Publications

2016

• 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
• 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
• 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
• Dynamic stability of biaxially strained thin sheets under high strain-rates: response to local perturbations
  
  • Wen Guangyang
  • Triantafyllidis Nicolas
  International Journal of Fracture Mechanics, Springer-Verlag, 2016, 200, pp.99-113. (10.1007/s10704-016-0123-9)
  DOI : 10.1007/s10704-016-0123-9
• A methodology for the estimation of the effective yield function of isotropic composites
  
  • Papadioti I
  • Danas Kostas
  • Aravas N.
  International Journal of Solids and Structures, Elsevier, 2016, 87, pp.120 - 138. In this work we derive a general model for N−phase isotropic, incompressible, rate-independent elasto-plastic materials at finite strains. The model is based on the nonlinear homogenization variational (or modified secant) method which makes use of a linear comparison composite (LCC) material to estimate the effective flow stress of the nonlinear composite material. The homogenization approach leads to an optimization problem which needs to be solved numerically for the general case of a N−phase composite. In the special case of a two-phase composite an analytical result is obtained for the effective flow stress of the elasto-plastic composite material. Next, the model is validated by periodic three-dimensional unit cell calculations comprising a large number of spherical inclusions (of various sizes and of two different types) distributed randomly in a matrix phase. We find that the use of the lower Hashin–Shtrikman bound for the LCC gives the best predictions by comparison with the unit cell calculations for both the macroscopic stress-strain response as well as for the average strains in each of the phases. The formulation is subsequently extended to include hardening of the different phases. Interestingly, the model is found to be in excellent agreement even in the case where each of the phases follows a rather different hardening response. (10.1016/j.ijsolstr.2016.02.022)
  DOI : 10.1016/j.ijsolstr.2016.02.022
• Experimental multiscale measurements for the mechanical identification of a cortical bone by digital image correlation
  
  • Nguyen Manh-Tu
  • Allain Jean-Marc
  • Gharbi Hakim
  • Desceliers Christophe
  • Soize Christian
  Journal of the mechanical behavior of biomedical materials, Elsevier, 2016, 63, pp.125-133. The implementation of the experimental methodology by optical measurements of mechanical fields, the development of a test bench, the specimen preparation, the experimental measurements, and the digital image correlation (DIC) method, have already been the object of research in the context of biological materials. Nevertheless, in the framework of the experimental identification of a mesoscopic stochastic model of the random apparent elasticity field, measurements of one specimen is required at both the macroscopic scale and the mesoscopic scale under one single loading. The nature of the cortical bone induces some difficulties, as no single speckled pattern technique is available for simultaneously obtaining the displacement at the macroscopic scale and at the mesoscopic scale. In this paper, we present a multiscale experimental methodology based on (i) an experimental protocol for one specimen of a cortical bone, (ii) its measuring bench, (iii) optical field measurements by DIC method, (iv) the experimental results, and (v) the multiscale experimental identification by solving a statistical inverse problem. (10.1016/j.jmbbm.2016.06.011)
  DOI : 10.1016/j.jmbbm.2016.06.011
• Shape Transformations of Epithelial Shells
  
  • Misra M
  • Audoly Basile
  • Kevrekidis I G
  • Shvartsman S y
  Biophysical Journal, Biophysical Society, 2016, 110 (7), pp.1670 - 1678. Regulated deformations of epithelial sheets are frequently foreshadowed by patterning of their mechanical properties. The connection between patterns of cell properties and the emerging tissue deformations is studied in multiple experimental systems, but the general principles remain poorly understood. For instance, it is in general unclear what determines the direction in which the patterned sheet is going to bend and whether the resulting shape transformation will be discontinuous or smooth. Here these questions are explored computationally, using vertex models of epithelial shells assembled from prism-like cells. In response to rings and patches of apical cell contractility, model epithelia smoothly deform into invaginated or evaginated shapes similar to those observed in embryos and tissue organoids. Most of the observed effects can be captured by a simpler model with polygonal cells, modified to include the effects of the apicobasal polarity and natural curvature of epithelia. Our models can be readily extended to include the effects of multiple constraints and used to describe a wide range of morphogenetic processes. (10.1016/j.bpj.2016.03.009)
  DOI : 10.1016/j.bpj.2016.03.009