Publications

2016

• A Dedicated DIC Methodology for Characterizing Plastic Deformation in Single Crystals
  
  • Wang Xiaogang
  • Witz Jean-François
  • El Bartali Ahmed
  • Oudriss Abdelali
  • Seghir Rian
  • Dufrenoy Philippe
  • Feaugas Xavier
  • Charkaluk Eric
  Experimental Mechanics, Society for Experimental Mechanics, 2016, 56 (7), pp.1155-1167. This paper focuses on the development of an appropriate Digital Image Correlation (DIC) methodology based on Image Registration and dedicated for characterizing the plastic deformation in single crystals. A pure nickel single crystal specimen is plastically deformed in tension and investigated by DIC technique. Based on the measured kinematic fields, the proposed method enables to identify the slip activity on the crystal surface and to locate precisely the slip band interfaces at microscale which behave as kinematic discontinuities. The computed displacement data are projected on a well-defined physical basis containing slip details, then the strain fields can be derived directly from a set of analytical functions. The possible errors in displacement induced by this projection approach are evaluated. Finally, some results of the evaluated strain fields are presented. It demonstrates that the developed DIC methodology allows quantitative characterization of a heterogeneous deformation process and promotes further relationships to be established between slip activity and strain field evolution in single crystals. (10.1007/s11340-016-0159-9)
  DOI : 10.1007/s11340-016-0159-9
• Quasi-static response, implicit scheme and incremental problem in gradient plasticity
  
  • Nguyen Quoc-Son
  Comptes Rendus. Mécanique, Académie des sciences (Paris), 2016, 344 (6), pp.439-447. This paper is devoted to the study of gradient plasticity at small strains. Some time-independent dissipative processes such as brittle damage can also be considered in the same framework. Our attention is focussed on the description of the constitutive equations, on the formulation of the governing equations in terms of the energy potential and the dissipation potential of the solid. A time-discretization by the implicit scheme of the evolution equation leads to the study of the incremental problem which is different from the rate problem. The increment of the response under an increment of the loads must satisfy a variational inequality and, if the energy potential is convex, an incremental minimum principle. In particular, a local minimum of the incremental minimum principle is a stable solution to the variational inequality. (10.1016/j.crme.2016.01.004)
  DOI : 10.1016/j.crme.2016.01.004
• Estimation from Moments Measurements for Amyloid Depolymerisation
  
  • Armiento Aurora
  • Doumic Marie
  • Moireau Philippe
  • Rezaei Human
  Journal of Theoretical Biology, Elsevier, 2016, 397, pp.68 - 88. Estimating reaction rates and size distributions of protein polymers is an important step for understanding the mechanisms of protein misfolding and aggregation, a key feature for amyloid diseases. This study aims at {setting} this framework problem when the experimental measurements consist in the time-dynamics of a moment of the population (\emph{i.e.} for instance the total polymerised mass, as in Thioflavine T measurements, or the second moment measured by Static Light Scattering). We propose a general methodology, and we solve the problem theoretically and numerically in the case of a depolymerising system. We then apply our method to experimental data of degrading oligomers, and conclude that smaller aggregates of ovPrP protein should be more stable than larger ones. This has an important biological implication, since it is commonly admitted that small oligomers constitute the most cytotoxic species during prion misfolding process. (10.1016/j.jtbi.2016.02.037)
  DOI : 10.1016/j.jtbi.2016.02.037
• Dispositif cardiaque
  
  • Chapelle Dominique
  • Imperiale Sebastien
  • Moireau Philippe
  • Laurin Alexandre
  , 2016.
• Three-dimensional full-field X-ray orientation microscopy
  
  • Viganò Nicola
  • Tanguy Alexandre
  • Hallais Simon
  • Dimanov Alexandre
  • Bornert Michel
  • Batenburg Kees Joost
  • Ludwig Wolfgang
  Scientific Reports, Nature Publishing Group, 2016, 6. A previously introduced mathematical framework for full-field X-ray orientation microscopy is for the first time applied to experimental near-field diffraction data acquired from a polycrystalline sample. Grain by grain tomographic reconstructions using convex optimization and prior knowledge are carried out in a six-dimensional representation of position-orientation space, used for modelling the inverse problem of X-ray orientation imaging. From the 6D reconstruction output we derive 3D orientation maps, which are then assembled into a common sample volume. The obtained 3D orientation map is compared to an EBSD surface map and local misorientations, as well as remaining discrepancies in grain boundary positions are quantified. The new approach replaces the single orientation reconstruction scheme behind X-ray diffraction contrast tomography and extends the applicability of this diffraction imaging technique to material micro-structures exhibiting sub-grains and/or intra-granular orientation spreads of up to a few degrees. As demonstrated on textured sub-regions of the sample, the new framework can be extended to operate on experimental raw data, thereby bypassing the concept of orientation indexation based on diffraction spot peak positions. This new method enables fast, three-dimensional characterization with isotropic spatial resolution, suitable for time-lapse observations of grain microstructures evolving as a function of applied strain or temperature. (10.1038/srep20618)
  DOI : 10.1038/srep20618
• Dirichlet-to-Neumann operator for diffraction problems in stratified anisotropic acoustic waveguides
  
  • Tonnoir Antoine
  Comptes Rendus. Mathématique, Académie des sciences (Paris), 2016. The purpose of this note is to construct a Dirichlet-to-Neumann operator for the diffraction problem in stratified anisotropic acoustic waveguides. The key idea consists in using an adapted change of coordinates that enables to recover the completeness and the orthogonality of the modes on " deformed " cross-sections of the waveguide. To cite this article: (10.1016/j.crma.2015.12.018)
  DOI : 10.1016/j.crma.2015.12.018
• Surface versus internal fatigue crack initiation in steel: Influence of mean stress
  
  • Gaur Vidit
  • Doquet Véronique
  • Persent Emmanuel
  • Mareau Charles
  • Roguet Eléonore
  • Kittel Jean
  International Journal of Fatigue, Elsevier, 2016, 82 (part 3), pp.437-448. Stress-controlled fatigue tests were run at different R ratios (¼ r min =r max) up to at most 3 million cycles on a 2.5%Cr–1%Mo steel (ASTM A182 F22) used in riser tubes connectors for offshore oil drilling. The fatigue lives, as well as the slope of the S–N curves were found to decrease with increasing R and the endurance limit to follow Gerber's parabola. Surface crack initiation without any defect involved, was most often observed for R = À1, À0.5 and 0, while an R ratio of 0.25 triggered crack initiation from either surface or internal pores or chemically inhomogeneous areas, leading, in the latter case, to fish-eye patterns for relatively low numbers of cycles. A further increase in R ratio to 0.5 promoted only defect-initiated surface cracks, while no fatigue fracture was observed within 10 million cycles above R ¼ 0:6. These transitions in crack initiation mode are discussed based on X-ray diffraction analyses of residual stresses, elastic–plastic F.E. computations on a unit cell model containing a pore and some fracture mechanics analyses, with a particular attention to environmental effects. (10.1016/j.ijfatigue.2015.08.028)
  DOI : 10.1016/j.ijfatigue.2015.08.028
• A homogenization based yield criterion for a porous Tresca material with ellipsoidal voids
  
  • Mbiakop Armel
  • Kostas Danas
  • Constantinescu Andrei
  International Journal of Fracture, Springer Verlag, 2016. This paper presents a rate-independent ana- lytical model for porous Tresca ( J3 -dependent) mate- rials containing general ellipsoidal voids. The model is based on the nonlinear variational homogenization method which uses a linear comparison material to esti- mate the response of the nonlinear porous solid and is denoted as “MVAR”. Specifically, the model is derived by an original approach starting from a novel porous single crystal model (Mbiakop et al. in Int J Solids Struct 64–65:100–119, 2015b, J Mech Phys Solids 84:436–467, 2015c) by considering the limiting case of infinite slip systems which leads readily to the cor- responding Tresca criterion. The MVAR yield surface is then validated using FEM on different unit-cells and various parameters including several porosity levels, several stress triaxiality ratios, different Lode angle and general void shapes and orientations. The MVAR model is found to be in good agreement with the finite element results for all cases considered in this study. Both the MVAR and the FEM computations reveal a strong sensitivity upon the microstructure anisotropy (void shape and orientation), and a dependence of the effective behavior on the third invariant of the applied stress. To the best knowledge of the authors, this is the first model in the literature that is able to deal with porous Tresca material and general void shapes and orientations. Moreover, the MVAR is used in a pre- dictive manner to investigate the complex response of porous Tresca cases with strong coupling between the J3-dependent matrix behavior and the (morphologi- cal) anisotropy induced by the shape and orientation of the voids. The simplicity of the present analytical study opens the possibility to adapt the present model to experimental results for various materials. (10.1007/s10704-015-0071-9)
  DOI : 10.1007/s10704-015-0071-9
• Two-scale homogenization to determine effective parameters of thin metallic-structured films
  
  • Marigo Jean-Jacques
  • Maurel Agnès
  Proceedings of the Royal Society A: Mathematical, Physical and Engineering Sciences, Royal Society, The, 2016, 472 (2192). We present a homogenization method based on matched asymptotic expansion technique to derive effective transmission conditions of thin structured films. The method leads unambiguously to effective parameters of the interface which define jump conditions or boundary conditions at an equivalent zero thickness interface. The homogenized interface model is presented in the context of electromagnetic waves for metallic inclusions associated with Neumann or Dirichlet boundary conditions for transverse electric or transverse magnetic wave polarization. By comparison with full-wave simulations, the model is shown to be valid for thin interfaces up to thicknesses close to the wavelength. We also compare our effective conditions with the two-sided impedance conditions obtained in transmission line theory and to the so-called generalized sheet transition conditions (10.1098/rspa.2016.0068)
  DOI : 10.1098/rspa.2016.0068
• Localization of deformation and loss of macroscopic ellipticity in microstructured solids
  
  • Santisi d'Avila Maria Paola
  • Triantafyllidis Nicolas
  • Wen Guangyang
  Journal of the Mechanics and Physics of Solids, Elsevier, 2016, 97, pp.275-298. (10.1016/j.jmps.2016.07.009)
  DOI : 10.1016/j.jmps.2016.07.009
• Stress gradient effects on the nucleation and propagation of cohesive cracks
  
  • Pham Tuan Hiep
  • Laverne Jérôme
  • Marigo Jean-Jacques
  Discrete and Continuous Dynamical Systems - Series S, American Institute of Mathematical Sciences, 2016, 9 (2), pp.557 - 584. The aim of the present work is to study the nucleation and propagation of cohesive cracks in two-dimensional elastic structures. The crack evolution is governed by Dugdale’s cohesive force model. Specifically, we investigate the stabilizing effect of the stress field non-uniformity by introducing a length l which characterizes the stress gradient in a neighborhood of the point where the crack nucleates. We distinguish two stages in the crack evolution: the first one where the entire crack is submitted to cohesive forces, followed by a second one where a non-cohesive part appears. Assuming that the material characteristic length dc associated with Dugdale’s model is small in comparison with the dimension L of the body, we develop a two-scale approach and, using the methods of complex analysis, obtain the entire crack evolution with the loading in closed form. In particular, we show that the propagation is stable during the first stage, but becomes unstable with a brutal crack length jump as soon as the non-cohesive crack part appears. We also discuss the influence of the problem parameters and study the sensitivity to imperfections. (10.3934/dcdss.2016012)
  DOI : 10.3934/dcdss.2016012
• Multiphysics and multiscale modelling, data-model fusion and integration of organ physiology in the clinic: ventricular cardiac mechanics
  
  • Chabiniok Radomir
  • Wang Vicky Y.
  • Hadjicharalambous Myrianthi
  • Asner Liya
  • Lee Jack
  • Sermesant Maxime
  • Kuhl Ellen
  • Young Alistair A.
  • Moireau Philippe
  • Nash Martyn P.
  • Chapelle Dominique
  • Nordsletten David
  Interface Focus, Royal Society publishing, 2016, 6 (2). With heart and cardiovascular diseases continually challenging healthcare systems worldwide, translating basic research on cardiac (patho)physiology into clinical care is essential. Exacerbating this already extensive challenge is the complexity of the heart, relying on its hierarchical structure and function to maintain cardiovascular flow. Computational modelling has been proposed and actively pursued as a tool for accelerating research and translation. Allowing exploration of the relationships between physics, multiscale mechanisms and function, computational modelling provides a platform for improving our understanding of the heart. Further integration of experimental and clinical data through data assimilation and parameter estimation techniques is bringing computational models closer to use in routine clinical practice. This article reviews developments in computational cardiac modelling and how their integration with medical imaging data is providing new pathways for translational cardiac modelling. (10.1098/rsfs.2015.0083)
  DOI : 10.1098/rsfs.2015.0083
• Asymptotic analysis of a linear isotropic elastic composite reinforced by a thin layer of periodically distributed isotropic parallel stiff fibres.
  
  • Bellieud Michel
  • Geymonat Giuseppe
  • Krasucki Françoise
  Journal of Elasticity, Springer Verlag, 2016, 122, pp.43-74. We present some mathematical convergence results using a two-scale method for a linear elastic isotropic medium containing one layer of parallel periodically distributed heterogeneities located in the interior of the whole domain around a plane surface \Sigma. The aim of this paper is to study the situation when the rigidity of the linearly isotropic elastic fibres is 1/ \epsilon ^ m the rigidity of the surrounding linearly isotropic elastic material. We use a two-scale convergence method adapted to the geometry of the problem (layer of fibres). In the models obtained \Sigma behaves for m = 1 as a "material surface" without membrane energy in the direction of the plane orthogonal to the direction of the fibres. For m = 3 the "material surface" has no bending energy in the direction orthogonal to the fibres. (10.1007/s10659-015-9532-7)
  DOI : 10.1007/s10659-015-9532-7
• Bowed strings instruments
  
  • Boutillon Xavier
  , 2016. (10.1007/978-1-4939-3679-3)
  DOI : 10.1007/978-1-4939-3679-3
• Elastic rods with incompatible strain: Macroscopic versus microscopic buckling
  
  • Lestringant Claire
  • Audoly Basile
  Journal of the Mechanics and Physics of Solids, Elsevier, 2016, 103, pp.40-71. We consider the buckling of a long prismatic elastic solid under the combined effect of a pre-stress that is inhomogeneous in the cross-section, and of a prescribed displacement of its endpoints. A linear bifurcation analysis is carried out using different structural models (namely a double beam, a rectangular thin plate, and a hyper-elastic prismatic solid in 3-d): it yields the buckling mode and the wavenumber q c that are first encountered when the end-to-end displacement is progressively decreased with fixed pre-stress. For all three structural models, we find a transition from a long-wavelength (q c = 0) to a short-wavelength first buckling mode (q c = 0) when the inhomogeneous pre-stress is increased past a critical value. A method for calculating the critical inhomogeneous pre-stress is proposed based on a small-wavenumber expansion of the buckling mode. Overall, our findings explain the formation of multiple perversions in elastomer strips, as well as the large variations in the number of perversions as a function of pre-stress and cross-sectional geometry, as reported by Liu et al. (10.1016/j.jmps.2016.12.001)
  DOI : 10.1016/j.jmps.2016.12.001
• Freedericksz instability for the twisted nematic device: A three-dimensional analysis
  
  • Sfyris George
  • Danas Kostas
  • Wen Guangyang
  • Triantafyllidis Nicolas
  Physical Review E, American Physical Society (APS), 2016, 94, pp.012704-1 – 012704-12. (10.1103/PhysRevE.94.012704)
  DOI : 10.1103/PhysRevE.94.012704
• Age-related changes in intraventricular kinetic energy: a physiological or pathological adaptation?
  
  • Wong James
  • Chabiniok Radomir
  • Devecchi Adelaide
  • Dedieu Nathalie
  • Sammut Eva
  • Schaeffter Tobias
  • Razavi Reza
  AJP - Heart and Circulatory Physiology, American Physiological Society, 2016, 310, pp.747 - 755. Aging has important deleterious effects on the cardiovascular system. We sought to compare intraventricular kinetic energy (KE) in healthy subjects of varying ages with subjects with ventricular dysfunction to understand if changes in energetic momentum may predispose individuals to heart failure. Four-dimensional flow MRI was acquired in 35 healthy subjects (age: 1– 67 yr) and 10 patients with left ventricular (LV) dysfunction (age: 28 –79 yr). Healthy subjects were divided into age quartiles (1st quartile: 16 yr, 2nd quartile: 17–32 yr, 3rd quartile: 33– 48 yr, and 4th quartile: 49 – 64 yr). KE was measured in the LV throughout the cardiac cycle and indexed to ventricular volume. In healthy subjects, two large peaks corresponding to systole and early diastole occurred during the cardiac cycle. A third smaller peak was seen during late diastole in eight adults. Systolic KE (P 0.182) and ejection fraction (P 0.921) were preserved through all age groups. Older adults showed a lower early peak diastolic KE compared with children (P 0.0001) and young adults (P 0.025). Subjects with LV dysfunction had reduced ejection fraction (P 0.001) and compared with older healthy adults exhibited a similar early peak diastolic KE (P 0.142) but with the addition of an elevated KE in diastasis (P 0.029). In healthy individuals, peak diastolic KE progressively decreases with age, whereas systolic peaks remain constant. Peak diastolic KE in the oldest subjects is comparable to those with LV dysfunction. Unique age-related changes in ventric-ular diastolic energetics might be physiological or herald subclin-ical pathology. (10.1152/ajpheart.00075.2015)
  DOI : 10.1152/ajpheart.00075.2015
• Thermomechanical Coupling & Dynamic Crack Propagation
  
  • Soumahoro Z
  • Maigre H.
  Universal Journal of Fracture Mechanics, 2016, 4, pp.70 - 85. Thermomechanical coupling is studied in dynamic fracture mechanisms. The crack propagation problem is formalized within the framework of the thermodynamics of irreversible processes to determine their essential parameters and couplings. The analytical approach developed herein is based on the linear elastic mechanics of fracture, including the inertial effects and thermomechanical couplings. For a one-dimensional restriction, the mechanics and thermics equations are uncoupled in the entire structure, with simplified thermomechanical coupling at the crack-tip. This study aims to determine the criteria for fracture initiation and propagation in the presence of heating and inertial effects.
• Gradient damage modeling of brittle fracture in an explicit dynamics context
  
  • Li Tianyi
  • Marigo Jean-Jacques
  • Guilbaud Daniel
  • Potapov Serguei
  International Journal for Numerical Methods in Engineering, Wiley, 2016. In this contribution we propose a dynamic gradient damage model as a phase-field approach for studying brutal fracture phenomena in quasi-brittle materials under impact-type loading conditions. Several existing approaches to account for the tension-compression asymmetry of fracture behavior of materials are reviewed. A better understanding of these models is provided through a uniaxial traction experiment. We then give an efficient numerical implementation of the model in an explicit dynamics context. Simulations results obtained with parallel computing are discussed both from a computational and physical point of view. Different damage constitutive laws and tension-compression asymmetry formulations are compared with respect to their aptitude to approximate brittle fracture. (10.1002/nme.5262)
  DOI : 10.1002/nme.5262
• Gradient damage models and their use in brittle fracture
  
  • Marigo Jean-Jacques
  • Maurini Corrado
  • Pham Kim
  , 2016. 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.
• Partial LVAD restores ventricular outputs and normalizes LV but not RV stress distributions in the acutely failing heart in silico
  
  • Sack Kevin L.
  • Baillargeon Brian
  • Acevedo-Bolton Gabriel
  • Genet Martin
  • Rebelo Nuno
  • Kuhl Ellen
  • Klein Liviu
  • Weiselthaler Georg M.
  • Burkhoff Daniel
  • Franz Thomas
  • Guccione Julius M.
  The International Journal of Artificial Organs, SAGE Publications, 2016, 39 (8), pp.421 - 430. Purpose: Heart failure is a worldwide epidemic that is unlikely to change as the population ages and life expectancy increases. We sought to detail significant recent improvements to the Dassault Systèmes Living Heart Model (LHM) and use the LHM to compute left ventricular (LV) and right ventricular (RV) myofiber stress distributions under the following 4 conditions: (1) normal cardiac function; (2) acute left heart failure (ALHF); (3) ALHF treated using an LV assist device (LVAD) flow rate of 2 L/min; and (4) ALHF treated using an LVAD flow rate of 4.5 L/min. Methods and Results: Incorporating improved systolic myocardial material properties in the LHM resulted in its ability to simulate the Frank-Starling law of the heart. We decreased myocardial contractility in the LV myocardium so that LV ejection fraction decreased from 56% to 28%. This caused mean LV end diastolic (ED) stress to increase to 508% of normal, mean LV end systolic (ES) stress to increase to 113% of normal, mean RV ED stress to decrease to 94% of normal and RV ES to increase to 570% of normal. When ALHF in the model was treated with an LVAD flow rate of 4.5 L/min, most stress results normalized. Mean LV ED stress became 85% of normal, mean LV ES stress became 109% of normal and mean RV ED stress became 95% of normal. However, mean RV ES stress improved less dramatically (to 342% of normal values). Conclusions: These simulations strongly suggest that an LVAD is effective in normalizing LV stresses but not RV stresses that become elevated as a result of ALHF. (10.5301/ijao.5000520)
  DOI : 10.5301/ijao.5000520
• Mechanical Behavior of a Bacillus subtilis Pellicle
  
  • Hollenbeck Emily C
  • Douarche Carine
  • Allain Jean-Marc
  • Roger Philippe C
  • Regeard Christophe G
  • Cegelski Lynette C
  • Fuller Gerald G
  • Raspaud Eric C
  Journal of Physical Chemistry B, American Chemical Society, 2016, 120 (26), pp.6080–6088. Bacterial biofilms consist of a complex network of biopolymers embedded with microorganisms, and together these components form a physically robust structure that enables bacteria to grow in a protected environment. This structure can help unwanted biofilms persist in situations ranging from chronic infection to the biofouling of industrial equipment, but under certain circumstances it can allow the biofilm to disperse and colonize new niches. Mechanical properties are therefore a key aspect of biofilm life. In light of the recently discovered growth-induced compressive stress present within a biofilm, we studied the mechanical behavior of Bacillus subtilis pellicles, or biofilms at the air−liquid interface, and tracked simultaneously the force response and macroscopic structural changes during elongational deformations. We observed that pellicles behaved viscoelastically in response to small deformations, such that the growth-induced compressive stress was still present, and viscoplastically at large deformations, when the pellicles were under tension. In addition, by using particle imaging velocimetry we found that the pellicle deformations were nonaffine, indicating heterogeneous mechanical properties with the pellicle being more pliable near attachment surfaces. Overall, our results indicate that we must consider not only the viscoelastic but also the viscoplastic and mechanically heterogeneous nature of these structures to understand biofilm dispersal and removal. (10.1021/acs.jpcb.6b02074)
  DOI : 10.1021/acs.jpcb.6b02074
• The surprising dynamics of a chain on a pulley: lift off and snapping
  
  • Brun Pierre-Thomas
  • Audoly Basile
  • Goriely Alain
  • Vella Dominic
  Proceedings of the Royal Society A: Mathematical, Physical and Engineering Sciences, Royal Society, The, 2016, 472 (2190). The motion of weights attached to a chain or string moving on a frictionless pulley is a classic problem of introductory physics used to understand the relationship between force and acceleration. Here, we consider the dynamics of the chain when one of the weights is removed and, thus, one end is pulled with constant acceleration. This simple change has dramatic consequences for the ensuing motion: at a finite time, the chain 'lifts off' from the pulley and the free end subsequently accelerates faster than the end that is pulled. Eventually, the chain undergoes a dramatic reversal of curvature reminiscent of the crack, or snap, of a whip. We combine experiments, numerical simulations, and theoretical arguments to explain key aspects of this dynamical problem. (10.1098/rspa.2016.0187)
  DOI : 10.1098/rspa.2016.0187
• Homogenization of ultrathin metallo-dielectric structures leading to transmission conditions at an equivalent interface
  
  • Maurel Agnes
  • Marigo Jean-Jacques
  • Ourir Abdelwaheb
  Journal of the Optical Society of America B, Optical Society of America, 2016, 33 (5). We present a method of homogenization of thin metallo-dielectric structures as used in the design of artificial surfaces , or metasurfaces. The approach is based on a so-called matched asymptotic expansion technique, leading to parameters being characteristic of an equivalent interface associated to jump conditions. It is applied to an array of metal strips on top of a metal-backed dielectric slab, with the strips having a small but possibly finite thickness. Solving the equivalent problem provides explicit expressions (i) of the reflection coefficient for a wave at oblique incidence and (ii) of the dispersion relation of the surface waves. The results are shown to be in agreement with results coming from the transmission line theory in the limit of vanishing thickness of the metallization and for normal incidence of the wave. The influence of the finite thickness of the metallization is exemplified and validated by comparison with full wave simulations. (10.1364/JOSAB.33.000947)
  DOI : 10.1364/JOSAB.33.000947
• Micromechanical modeling for the probabilistic failure prediction of stents in high-cycle fatigue
  
  • Guerchais Raphael
  • Scalet Giulia
  • Constantinescu Andrei
  • Auricchio Ferdinando
  International Journal of Fatigue, Elsevier, 2016, 87, pp.405-417. The present paper introduces a methodology for the high-cycle fatigue design of balloon-expandable stents. The proposed approach is based on a micromechanical model coupled with a probabilistic methodology for the failure prediction of stents. This allows to account for material heterogeneity and scatter, to introduce a fatigue criterion able to consider stress gradients, and to perform a probabilistic analysis to obtain general predictions from a limited number of realizations of microstructures investigated. Numerical simulations have allowed to highlight the noteworthy characteristics of the mechanical response in the stent as well as the heterogeneity of the mechanical fields due to stress concentrations in the unit cell geometry and to strain incompatibilities between the grains induced by the anisotropy of their mechanical behavior. The predicted survival probability of the stent is in accordance with the experimental data from the literature. Moreover, the influence of the amplitude of the arterial pressure on the fatigue strength of the stent has been evaluated. (10.1016/j.ijfatigue.2016.02.026)
  DOI : 10.1016/j.ijfatigue.2016.02.026