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International audience; Nanocelluloses are seen as the basis of high-performance materials from renewable sources, enabling a bio-based sustainable future. Unsurprisingly, research has initially been focused on the design of new material concepts and less on new and adapted fabrication processes that would allow large-scale industrial production and widespread societal impact. In fact, even the processing routes for making nanocelluloses and the understanding on how the mechanical action fibrillates plant raw materials, albeit chemically or enzymatically pre-treated, are only rudimentary and have not evolved significantly during the past three decades. To address the challenge of designing cellulose comminution processes for a reliable and predictable production of nanocelluloses, we engineered a study setup, referred to as Hyper Inertia Microfluidizer, to observe and quantify phenomena at high speeds and acceleration into microchannels, which is the underlying flow in homogenization. We study two different channel geometries, one with acceleration into a straight channel and one with acceleration into a 90°bend, which resembles the commercial equipment for microfluidization. With the purpose of intensification of the nanocellulose production process, we focused on an efficient first pass fragmentation. Fibers are strained by the extensional flow upon acceleration into the microchannels, leading to buckling deformation and, at a higher velocity, fragmentation. The treatment induces sites of structural damage along and at the end of the fiber, which become a source for nanocellulose. Irrespectively on the treatment channel, these nanocelluloses are fibril-agglomerates, which are further reduced to smaller sizes. In a theoretical analysis, we identify fibril delamination as failure mode from bending by turbulent fluctuations in the flow as a comminution mechanism at the nanocellulose scale. Thus, we argue that intensification of the fibrillation can be achieved by an initial efficient fragmentation of the cellulose in smaller fragments, leading to a larger number of damaged sites for the nanocellulose production. Refinement of these nanocelluloses to fibrils is then achieved by an increase in critical bending events, i.e., decreasing the turbulent length scale and increasing the residence time of fibrils in the turbulent flow.

Lead halide perovskite nanocrystals (NCs) have been widely investigated owing to their potential applications as optoelectronic devices. However, these materials suffer from poor water stability, which make them impossible to be applied in biomedicine. Here, insoluble CsPbBr3/CsPb2Br5 composite NCs were successfully synthesized via simple water-assisted chemical transformation of perovskite NCs. Water plays two key roles in this synthesis: (i) stripping CsBr from CsPbBr3/Cs4PbBr6 and (ii) modifying the coordination number of Pb2+ (six in CsPbBr3 and Cs4PbBr6 vs eight in CsPb2Br5). The as-prepared CsPbBr3/CsPb2Br5 composite NCs not only retain the photoluminescence quantum yield (up to 80%) and a narrow full width to half-maximum of 16 nm, but also present excellent water stability and low cytotoxicity. With these properties, the CsPbBr3/CsPb2Br5 composite NCs were demonstrated as efficient fluorescent probes in live HeLa cells. We believe that our finding not only provides a new method to prepare insoluble, narrow-band, and brightly luminescent CsPbBr3/CsPb2Br5 composite NCs, but also extend the potential applications of lead halides in biomedicine.

International audience; The aim of this work is to estimate the parameters of elastoplastic and damage laws for nodular graphite cast iron from a cyclic uniaxial test on a dog-bone sample. The paper focuses on the identification of material parameters coupling finite element models and full-field measurements. The gap between the measured and simulated data is used to estimate the quality of the proposed constitutive postulates. Last, a cyclic uniaxial experiment is carried out in a lab tomograph to reveal the damage micro-mechanism. Digital volume correlation is used to measure displacement fields in the bulk of the sample. The correlation residuals are used to detect the damage mechanism occurring in the heterogeneous microstructure of the material.

International audience; Background: The goal of the present study is to illustrate the full integration of sensor and imaging data into numerical procedures for the purpose ofidentification of constitutive laws and their validation. The feasibility of such approaches is proven in the context of in situ tests monitored by tomography.The bridging tool consists of spatiotemporal (i.e., 4D) analyses with dedicated (integrated) correlation algorithms.Method: A tensile test on nodular graphite cast iron sample is performed within a lab tomograph. The reconstructed volumes are registered via integrated digital volume correlation (DVC) that incorporates a finite element modeling of the test, thereby performing a mechanical integration in 4D registration of a series of 3D images. In the present case a non-intrusive procedure is developed in which the 4D sensitivity fields are obtained with a commercial finite element code, allowing for a large versatility in meshing and incorporation of complex constitutive laws. Convergence studies can thus be performed in which the quality of the discretization is controlled both for the simulation and the registration.Results: Incremental DVC analyses are carried out with the scans acquired during the in situ mechanical test. For DVC, the mesh size results from acompromise between measurement uncertainties and its spatial resolution. Conversely, a numerically good mesh may reveal too fine for the considered material microstructure. With the integrated framework proposed herein, 4D registrations can be performed and missing boundary conditions of the reference state as well as mechanical parameters of an elastoplastic constitutive law are determined in fair condition both for DVC and simulation.

International audience; In this paper the nonlinear behavior of nodular graphite cast iron is investigated under in-plane biaxial loading regimes. Multiaxial experiments enable a wide variety of loading paths to be investigated. In the presented study two loading histories are proposed, namely, equibiaxial and " snail " loading paths corresponding to proportional and nonproportional regimes. The center gauge zone of the testing cruciform specimen is observed. Mean strain fluctuations are measured and presented in order to determine the material response for the prescribed biaxial loading paths. For the equibiaxial and snail loading histories finite element model updating will couple 2D-DIC measurements with 3D-FE analyses to calibrate material parameters describing elasticity, isotropic or kinematic hardening.

International audience; Enhancing the thermal conductivity and reducing the thermal expansion for electronic packaging applications can be achieved by compositing carbon nanofibers in copper-matrices. Though achieving these optimal thermal properties is theoretically possible, such composites are currently not available due to many unresolved practical problems. Conventional compositing processes are incapable of obtaining the desired fiber distribution while controlling the fiber–matrix interfaces for effective heat and load transfers. In this paper, three different powder metallurgy based processes are presented; two based on conventional techniques and the third a relatively new method. The first method is basically the conventional powder metallurgy process. The second and the third methods are also powder metallurgy processes with different ways of modifying the surface of the fibers using either electroless coating or the novel salt decomposition method. It is shown that the salt decomposition method is capable of achieving the desired high thermal conductivity values while the thermal expansion values remain the same in all the three processes.

We develop a Thurston-like theory to characterize geometrically finite rational maps, then apply it to study pinching and plumbing deformations of rational maps. We show that in certain conditions the pinching path converges uniformly and the quasiconformal conjugacy converges uniformly to a semi-conjugacy from the original map to the limit. Conversely, every geometrically finite rational map with parabolic points is the landing point of a pinching path for any prescribed plumbing combinatorics., Comment: 78 pages, 6 figures

International audience; By construction digital image correlation is an ill-posed problem. To circumvent this di culty, the regularization is often performed implicitly through the kinematic basis chosen to express the sought displacement fields. Conversely, a priori information on the mechanical behavior of the studied material is often available. It is proposed to evaluate the gain to be expected from such a mechanical assistance, namely, the measured displacement not only satis es as best as possible the gray level conservation, but also mechanical admissibility.

International audience

International audience; Nano‐submodeling is an approach that enables insertion of nano‐refined submodel (atomistic) in the global model (continuum). In this work analysis of the spurious effects that may arise in the concurrent atomistic‐to‐continuum coupling is performed. The coupling is based on the overlapping domain decomposition (ODD) method called bridging domain [1, 2] (similar is Arlequin [3] method) where different models are overlapped and the displacements compatibility is enforced via Lagrange multipliers (LM). Some coupling options such as energy weighting, coupling zone geometry and LM field interpolation are tested.

International audience; With the inclusion of all terms up to the third-order in the membrane strain to consider the geometric nonlinearity in deformation, a third-order implicit model and a fourth-order explicit model for the vibration transformation between extensional and flexural modes in thin-walled cylindrical shells are established and solved numerically. Numerical instability is observed in numerical solutions based on the explicit model. It is found that such numerical instability does not result from the accumulated numerical errors in the numerical integration process, but from the neglecting of higher order terms in the formulation of the problem. With the inclusion of all terms up to the fourth-order in the strain energy, the explicit model can predict a stable vibration history with periodic mode transformations between the two modes. The same 2:1 internal resonance of the vibration mode transformation is predicted by both models. As flexural stress growth is concerned, the two models matches very well during the first group of peaks but lag in phase gradually appears in later group of stress peaks based on the implicit model. This is understood to be resulting from the limited number of terms included in the series expansions based on the explicit model, which allows the most likely excited flexural mode get a larger share of energy transferred from the principle mode and as a result flexural stress arrives at peaks earlier based on the explicit model.

International audience; The present paper is devoted to the analysis of the contact/impact problems with Coulomb friction and large deformation between two hyperelastic bodies of Gent model. The total Lagrangian formulation is adopted to describe the geometrically non-linear behavior. For the finite element implementation, the explicit expression of the incremental law of Gent model is derived. A first order algorithm is applied for the numerical integration of the time-discretized equation of motion. Efficiency and accuracy of the resulting method is illustrated on a two-dimensional static contact problem and a three-dimensional dynamic contact problem as compared with ANSYS simulations.

International audience; L'objet de cette communication est de présenter, de manière succincte, un prototype du logiciel FER/Mesh-un logiciel de génération automatique de maillages pour le calcul par éléments finis. L'algorithme du maillage est basé sur la méthode frontale. Ce logiciel est développé sur PC en langage C++ avec le support graphique OpenGL. Une organisation orientée-objets en langage C++, ayant une architecture souple et ouverte, facilite des nouveaux développements futurs.

International audience; Many applications in the electronic industry require an optimum combination of thermal transport properties (e.g., high thermal conductivity for a given coefficient of thermal expansion). This combination cannot possibly be obtained using a single material and hence requires judicious selection of matrix material with appropriate distribution of the second phase to form a composite. Such a composite provide the combination of thermal properties required for a given application. The properties of matrix and reinforcing materials, as well as shape, size and relative volume fraction and spatial distribution of the reinforcing phase, all play key roles in determining the overall thermal properties of the composite. In this work, we present an analytical model based on Eshelby's tensor for determining the coefficient of thermal expansion () and thermal conductivity (k) of composites. We are able to account for the effect of different fiber shapes and volume fractions; in addition, we have included the effect of orientation distribution of the fibers (inhomogeneities) on the composite thermal properties. The calculated values compare favorably with the available experimental data.

pp. 1-7; International audience; We investigate subgrid modeling of anisotropic rotating turbulence with a dynamic equation of structure functions of the filtered velocity field. The local volume-averaged structure function equation of rotating turbulence is introduced and an eddy viscosity subgrid model is obtained. The resulting subgrid model is similar to that of the study of Cui et al. [Phys. Fluids 16, 2835 (2004)]. It is directly related to the transfer term: the third-order structure function. This term can be computed dynamically during large eddy simulations (LES). Tests are successfully carried out in LES of decaying, rotating, homogeneous turbulence at high Reynolds numbers. Results are in excellent agreement when compared with those of Cambon et al. [J. Fluid Mech. 337, 303 (1997)].

International audience; The velocity increment (VI) model, which was introduced by Brun et al., is improved by employing the Kolmogorov equation of filtered velocity in this paper. This model has two different formulations: a dynamic formulation and a simplified constant form in high Reynolds number turbulence. A priori tests in isotropic turbulence and wall-bounded turbulence are performed. A posteriori tests of decaying turbulence and channel Poiseuille flow are made to testify the model performance, especially on the energy backscatter. The simple constant coefficient formulation has good performance, and avoids the ensemble average operation, which exists in other subgrid models. This constant improved VI model is particularly proposed in complicated large-eddy simulation projects.

In eukaryotes, DNA replication initiation requires assembly and activation of the minichromosome maintenance (MCM) 2-7 double hexamer (DH) to melt origin DNA strands. However, the mechanism for this initial melting is unknown. Here, we report a 2.59-Å cryo-electron microscopy structure of the human MCM-DH (hMCM-DH), also known as the pre-replication complex. In this structure, the hMCM-DH with a constricted central channel untwists and stretches the DNA strands such that almost a half turn of the bound duplex DNA is distorted with 1 base pair completely separated, generating an initial open structure (IOS) at the hexamer junction. Disturbing the IOS inhibits DH formation and replication initiation. Mapping of hMCM-DH footprints indicates that IOSs are distributed across the genome in large clusters aligning well with initiation zones designed for stochastic origin firing. This work unravels an intrinsic mechanism that couples DH formation with initial DNA melting to license replication initiation in human cells.