integrate roughness-induced friction in the estimation of the effective loading path from the nominal one. This approach was validated on sequential mixed-mode I + II & III experiments.
• Strength of Materials and Structural Components
  
  • Abbadi Mohammed
  • Nouari Mohammed
  • El Otmani Rabie
  , 2019, pp.224. This special issue contains articles from the field of the strength of materials and structural components, additive manufacturing, and testing and modeling methods in materials science. We hope this volume will be interesting for many engineers from the area of machinery.
• Energy Decay and Stability of a Perfectly Matched Layer For the Wave Equation
  
  • Baffet Daniel Henri
  • Grote Marcus J.
  • Imperiale Sébastien
  • Kachanovska Maryna
  Journal of Scientific Computing, Springer Verlag, 2019. In [25, 26], a PML formulation was proposed for the wave equation in its standard second-order form. Here, energy decay and L 2 stability bounds in two and three space dimensions are rigorously proved both for continuous and discrete formulations. Numerical results validate the theory. (10.1007/s10915-019-01089-9)
  DOI : 10.1007/s10915-019-01089-9
• Identification of the material behavior of adhesive joints under dynamic multiaxial loadings
  
  • Janin Anthony
  • Constantinescu Andrei
  • Weisz-Patrault Daniel
  • Nevière Robert
  • Stackler Matthieu
  • Albouy William
  International Journal of Impact Engineering, Elsevier, 2019. This paper presents a numerical inverse method dedicated to the characterization of adhesive joints under multiaxial and dynamic loading conditions. The properties under scrutiny are the constitutive behavior of the joint as well as the final fracture surface. The experimental setup consists of a Split Hopkinson Pressure Bar system and local strain measurements performed by Digital Image Correlation (DIC) as well as a novel specific sandwich specimen denoted as DODECA. The direct numerical model is an original Finite Element computation combining 3D and 1D elements for an optimal handling of wave reflection and interfaces. It further provides an optimal compromise between computation time and accuracy. The identification method is based on the Finite Element Model Updating method (FEMU). Material parameters are identified for three different multiaxial loading conditions and presented as yield and fracture surfaces in the space of equivalent von Mises stress vs hydrostatic stress. As a complement of the analysis, uncertainties and confidence of the identified parameters are estimated with precise qualitative tools. (10.1016/j.ijimpeng.2019.103355)
  DOI : 10.1016/j.ijimpeng.2019.103355
• Design and testing of 3D-printed micro-architectured polymer materials exhibiting a negative Poisson’s ratio
  
  • Agnelli Filippo
  • Constantinescu Andrei
  • Nika Grigor
  Continuum Mechanics and Thermodynamics, Springer Verlag, 2019. This work proposes the complete design cycle for several auxetic materials where the cycle consists of three steps (i) the design of the micro-architecture, (ii) the manufacturing of the material and (iii) the testing of the material. We use topology optimization via a level-set method and asymptotic homogenization to obtain periodic micro-architectured materials with a prescribed effective elasticity tensor and Poisson's ratio. The space of admissible micro-architectural shapes that carries orthotropic material symmetry allows to attain shapes with an effective Poisson's ratio below − 1. Moreover, the specimens were manufactured using a commercial stereolithography Ember printer and are mechanically tested. The observed displacement and strain fields during tensile testing obtained by digital image correlation match the predictions from the finite element simulations and demonstrate the efficiency of the design cycle. (10.1007/s00161-019-00851-6)
  DOI : 10.1007/s00161-019-00851-6
• Mechanical and Imaging Models-based Image Registration
  
  • Škardová Kateřina
  • Rambausek Matthias
  • Chabiniok Radomir
  • Genet Martin
  , 2019. Image registration plays an increasingly important role in many fields such as biomedical or mechanical engineering. Generally speaking, it consists in deforming a (moving) source image to match a (fixed) template image. Many approaches have been proposed over the years; if new model-free machine learning-based approaches are now beginning to provide robust and accurate results, extracting motion from images is still most commonly based on combining some statistical analysis of the images intensity and some model of the underlying deformation as initial guess or regularizer. These approaches may be efficient even for complex type of motion; however, any artifact in the source image (e.g., partial voluming, local decrease of signal-to-noise ratio or even local signal void), drastically deteriorates the registration. This paper introduces a novel approach of extracting motion from biomedical image series, based on a model of the imaging modality. It is, to a large extent, independent of the type of model and image data-the prerequisite is to incorporate biomechanical constraints into the motion of the object (organ) of interest and being able to generate data corresponding to the real image, i.e., having an imaging model at hand. We will illustrate the method with examples of synthetically generated 2D tagged magnetic resonance images. (10.1007/978-3-030-32040-9_9)
  DOI : 10.1007/978-3-030-32040-9_9
• Guided active particles
  
  • Garcia-Garcia Reinaldo
  • Collet Pierre
  • Truskinovsky Lev
  Physical Review E, American Physical Society (APS), 2019, 100 (4). (10.1103/PhysRevE.100.042608)
  DOI : 10.1103/PhysRevE.100.042608
• Fatigue crack growth in compacted and spheroidal graphite cast irons
  
  • Hosdez Jérôme
  • Limodin N.
  • Najjar Denis
  • Witz Jean-François
  • Charkaluk E.
  • Osmond Pierre
  • Forre Agathe
  • Szmytka F.
  International Journal of Fatigue, Elsevier, 2019, pp.105319. The present paper focuses on the fatigue life of a Compacted Graphite cast Iron (CGI) as compared to a Spheroidal Graphite cast Iron (SGI). Fatigue crack growth laws have been determined with digital image correlation. Main difference between the materials is that cracks propagate faster in the CGI than in the SGI. X-ray tomography was also used in order to assess graphite morphologies and crack shapes. A complex morphology was observed for the vermicular graphite with rounded edges that limit notch effects. The crack spreads easily in CGI via a quasi cleavage mechanism and by propagating through graphite mainly by breaking the vermicules. (10.1016/j.ijfatigue.2019.105319)
  DOI : 10.1016/j.ijfatigue.2019.105319
• Modeling and analysis of cell population dynamics : application to the early development of ovarian follicles
  
  • Robin Frédérique
  , 2019. This thesis aims to design and analyze population dynamics models dedicated to the dynamics of somatic cells during the early stages of ovarian follicle growth. The model behaviors are analyzed through theoretical and numerical approaches, and the calibration of parameters is performed by proposing maximum likelihood strategies adapted to our specific dataset. A non-linear stochastic model, that accounts for the joint dynamics of two cell types (precursors and proliferative), is dedicated to the activation of follicular growth. In particular, we compute the extinction time of precursor cells. A rigorous finite state projection approach is implemented to characterize the system state at extinction. A linear multitype age-structured model for the proliferative cell population is dedicated to the early follicle growth. The different types correspond here to the spatial cell positions. This model is of decomposable kind; the transitions are unidirectional from the first to the last spatial type. We prove the long-term convergence for both the stochastic Bellman-Harris model and the multi-type McKendrick-VonFoerster equation. We adapt existing results in a context where the Perron-Frobenius theorem does not apply, and obtain explicit analytical formulas for the asymptotic moments of cell numbers and stable age distribution. We also study the well-posedness of the inverse problem associated with the deterministic model.