Publications

2016

• Dimensionnement en fatigue multiaxiale des toiles de roues ferroviaires sous sollicitations multi-paramètres à amplitude variable
  
  • Roux Clément
  , 2016. L’objectif principal de cette thèse est de développer une méthodologie qui permette de définir des sollicitations simplifiées représentatives des sollicitations réelles rencontrées en exploitation, c’est-à-dire équivalentes en sévérité vis-à-vis du phénomène d’amorçage de fissure par fatigue mécanique. Cette méthodologie doit être adaptée aux problèmes multi-paramètres car les sollicitations subies par les roues ferroviaires sont multidimensionnelles (chargement vertical et latéral) et indépendantes. Enfin, la thèse vise aussi à proposer une approche fiabiliste globale du problème de fatigue des roues, qui pourra être une extension de la méthode Contrainte-Résistance aux cas des sollicitations multi-entrée. Un critère de fatigue pour l’acier des roues est identifié à partir des nouveaux essais.
• Journal of the Mechanics and Physics of Solids, Volume 97, (December 2016) SI:Pierre Suquet Symposium
  
  • Triantafyllidis Nicolas
  , 2016, 97.
• Combined AFM, SEM and crystal plasticity analysis of grain boundary sliding in titanium at room temperature
  
  • Doquet Véronique
  • Barkia B
  Mechanics of Materials, Elsevier, 2016, 103, pp.18-28. Grain boundary sliding (GBS) was observed at room-temperature, at a very early stage of tensile tests on Grade 2 and Grade 4 titanium specimens covered with fiducial grids. AFM images were also repeatedly captured to monitor the evolution with strain of the out-of-plane component of GBS. Sharp gradients in sliding were observed along some grain boundaries, which proved that GBS should not be considered as a rigid-body motion. Grade 4 Ti was less prone to GBS than Grade 2, which was attributed to a pinning effect of βphase particles present along the grain boundaries. Contrary to what was reported for Zinc and Magnesium alloys, GBS was not found to contribute significantly to the global plastic strain in Titanium at room temperature. Crystal viscoplasticity finite element simulations based on real microstructure were run to compute the shear stress along the grain boundaries for various strain rates. The shear stress along the boundaries that slid was found to be substantially higher than the mean shear stress over all boundaries. However, a high shear stress was not sufficient to trigger GBS, also influenced by other factors. A change in strain rate not only modified the mean shear stress on all grain boundaries, but also the relative values of the shear stress on the grain boundaries, which was attributed to the strain-rate dependence of slip activities in each grain. (10.1016/j.mechmat.2016.09.001)
  DOI : 10.1016/j.mechmat.2016.09.001
• High-frequency performance of ferromagnetic shape memory alloys
  
  • Pascan Oana-Zenaida
  • He Y. J.
  • Moumni Ziad
  • Zhang Weihong
  Annals of Solid and Structural Mechanics, Springer Berlin Heidelberg, 2016, 8 (1-2), pp.17 - 25. (10.1007/s12356-016-0045-2)
  DOI : 10.1007/s12356-016-0045-2
• Localization in the piano soundboard
  
  • Lefebvre Gautier
  • Filoche Marcel
  • Boutillon Xavier
  , 2016, 140 (4), pp.3255 - 3255. The soundboard is the complex plane structure that radiates the piano sound. We focus on its structural vibrations. The arrangement of the stiffeners (parallel bars) displays some disorder (height, separation distance) that has been shown responsible for the localization of the vibrations above a precise frequency (Ege 2013, JSV; Chaigne 2013, JASA) By means of a finite-element method, we investigate the localization properties of the eigenmodes of a simplified soundboard model. The localization occurs close to the band-edge of the first Brillouin zone, indicating the importance of a slightly disordered structure, similarly to photonic or phononic crystals with defects (John 1987, PRL). For structural parameters typical of a piano soundboard, this phenomenon seems to force the soundboard to remain in the subsonic regime in a very large frequency range. Moreover, by the adaptation of the theory of the localization landscape (Filoche 2012 PNAS), we compute a dual landscape (Lyra 2015 EPL) for the high-frequency modes of a stiffened orthotropic thin plate. This enables to predict the position of localized eigenmodes. [Work supported by the ANR grant ANR-14-CE07-0014.] (10.1121/1.4970305)
  DOI : 10.1121/1.4970305
• Strength properties of nanoporous materials: Theoretical analyses and Molecular Dynamics computations
  
  • Brach Stella
  , 2016. Since the recent arising of advanced nano-technologies, as well as of innovative engineering design approaches, nanoporous materials have been extensively studied in the last two decades, leading to a considerable worldwide research interest in both industrial and academic domains. Generally characterised by high specific surface area, uniform pore size and rich surface chemistry, nanoporous materials have allowed for the development of challenging ultra-high performance devices with tailorable properties, finding widespread application in several technical fields, including civil and environmental engineering, petroleum and chemical industries, aeronautics and biomechanics. In order to fulfil to these promising applications, one of the most fundamental research aspect consists in characterising and predicting the strength properties of these materials, as dependent on the size of voids. Since the current lack of an exhaustive benchmarking evidence, as well as of a comprehensive theoretical modelling, the central purpose of the present thesis consisted in: • investigating strength properties of in-silico nanoporous samples via Molecular Dynamics computations. In detail, a parametric analysis with respect to the void radius and for different porosity levels has been carried out, by considering different loading paths with a wide range of triaxiality scenarios. As a result, the influence of void-size effects on the computed strength properties has been clearly quantified, also highlighting the dependence of the predicted material strength domain on the three stress invariants; • establishing engineering-oriented theoretical models able to predict macroscopic strength properties of nanoporous materials, by properly accounting for void-size effects. To this end, theoretical approaches based on both non-linear homogeniza- tion techniques and kinematic limit-analysis strategies have been proposed. As a result, closed-form macroscopic strength criteria have been derived, allowing for a consistent description of void-size effects and taking into account different local plastic behaviours.
• Observers for data assimilation - Applications to cardiac modeling
  
  • Moireau Philippe
  , 2016. My work focuses on inverse problems and in particular identification problems and more generally data assimilation problems with applications in particular to the cardiovascular system. In this context, I propose original sequential estimation approach also known as observers. My contributions consist of the design of observer feedback, the analysis of the resulting observers, the definition of joint state and parameter estimation strategy, the discretization of these sequential estimators and their application in a real data context. In parallel of this "inverse problem" activity, I contribute to the improvement of mathematical and mechanical models of the cardiac contraction, to their discretization and to their analysis. This second aspect of my research is complementary of the first one as the data assimilation success deeply depends on adequate modeling ingredients.
• Patient-Specific Computational Analysis of Ventricular Mechanics in Pulmonary Arterial Hypertension
  
  • Xi Ce
  • Latnie Candace
  • Tan Ju Le
  • Wall Samuel T.
  • Genet Martin
  • Zhong Liang
  • Lee Lik Chuan
  • Zhao Xiaodan
  Journal of Biomechanical Engineering, American Society of Mechanical Engineers, 2016, 138 (11). Patient-specific biventricular computational models associated with a normal subject and a patient diagnosed with pulmonary arterial hypertension (PAH) were developed to investigate the effects of this disease on ventricular mechanics. These models were developed using geometry reconstructed from magnetic resonance (MR) images, and constitutive descriptors of passive and active mechanics. Model parameter values associated with ventricular mechanical properties and myofiber architecture were obtained by fitting the models with the corresponding measured pressure-volume loops and the circumferential strain calculated from the MR images using a hyperelastic warping method. Our results shows that the peak right ventricle (RV) pressure was substantially higher in the PAH patient when compared to the normal (65 mmHg vs. 20 mmHg). Circumferential strain (E cc) and ejection fraction (EF) were comparatively lower in both the left ventricle (LV) and RV of the PAH patient (LV EF: 39% vs. 66% and RV EF: 18% vs. 64%; LV E cc :-2.1% vs-9.4% and RV E cc-6.8% vs.-13.2%). On the other hand, passive stiffness, contractility and myofiber stress were all found to be substantially increased in the PAH patient in both the RV and the left ventri-cle (LV). Septum in the PAH patient was also found to possess a smaller curvature than the LV free wall. Simulations using the PAH model with varying RV preload and afterload revealed an approximately linear relationship between the septum curvature and the transseptal pressure gradient at early-diastole and end-systole, respectively. These findings suggest that PAH can induce LV remodeling and measurements of sep-tum curvature may be useful in quantifying the transseptal pressure gradient in PAH patients. (10.1115/1.4034559)
  DOI : 10.1115/1.4034559
• Elasto-Plastic Parameter Identification in a Recrystallized Iron Specimen of Heterogeneous Grain Size
  
  • Baudoin Pierre
  • Magnier Vincent
  • Witz Jean-Francois
  • El Bartali Ahmed
  • Dufrenoy Philippe
  • Charkaluk Eric
  , 2016.
• Analyse de la rupture dynamique fragile via les modèles d'endommagement à gradient : principes variationnels et simulations numériques
  
  • Li Tianyi
  , 2016. Une bonne tenue mécanique des structures du génie civil en béton armé sous chargements dynamiques sévères est primordiale pour la sécurité et nécessite une évaluation précise de leur comportement en présence de propagation dynamique de fissures. Dans ce travail, on se focalise sur la modélisation constitutive du béton assimilé à un matériau élastique-fragile endommageable. La localisation des déformations sera régie par un modèle d'endommagement à gradient où un champ scalaire réalise une description régularisée des phénomènes de rupture dynamique. La contribution de cette étude est à la fois théorique et numérique. On propose une formulation variationnelle des modèles d'endommagement à gradient en dynamique. Une définition rigoureuse de plusieurs taux de restitution d'énergie dans le modèle d'endommagement est donnée et on démontre que la propagation dynamique de fissures est régie par un critère de Griffith généralisé. On décrit ensuite une implémentation numérique efficace basée sur une discrétisation par éléments finis standards en espace et la méthode de Newmark en temps dans un cadre de calcul parallèle. Les résultats de simulation de plusieurs problèmes modèles sont discutés d'un point de vue numérique et physique. Les lois constitutives d'endommagement et les formulations d'asymétrie en traction et compression sont comparées par rapport à leur aptitude à modéliser la rupture fragile. Les propriétés spécifiques du modèle d'endommagement à gradient en dynamique sont analysées pour différentes phases de l'évolution de fissures : nucléation, initiation, propagation, arrêt, branchement et bifurcation. Des comparaisons avec les résultats expérimentaux sont aussi réalisées afin de valider le modèle et proposer des axes d'amélioration.
• Estimation of passive and active properties in the human heart using 3D tagged MRI
  
  • Asner Liya
  • Hadjicharalambous Myrianthi
  • Chabiniok Radomir
  • Devis Peresutti ·
  • Sammut Eva
  • Wong James
  • Carr-White Gerald
  • Chowienczyk Philip
  • Lee Jack
  • King Andrew D.
  • Smith Nicolas P.
  • Razavi Reza
  • David Nordsletten ·
  Biomechanics and Modeling in Mechanobiology, Springer Verlag, 2016, 15 (5), pp.1121-1139. Advances in medical imaging and image processing are paving the way for personalised cardiac biomechani-cal modelling. Models provide the capacity to relate kinematics to dynamics and—through patient-specific modelling— derived material parameters to underlying cardiac muscle pathologies. However, for clinical utility to be achieved, model-based analyses mandate robust model selection and parameterisation. In this paper, we introduce a patient-specific biomechanical model for the left ventricle aiming to balance model fidelity with parameter identifiability. Using non-invasive data and common clinical surrogates, we illustrate unique identifiability of passive and active parameters over the full cardiac cycle. Identifiability and accuracy of the estimates in the presence of controlled noise are verified with a number of in silico datasets. Unique parametrisation is then obtained for three datasets acquired in vivo. The model predictions show good agreement with the data extracted from the images providing a pipeline for personalised biomechan-ical analysis. (10.1007/s10237-015-0748-z)
  DOI : 10.1007/s10237-015-0748-z
• Mechanical modeling and numerical methods for poromechanics : Application to myocardium perfusion
  
  • Burtschell Bruno
  , 2016. This thesis is dedicated to the development of numerical methods for poromechanics, and to their application in a cardiac modeling context. It is motivated by the introduction into existing cardiac models of the coronary network that perfuses the myocardium, to better describe coronary vascular diseases.Drawing our inspiration from existing works, we propose a perfused heart model, and a 0D reduction allowing the cost-effective reproduction of a realistic cardiac cycle with perfusion mass and pressure. The model derived illustrates physiological phenomena inaccessible in former models, and with great clinical application potential, such as vasodilatation and coronary diseases.The integration of a porous compartment to represent the perfused myocardium within 3D models is more challenging. Relying on splitting time schemes established for fluid-structure interaction to model blood vessels, we propose a semi-implicit discretization of a general poromechanics formulation, satisfying a discrete energy balance. In order to illustrate and validate our approach, we reproduce in the finite element software FreeFem++ classical swelling and drainage 2D test cases, and we monitor the discrete energy balance.Finally, motivated by the study of spatial discretization aspects of our problem, we establish in a linear framework a conditional total convergence result. This enables us to propose a computational method easy to implement and presenting good stability results. FreeFem++ enables us again to validate our results, illustrating numerical pathologies associated with incompressibility, and their efficient treatment with the proposed strategies, first in a linear framework and then in a more general situation.
• Front observer for data assimilation of electroanatomical mapping data for a numerical atrial model
  
  • Gérard A
  • Collin Annabelle
  • Bayer J
  • Frontera A
  • Moireau Philippe
  • Coudière Yves
  , 2016. The purpose of our work is to personalize an atrial model of the propagation of the action potential, based on electrical catheter data with the help of the data assimilation approach introduced in [Collin & Al, Journal of Computational Physics 2015]. The originality of the approach is to indroduce a Luenberger observer of a surface atrial model of the propagation which can pursue - like in classical Kalman filtering approach - the actual activation front reconstructed from catheter data. Moreover, this approach may account for the breakthrough of new activation fronts at anytime with an additional topological gradient term. In the present work, we adapt this approach to the bilayer surface atrial model of th propagation of action potentials [Labarthe & Al, Europace 2014], and evaluated for the first time on a real patient's dataset. First, the model was geometrically fit to the patient's data. A fiber architecture was added to the geometry. Then an initial electrophysiological state was guessed, and the model was run with the Luenberger filter for some catheter data acquired during a CARTO procedure. All along the simulation, the filter corrects the action potential so as to track CARTO local activation times, while preserving a biophysical behavior. With this technique, we are able to reconstruct smooth activation maps over the whole atrial surfaces. This promising technique may also allow to reconstruct velocity fields and directions, phase map and possibly give information on repolarization. This work results from a collaborative project carried out during a training session at CEMRACS 2016 in Marseille, Luminy.
• Stability and Localization of Deformation in Finitely Strained Solids and Structures : Static and High Strain Rate Dynamic Aspects
  
  • Wen Guangyang
  , 2016. Localization of deformation in finitely strained ductile solids is the instability mechanism leading to their failure by rupture. This phenomenon occurs under static and dynamic loading conditions. It can appear in the bulk of solids, in which case it is referred to as a material instability phenomenon or in a structure, in which case one talks about a structural instability problem. The thesis at hand studies localization in the material and structural context, both under static and dynamic conditions, using a common tool: the evolution of a geometrically localized perturbation.An introduction to the localization of deformation problem in solid mechanics is presented in Chapter 1. The material instability aspect of localization of deformation in microstructured solids under quasistatic loading and its connection to macroscopic ellipticity – the continuum criterion for the presence of a discontinuous strain field – are addressed in Chapter 2. In this part we show the connection of the homogenized post-bifurcation response to the presence or absence of a localized deformation field in an infinite, fiber-reinforced composite under plane strain compression. The material instability aspect of localization of deformation under dynamic loading, i.e. where inertia becomes important, is addressed in Chapter 3. In this part we study the influence cones for the wave propagation emerging from a point perturbation in an infinite, biaxially strained plate whose constitutive response loses ellipticity at finite level of strain. The structural instability aspect of localization of deformation is investigated under dynamic loading conditions in Chapter 4 by studying the dynamic compression on an electromagnetically loaded metallic ring. In contrast to the quasistatic case, where a global failure mode is observed, the failure pattern for the rapidly compressed ring shows highly localized deformation areas.
• Homogenization of sound hard metamaterials in the time domain
  
  • Maurel A
  • Marigo J.-J
  • Lombard B
  , 2016. We present a homogenization method based on a matched asymptotic expansion technique for sound hard materials structured at subwavelength scale. Considering the wave equation in the time domain, jump conditions are derived for the acoustic pressure and the normal velocity across an equivalent interface with non zero thickness. These jump conditions are implemented in a numerical scheme and compared to the results of the direct problem.
• Plasticité et Rupture
  
  • Marigo Jean-Jacques
  , 2016, pp.265. Ce cours a un double objectif: 1. renforcer les notions de mécanique des milieux continus; 2. présenter les concepts fondamentaux de la plasticité et de la rupture, premiers exemples du comportement irréversible des solides. En conséquence, le cours sera divisé en trois parties: une consacrée à l'élasticité, une deuxième à la plasticité et la troisième à la mécanique de la rupture. Partie Elasticité: On commencera par un rappel sur les concepts fondamentaux de la mécanique des milieux continus tridimensionnels et du comportement élastique, en se limitant pour l'essentiel au cadre des petites perturbations. Partie Plasticité: On introduira ensuite le comportement élasto-plastique en l'illustrant sur les deux modèles fondamentaux basés sur les critères de Von Mises et de Tresca . On s'intéressera ensuite au calcul de structures élasto-plastiques en mettant en évidence les phénomènes d'écrouissage structurel, de charge limite ou de contraintes résiduelles. De nombreux exemples d'application viendront illustrer la démarche. Partie Rupture: Après avoir mis en évidence la réalité et l'importance des phénomènes de fissuration à partir d'incidents survenus et de résultats expérimentaux, on se consacrera à leur modélisation à l'échelle macroscopique. La construction des lois d'évolution des fissures se basera sur des concepts énergétiques associés à des principes physiques fondamentaux. Cela débouchera sur des modèles "mathématiques" destinés aux ingénieurs pour calculer et dimensionner des structures. De nombreux exemples viendront illustrer les différents concepts théoriques et les phénomènes physiques associés. On s'efforcera aussi de montrer que ce domaine de la mécanique des solides est en pleine évolution, la compréhension et la modélisation fine des phénomènes de localisation de la déformation, de fissuration, de fatigue et de ruine des matériaux et des structures nécessitant des travaux de recherche faisant appel aux outils expérimentaux et théoriques les plus sophistiqués. MOTS CLÉS Elasticité: comportement thermo-élastique, loi de Hooke, coefficients élastiques et potentiel élastique, problème aux limites, formulation variationnelle de l'équilibre, théorèmes de l'énergie potentielle et de l'énergie complémentaire, méthodes d'encadrement et méthodes d'approximation. Plasticité: déformations élastiques et déformations plastiques, critère de plasticité, loi d'écoulement, règle de normalité, critères de Von Mises et de Tresca, écrouissage, charge limite, contraintes résiduelles. Rupture fragile: concentration de contraintes, singularités des contraintes, ténacité, taille critique de défauts et taux de restitution de l'énergie, critère de propagation, principes énergétiques, méthodes numériques. Niveau requis : Ce cours peut être suivi sans prérequis, mais il est vivement conseillé d'avoir suivi au préalable un cours d'introduction à la Mécanique des Milieux Continus
• An overview of the modelling of fracture by gradient damage models
  
  • Marigo Jean-Jacques
  • Maurini Corrado
  • Pham Kim
  Meccanica, Springer Verlag, 2016, 51 (12), pp.3107–3128. The paper is devoted to gradient damage models which allow us to describe all the process of degradation of a body including the nucleation of cracks and their propagation. The construction of such model follows the variational approach to fracture and proceeds into two stages: (i) definition of the energy; (ii) formulation of the damage evolution problem. The total energy of the body is defined in terms of the state variables which are the displacement field and the damage field in the case of quasi-brittle materials. That energy contains in particular gradient damage terms in order to avoid too strong damage localizations. The formulation of the damage evolution problem is then based on the concepts of irreversibility, stability and energy balance. That allows us to construct homogeneous as well as localized damage solutions in a closed form and to illustrate the concepts of loss of stability, of scale effects, of damage localization, and of structural failure. Moreover, the variational formulation leads to a natural numerical method based on an alternate minimization algorithm. Several numerical examples illustrate the ability of this approach to account for all the process of fracture including a 3D thermal shock problem where the crack evolution is very complex. (10.1007/s11012-016-0538-4)
  DOI : 10.1007/s11012-016-0538-4
• Carbon nanotubes micromechanical and chemical sensors: Reproducibility, reliability analysis and deployment in real use cases
  
  • Lebental Bérengère
  • Bodelot Laurence
  , 2016, pp.1p. While today's galloping urbanization weighs heavily on both People and Environment and while Climate Change increases natural risks worldwide, Internet of Things Technologies stand at the forefront of the efforts toward Greener Cities. Nanosensors fully integrated into wireless sensor node may become instrumental in this field because of their small size, low cost, versatility and low power consumption. Potential applications are environmental monitoring, structural health monitoring, energy performances monitoring or people exposure monitoring. Challenges range from the manufacturing of the sensors with high reproducibility to their full integration into communicating devices, including ensuring device reliability in complex and harsh environmental conditions. Carbon nanotubes (CNT) especially are choice material toward this goal, as they have demonstrated both a very strong sensitivity to a variety of environmental parameters and a strong resilience to chemical and mechanical stress. In the present paper, we present a full proof of concept of the use of carbon nanotubes micromechanical and chemical sensors practical use cases, from the reproducible fabrication of the carbon nanotubes sensors [1] to their practical deployment in real life [2]. Two specific examples are discussed, infrastructure durability monitoring and water quality monitoring. Specific methodologies for reliability analysis of carbon-based nanomaterials are also discussed.
• Accurate and consistent automatic seismocardiogram annotation without concurrent ECG
  
  • Laurin Alexandre
  • Khorsow Farzad
  • Blaber Andrew
  • Tavakolian Kouhyar
  Physiological Measurement, IOP Publishing, 2016, 37 (9). Seismocardiography (SCG) is the measurement of vibrations in the sternum caused by the beating of the heart. Precise cardiac mechanical timings that are easily obtained from SCG are critically dependent on accurate identification of fiducial points. So far, SCG annotation has relied on concurrent ECG measurements. An algorithm capable of annotating SCG without the use any other concurrent measurement was designed. We subjected 18 participants to graded lower body negative pressure. We collected ECG and SCG, obtained R peaks from the former, and annotated the latter by hand, using these identified peaks. We also annotated the SCG automatically. We compared the isovolumic moment timings obtained by hand to those obtained using our algorithm. Mean ± confidence interval of the percentage of accurately annotated cardiac cycles were ± 97.2 3.7%, ± 93.0 4.6%, ± 76.9 14.9%, ± 61.6 17.2%, and ± 65.0 14.0% for levels of negative pressure 0, −20, −30, −40, and −50 mmHg. LF/HF ratios, the relative power of low-frequency variations to high-frequency variations in heart beat intervals, obtained from isovolumic moments were also compared to those obtained from R peaks. The mean differences ± confidence interval were ± 0.16 0.18, − ± 0.04 0.46, ± 1.02 0.70, ± 1.22 0.61, and ± 2.31 1.09 for increasing levels of negative pressure. The accuracy and consistency of the algorithm enables the use of SCG as a standalone heart monitoring tool in healthy individuals at rest, and could serve as a basis for an eventual application in pathological cases. (10.1088/0967-3334/37/9/1588)
  DOI : 10.1088/0967-3334/37/9/1588
• Influence of pores on crack initiation in monotonic tensile and cyclic loadings in lost foam casting A319 alloy by using 3D in-situ analysis
  
  • Wang Long
  • Limodin Nathalie
  • El Bartali Ahmed
  • Witz Jean-Francois
  • Seghir Rian
  • Buffiere Jean-Yves
  • Charkaluk Eric
  Materials Science and Engineering, Elsevier, 2016, 673, pp.362-372. Lost Foam Casting (LFC) process is replacing the conventional gravity Die Casting (DC) process in automotive industry for the purpose of geometry optimization, cost reduction and consumption control. However, due to lower cooling rate, LFC produces in a coarser microstructure that reduces fatigue life. In order to study the influence of the casting microstructure of LFC Al-Si alloy on damage micromechanisms under monotonic tensile loading and Low Cycle Fatigue (LCF) at room temperature, an experimental protocol based on the three dimensional (3D) in-situ analysis has been set up and validated. This paper focuses on the influence of pores on crack initiation in monotonic and cyclic tensile loadings. X-ray Computed Tomography (CT) allowed the microstructure of material being characterized in 3D and damage evolution being followed in-situ also in 3D. Experimental and numerical mechanical fields were obtained by using Digital Volume Correlation (DVC) technique and Finite Element Method (FEM) simulation respectively. Pores were shown to have an important influence on strain localization as large pores generate enough strain localization zones for crack initiation both in monotonic tensile and cyclic loadings. (10.1016/j.msea.2016.07.036)
  DOI : 10.1016/j.msea.2016.07.036
• Crack Tip Equation of Motion in Dynamic Gradient Damage Models
  
  • Li Tianyi
  • Marigo Jean-Jacques
  Journal of Elasticity, Springer Verlag, 2016. We propose in this contribution to investigate the link between the dynamic gradient damage model and the classical Griffith's theory of dynamic fracture during the crack propagation phase. To achieve this main objective, we first rigorously reformulate two-dimensional linear elastic dynamic fracture problems using variational methods and shape derivative techniques. The classical equation of motion governing a smoothly propagating crack tip follows by considering variations of a space-time action integral. We then give a variationally consistent framework of the dynamic gradient damage model. Owing to the analogies between the variational ingredients of these two models and under some basic assumptions concerning the damage band structuration, one obtains a generalized Griffith criterion which governs the crack tip evolution within the non-local damage model. Assuming further that the internal length is small compared to the dimension of the body, the previous criterion leads to the classical Griffith's law through a separation of scales between the outer linear elastic domain and the inner damage process zone. (10.1007/s10659-016-9595-0)
  DOI : 10.1007/s10659-016-9595-0
• Micro-scale measurements of plasticstrain field, and local contributions of slip and twinning inTWIP steels during in situ tensile tests
  
  • H.K. Yang
  • Doquet Véronique
  • Z.F. Zhang
  Materials Science and Engineering: A, Elsevier, 2016, 672, pp.7-14. In-situ tensile tests were carried out on Fe22Mn0.6C and Fe22Mn0.6C3Al (wt. %) twinning-induced plasticity (TWIP) steels specimens covered with gold micro-grids. High resolution atomic force microscopy (AFM) and scanning electron microscope (SEM) images were periodically captured. The latter were used for measurements of the plastic strain field, using digital image correlation (DIC). Although no meso-scale localization bands appeared, some areas were deformed three times more than average. Plastic deformation inside the grains was more heterogeneous in Fe22Mn0.6C, but at meso-scale, the degree of strain heterogeneity was not higher, at least up to 12% strain. Plastic deformation started from grain boundaries or annealing twin boundaries in both materials, due to a high elastic anisotropy of the grains. An original method based on DIC was developed to estimate the twin fraction in grains that exhibit a single set of slip/twin bands. Deformation twinning accommodated 60 to 80% of the plastic strain in some favourably oriented grains, from the onset of plastic flow in Fe22Mn0.6C, but was not observed in the Al-bearing steel until 12% strain. The back stress was important in both materials, but significantly higher in Fe22Mn0.6C. (10.1016/j.msea.2016.06.064)
  DOI : 10.1016/j.msea.2016.06.064
• Eidolon: Visualization and Computational Framework for Multi-Modal Biomedical Data Analysis
  
  • Kerfoot Eric
  • Fovargue Lauren
  • Rivolo Simone
  • Shi Wenzhe
  • Rueckert Daniel
  • Nordsletten David
  • Lee Jack
  • Chabiniok Radomir
  • Razavi Reza
  , 2016. Biomedical research, combining multi-modal image and geometry data, presents unique challenges for data visualization, processing , and quantitative analysis. Medical imaging provides rich information , from anatomical to deformation, but extracting this to a coherent picture across image modalities with preserved quality is not trivial. Addressing these challenges and integrating visualization with image and quantitative analysis results in Eidolon, a platform which can adapt to rapidly changing research workflows. In this paper we outline Eidolon, a software environment aimed at addressing these challenges, and discuss the novel integration of visualization and analysis components. These capabilities are demonstrated through the example of cardiac strain analysis , showing the Eidolon supports and enhances the workflow.
• Experimental investigation of kinematic and thermal localizations of perforated plate under plastic deformations
  
  • Li L
  • Martel C
  • El Bartali Ahmed
  • Witz Jean-Francois
  • Charkaluk Eric
  , 2016.
• Numerical investigation of dynamic brittle fracture via gradient damage models
  
  • Li Tianyi
  • Marigo Jean-Jacques
  • Guilbaud Daniel
  • Potapov Serguei
  Advanced Modeling and Simulation in Engineering Sciences, Springer, 2016, 3, pp.26. Background: Gradient damage models can be acknowledged as unified framework of dynamic brittle fracture. As a phase-field approach to fracture, they are gaining popularity over the last few years in the computational mechanics community. This paper concentrates on a better understanding of these models. We will highlight their properties during the initiation and propagation phases of defect evolution. Methods: The variational ingredients of the dynamic gradient damage model are recalled. Temporal discretization based on the Newmark-β scheme is performed. Several energy release rates in gradient damage models are introduced to bridge the link from damage to fracture. Results and discussion: An antiplane tearing numerical experiment is considered. It is found that the phase-field crack tip is governed by the asymptotic Griffith's law. In absence of unstable crack propagation, the dynamic gradient damage model converges to the quasi-static one. The defect evolution is in quantitative accordance with the linear elastic fracture mechanics predictions. Conclusion: These numerical experiments provide a justification of the dynamic gradient damage model along with its current implementation, when it is used as a phase-field model for complex real-world dynamic fracture problems. (10.1186/s40323-016-0080-x)
  DOI : 10.1186/s40323-016-0080-x