Publications

2016

• 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
• 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
• 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
• Non-linear simulation of coiling accounting for roughness of contacts and multiplicative elastic-plastic behavior
  
  • Weisz-Patrault Daniel
  • Ehrlacher Alain
  • Legrand Nicolas
  International Journal of Solids and Structures, Elsevier, 2016, 94-95, pp.1-20. In this paper numerical simulations of coiling (winding of a steel strip on itself) and uncoiling are developed. Initial residual stress field is taken into account as well as roughness of contacts and elastic-plastic behavior at finite strains, considering the Tresca yield function and isotropic hardening. The main output is the residual stress field due to plastic deformations during the process. This enables to quantify additional flatness defects. The presented coiling simulation relies on a modeling strategy that consists in dividing each time step into two sub-steps. Each sub-step can be solved semi-analytically and numerical optimizations enable to obtain a general solution. Thus reasonable computation times are reached and parametric studies can be performed in order to develop coiling strategies considering the process parameters. Comparisons with previous models from the literature are presented. Moreover the comparison with a Finite Element simulation presents the same order of magnitude, however it shows that direct computations using classical FE codes are difficult to perform in terms of computation times and stability if an explicit integration scheme is chosen. Numerical results are also given in order to determine the effect of some parameters such as roughness, yield stress, applied force, strip crown or mandrel's radius. (10.1016/j.ijsolstr.2016.05.012)
  DOI : 10.1016/j.ijsolstr.2016.05.012
• In situ 3D characterization of high temperature fatigue damage mechanisms in a cast aluminum alloy using synchrotron X-ray tomography
  
  • Dezecot Sébastien
  • Buffiere Jean-Yves
  • Köster Alain
  • Maurel Vincent
  • Szmytka Fabien
  • Charkaluk Eric
  • Dahdah Nora
  • El Bartali Ahmed
  • Limodin Nathalie
  • Witz Jean-Francois
  Scripta Materialia, Elsevier, 2016, 113, pp.254-258. Fatigue tests were performed at 250 °C on a cast AlSi7Cu3Mg aluminum alloy and monitored with Synchrotron in situ X-ray tomography in order to understand the micro-mechanisms of crack initiation and propagation. The analysis of the 3D images reveals that internal shrinkage pores are responsible for the main crack initiation. Crack propagation is mainly due to the complex and highly interconnected network of hard particles of the eutectic regions. (10.1016/j.scriptamat.2015.11.017)
  DOI : 10.1016/j.scriptamat.2015.11.017
• Post-bifurcation and stability of a finitely strained hexagonal honeycomb subjected to equi-biaxial in-plane loading
  
  • Combescure Christelle
  • Henry Pierre
  • Elliott Ryan
  International Journal of Solids and Structures, Elsevier, 2016, 8889 (24), pp.296 - 318. The buckling and crushing mechanics of cellular honeycomb materials is an important engineering problem. Motivated by the pioneering experimental and numerical studies of Papka and Kyriakides (1994, 1999a,b), we review the literature on finitely strained honeycombs subjected to in-plane loading and identify two open questions: (i) How does the mechanical response of the honeycomb depend on the applied loading device? and (ii) What can the Bloch wave representation of all bounded perturbations contribute to our understanding of the stability of post-bifurcated equilibrium configurations? To address these issues we model the honeycomb as a two-dimensional infinite perfect periodic medium. We use analytical group theory methods (as opposed to the more common, but less robust, imperfection method) to study the honeycomb's bifurcation behavior under three different far-field loadings that produce (initially) the same equi-biaxial contractive dilatation. Using an FEM discretization of the honeycomb walls (struts), we solve the equilibrium equations to find the principal and bifurcated equilibrium paths for each of the three loading cases. We evaluate the structure's stability using two criteria: rank-one convex-ity of the homogenized continuum (long wavelength perturbations) and Bloch wave stability (bounded perturbations of arbitrary wavelength). We find that the post-bifurcation behavior is extremely sensitive to the applied loading device, in spite of a common principal solution. We confirm that the flower mode is always unstable, as previously reported. However, our (first ever) Bloch wave stability analysis of the post-bifurcated equilibrium paths shows that the flower mode is stable for all sufficiently short wavelength perturbations. This new result provides a realistic explanation for why this mode has been observed in the finite size specimen experiments of Papka and Kyriakides (1999a). (10.1016/j.ijsolstr.2016.02.016)
  DOI : 10.1016/j.ijsolstr.2016.02.016