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International audience; Benefiting from the recent advances of synchrotron X-ray nanoprobes, we demonstrate three-dimensional (3D) correlative nano-imaging on malaria-infected human red blood cells. By combining X-ray fluorescence to-mography and phase contrast nanotomography on the same cell with sub-100 nm pixel size, we establish a routine workflow from the data acquisition, data processing to tomographic reconstruction. We quantitatively compare the elemental volumes obtained with different reconstruction methods, the total variation minimization giving the most satisfactory results. We reveal elemental correlations in different cell compartments more reliably on reconstructions as opposed to 2D projections. Finally, we determine for the first time the 3D mass fraction maps of multiple elements at sub-cellular level. The estimated total number of Fe atoms and the total mass of the red blood cell show very good agreement with previously reported values.

Chemical utilization of vast fossil and renewable feedstocks of methane remains one of the most important challenges of modern chemistry. Herein, we report direct and selective methane photocatalytic oxidation at ambient conditions into carbon monoxide, which is an important chemical intermediate and a platform molecule. The composite catalysts on the basis of zinc, tungstophosphoric acid and titania exhibit exceptional performance in this reaction, high carbon monoxide selectivity and quantum efficiency of 7.1% at 362 nm. In-situ Fourier transform infrared and X-ray photoelectron spectroscopy suggest that the catalytic performance can be attributed to zinc species highly dispersed on tungstophosphoric acid /titania, which undergo reduction and oxidation cycles during the reaction according to the Mars–van Krevelen sequence. The reaction proceeds via intermediate formation of surface methyl carbonates., While methane has become an increasingly important chemical feedstock, its selective conversion to other chemicals has proven challenging. Here, authors demonstrate a photocatalyst based on earth-abundant elements that selectively converts methane to carbon monoxide under mild conditions.

International audience; The vibro-acoustic modelling of a panel-cavity system is of prime importance for the building industries, exemplified by the noise insulation of single or double skin façade. The vibro-acoustic analysis of such systems in the mid-to-high frequency range is computational costly and technically challenging due to the complex wavelength composition. In the present paper, the Condensed Transfer Function (CTF) approach is revisited to tackle this problem. It is demonstrated that the calculation efficiency of the CTF method can be greatly increased by properly selecting the Condensation Functions (CFs) and exploiting their physical characteristics. In particular, owing to their spatial wavy features, the complex exponential functions can better match the structural wavelength variations so that the velocity on the plate-cavity interface can be described by using a much reduced CF set as compared with the gate functions which are widely used in the previous works. Numerical results show a piecewise convergence behavior of the calculation which is further exploited for establishing a criterion for the truncation of the CFs. The proposed criterion allows the determination of a sub-set of the CFs for any prescribed frequency band for the calculation of the system response in a progressive and piecewise manner, resulting in a great increase in the computational efficiency.

We consider a discrete time financial market with proportional transaction costs under model uncertainty, and study a num\'eraire-based semi-static utility maximization problem with an exponential utility preference. The randomization techniques recently developed in \cite{BDT17} allow us to transform the original problem into a frictionless counterpart on an enlarged space. By suggesting a different dynamic programming argument than in \cite{bartl2016exponential}, we are able to prove the existence of the optimal strategy and the convex duality theorem in our context with transaction costs. In the frictionless framework, this alternative dynamic programming argument also allows us to generalize the main results in \cite{bartl2016exponential} to a weaker market condition. Moreover, as an application of the duality representation, some basic features of utility indifference prices are investigated in our robust setting with transaction costs., Comment: Final version, to appear in Mathematics of Operations Research

International audience; Variations of the lattice parameter can significantly change the properties of a material, and, in particular, its electronic behaviour. In the case of graphene, however, variations of the lattice constant with respect to graphite have been limited to less than 2.5% due to its well-established high in-plane stiffness. Here, through systematic electronic and lattice structure studies, we report regions where the lattice constant of graphene monolayers grown on copper by chemical vapour deposition increases up to ~7.5% of its relaxed value. Density functional theory calculations confirm that this expanded phase is energetically metastable and driven by the enhanced interaction between the substrate and the graphene adlayer. We also prove that this phase possesses distinctive chemical and electronic properties. The inherent phase complexity of graphene grown on copper foils revealed in this study may inspire the investigation of possible metastable phases in other seemingly simple heterostructure systems.

The rapidly intensifying process of ocean acidification (OA) due to anthropogenic CO2 is not only depleting carbonate ions necessary for calcification but also causing acidosis and disrupting internal pH homeostasis in several marine organisms. These negative consequences of OA on marine calcifiers, i.e. oyster species, have been very well documented in recent studies; however, the consequences of reduced or impaired calcification on the end-product, shells or skeletons, still remain one of the major research gaps. Shells produced by marine organisms under OA are expected to show signs of dissolution, disorganized microstructure and reduced mechanical properties. To bridge this knowledge gap and to test the above hypothesis, we investigated the effect of OA on juvenile shells of the commercially important oyster species, Magallana angulata, at ecologically and climatically relevant OA levels (using pH 8.1, 7.8, 7.5, 7.2). In lower pH conditions, a drop of shell hardness and stiffness was revealed by nanoindentation tests, while an evident porous internal microstructure was detected by scanning electron microscopy. Crystallographic orientation, on the other hand, showed no significant difference with decreasing pH using electron back-scattered diffraction (EBSD). These results indicate the porous internal microstructure may be the cause of the reduction in shell hardness and stiffness. The overall decrease of shell density observed from micro-computed tomography analysis indicates the porous internal microstructure may run through the shell, thus inevitably limiting the effectiveness of the shell's defensive function. This study shows the potential deterioration of oyster shells induced by OA, especially in their early life stage. This knowledge is critical to estimate the survival and production of edible oysters in the future ocean.

International audience; A damage characterization framework based on the “pseudo-excitation” (PE) approach has recently been established, aimed at quantitatively identifying damage in beam-, plate-, and shell-like structural components. However, it is envisaged that the effectiveness of the PE approach can be restricted in practical implementation, due to the involvement of high-order derivatives of structural dynamic deflections, in which measurement noise and uncertainties can overwhelm the damage-associated signal features upon mathematical differentiation. In this study, the PE approach was revamped by introducing the weighted integration, whereby the prerequisite of satisfying the local equilibrium conditions was relaxed from “point-by-point” to “region-by-region”. The revamped modality was thus colloquially referred to as “weak formulation” of the PE approach, as opposed to its original version which is contrastively termed as “strong formulation”. By properly configuring a weight function, noise immunity of the PE approach was enhanced, giving rise to improved detection accuracy and precision even under noisy measurement conditions. Furthermore, the ‘weak formulation’ was extended to a series of coherent variants through partial integration, rendering a multitude of detection strategies by selecting measurement parameters and configurations. This endowed the PE approach with flexibility in experimental manipulability, so as to accommodate various detection requirements. As an application of the “weak formulation”, a continuous gauss smoothing (CGS)-based detection scheme was developed, and validated by localizing multiple cracks in a beam structure, showing fairly improved noise tolerance.

International audience; While the structural-acoustic coupling between flexible structures and closed acoustic cavities has been extensively studied in the literature, the modeling of structures coupled through open cavities, especially connected in cascade, is still a challenging task for most of the existing methods. The possible presence of micro-perforated panels (MPPs) in such systems adds additional difficulties in terms of both modeling and physical understanding. In this study, a sub-structuring methodology based on the Patch Transfer Function (PTF) approach with a Compound Interface treatment technique, referred to as CI-PTF method, is proposed, for dealing with complex systems involving cascade open/closed acoustic cavities and MPPs. The co-existence of apertures and solid/flexible/micro-perforated panels over a mixed separation interface is characterized using a compound panel subsystem, which enhances the systematic coupling feature of the PTF framework. Using several typical configurations, the versatility and efficiency of the proposed method is illustrated. Numerical studies highlight the physical understanding on the behavior of MPP inside a complex vibroacoustic environment, thus providing guidance for the practical design of such systems.

International audience; Unlike crystalline metals, which have a well-understood periodical structure, the amorphous structure of metallic glasses (MGs) is still poorly understood, particularly when such a structure rearranges itself at the nanoscale under external agitations. In this article, we provide compelling evidence obtained from a recently developed high-resolution atomic force microscopy (HRAFM) technique that reveals the nanoscale structural heterogeneity after thermal annealing in a Zr-Ni metallic glass. Through the HRAFM technique, we are able to uncover the annealing-induced fractal growth of the dense-packing phases in the binary MG thin film, which exhibits a fractal dimension of similar to 1.7, in line with a two-dimensional diffusion limited aggregation process. The current findings not only reveal the evolution process of atomic packing in the annealed MG thin film, but also shed light on the possible cooling rate effect on the atomic structure of MGs. (c) 2012 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

International audience; Experimental verifications have been performed on three engineering metals to verify recent methods proposed for extracting stress–strain curves from indentation tests. Their sensitivity to data errors is evaluated. Finally, the factors that might cause the inaccuracy and instability of the proposed methods are discussed, providing information that can be useful for further improving these methods.

High-energy synchrotron X-ray diffraction was used to investigate the isothermal precipitation of delta-hydride platelets in Zircaloy-4 at a range of temperatures relevant to reactor conditions, during both normal operation and thermal transients. From an examination of the rate kinetics of the precipitation process, precipitation slows with increasing temperature above 200 degrees C, due to a reduction in the thermodynamic driving force. A model for nucleation rate as a function of temperature was developed, to interpret the precipitation rates seen experimentally. While the strain energy associated with the misfit between hydrides and the matrix makes a significant contribution to the energy barrier for nucleation, a larger contribution arises from the interfacial energy. Diffusion distance calculations show that hydrogen is highly mobile in the considered thermal range and on the scale of inter-hydride spacing and it is not expected to be significantly rate limiting on the precipitation process that takes place under reactor operating conditions. (C) 2015 The Authors. Published by Elsevier B.V.

International audience; Modeling sound transmission among acoustic media through mixed separations, consisting of both rigid/flexible structures with apertures, is a challenging task. The coexistence of both structural and acoustic transmission paths through the same coupling surface adds system complexities, hampering the use of existing sub-structuring modeling techniques when the system configuration becomes complex. In the present work, a virtual panel treatment is proposed to model thin apertures involved in such complex vibroacoustic systems. The proposed virtual panel considers an aperture as an equivalent structural component, which can be integrated with the solid/flexible structure to form a unified compound interface. This allows handling the entire compound interface as a pure structural element, thus providing an efficient and versatile tool to tackle system complexities when using sub-structuring techniques. The accuracy and convergence of the method are investigated and validated, and the effective thickness range allowing for the virtual panel treatments is determined. The capability and the flexibility of the proposed formulation are demonstrated through several numerical examples, with underlying physics being explored.

International audience; In this article, we discuss the recent use of the high-resolution dynamic atomic force microscopy (DAFM) in mapping the nano-scale dynamical structural heterogeneity in thin film metallic-glasses (TFMGs). Our focus is laid on the major factors which can influence the structural contrast in the DAFM images, such as tip radius, free-amplitude, set-point amplitude and surface roughness. Finally, through a comparative study of different TFMGs and single-crystal silicon, we demonstrate that the DAFM technique is effective in distinguishing different nanostructures through their energy dissipation spectra.

International audience; Shearing tests and penetration tests were combined to determine the mechanical behavior of a stainless steel metal treated by surface mechanical attrition (SMA). SMAT is a usual treatment to strengthen the hardness of treated surface by severe mechanical strain, which generates also a gradient through the direction of thickness. This paper aimed at investigating the behavior of sheet metals treated by means of SMAT under impact loading. Double shearing tests at high strain rate were performed using Hopkinson bar. The experimental results demonstrated notable strain rate sensitivity and a good ductility without sacrificing the strength. An impact perforation test is also performed using inversed perforation test in order to study its sensibility of failure to the loading rate.

International audience; Previously, an inverse damage characterization framework was proposed by quantifying the perturbation to local dynamic equilibrium of a beam-like structure, showing advantages in some aspects over the traditional global vibration-based and local guided-wave-based methods. Residing on the plate theory, this framework was expanded to a two-dimensional domain. Inheriting the attributes of localized canvassing using high-order spatial derivatives this approach has proven effectiveness in quantitatively characterizing damage of small dimension, regardless of its number and type. In addition, the approach requires no benchmarks, baseline signals, global models, additional excitation sources, pre-modal analysis nor prior knowledge on structural boundary. A damage imaging algorithm using the quantified dynamic perturbation was further established, enabling presentation of damage characterization results in an intuitive and prompt manner. Integrating the detection capacities in one- and two-dimensional domains, a hybrid damage visualization strategy was developed, for systems comprising structural components of different types, various geometries and diverse boundary conditions. Two independent de-noising techniques (low-pass wavenumber filtering and adjustment of measurement density), together with a hybrid data fusion algorithm, were proposed as auxiliary means to enhance the robustness of the strategy in noisy measurement conditions. The strategy was applied experimentally to the evaluation of multi-damage in a plane structure comprising beam and plate components, showing satisfactory results. (C) 2013 Elsevier Ltd. All rights reserved.

Summary When simulating urban floods, most approaches have to simplify the topography of the city and cannot afford to include the obstacles located in the streets such as bus stops, trees and parked cars. The aim of the present paper is to investigate the error made when neglecting such singularities in a simple flooded 3-branch crossroad configuration with a specific concern regarding the error in discharge distribution to the downstream streets. Experimentally, the discharge distribution for 14 flows in which nine obstacles occupying 1/6 of the flow section are introduced one after the other is measured using electromagnetic flow-meters. The velocity field for one given flow is obtained using horizontal-PIV. Additionally, all these flows are computed using a CFD methodology. It appears that the modification in discharge distribution is mostly related to the location of the obstacles with regards to the intersection, the location of the separating interface and is strongly impacted by the Froude number of the inflow while the influence of the normalized water depth remains very limited. Overall, the change in discharge distribution induced by the obstacles remains lower than 15% of the inflow discharge even for high Froude number flows.

International audience; The micro-perforated panel (MPP) with a backing cavity is a well known efficient device for noise absorption. This device has been thoroughly studied in the experimental conditions of an acoustic tube (Kundt tube), in which the MPP is excited by a normal incident plane wave in one dimension. In an industrial situation, the efficiency of MPP may be influenced by the vibro-acoustic behaviour of the surrounding systems as well as excitation. To deal with this problem, a vibroacoustic formulation based on the Patch Transfer Functions (PTF) approach is proposed to model the behaviour of micro-perforated structure in a complex vibro-acoustic environment. PTF is a substructuring approach which allows assembling different vibro-acoustic subsystems through coupled surfaces. Upon casting micro-perforations and the flexibility of the MPP under transfer function framework, the proposed PTF formulation provides explicit representation of the coupling between subsystems and facilitates explanation of physical phenomenon. As an illustration example, application to a MPP with a backing cavity located in an infinite baffle is demonstrated. The proposed PTF formulation is finally validated through comparison with experimental measurements available in the literature.

International audience; In recognition of the obvious limitations of most global vibration-based and local guided-wave-based damage detection techniques, a novel inverse identification approach was developed by canvassing the local perturbance to equilibrium characteristics of the structural component under inspection. Characterized by high-order spatial derivatives, this approach has in particular proven sensitivity to structural damage. Most importantly, it requires neither benchmark structures nor baseline signals; neither global models nor additional excitation sources as long as the structure undergoes steady vibration under its normal operation. Independent of a global model, prior knowledge on structural boundary conditions is not compulsory. To minimize unavoidable influence of measurement noise on high-order spatial derivatives, various de-noising treatments, including wavenumber filtering, optimal selection of measurement configuration and hybrid information fusion were introduced independently. Using a simple beam as a representative structural component for illustration, relationships among vibration frequency, density of measurement points and size of detectable damage were explored, facilitating a judicious selection of measurement parameters. Proof-of-concept validation was numerically conducted, and then experimentally demonstrated using a scanning laser vibrometer. In principle, this proposed methodology is applicable to a complex system comprising various structural components, provided that the local equilibrium relationships of the components are known a priori. (C) 2011 Elsevier Ltd. All rights reserved.

International audience; In the present study, a method is presented combining surface nanocrystalline treatment (SMAT) and the co-rolling process. The aim of this duplex treatment is the development of a 316L stainless steel semi-massive multilayered bulk structure with improved yield and ultimate tensile strengths, while conserving an acceptable elongation to failure by optimizing the volume fraction and distribution of the nano-grains in the laminate. To characterize this composite structure, tensile tests as well as sharp nanoindentation tests were carried out to follow the local hardness evolution through the cross-section of the laminate. Furthermore, transmission electron microscope (TEM) observations were carried out to determine the correlation between the microstructure, the local hardness and the mechanical response of the structure.

Une structure granulaire est un assemblage de grains dont les tailles sont décrites par une distribution donnée. Les grains sont séparés par des interfaces appelées joints de grains. Plusieurs domaines de physique et d'ingénierie sont concernés par ces structures, notamment la métallurgie, la mécanique des roches, la biologie et l'élaboration de céments. Un échantillon de ce type de structure est souvent modélisé par un rectangle rempli de disques (grains) en deux dimensions ou par un parallélépipède contenant des boules en trois dimensions. La modélisation géométrique d'une telle structure consiste donc à déterminer la répartition de l'ensemble des disques (ou des boules) disjoints respectant la distribution de taille spécifiée. Dans ce rapport, un nouvel algorithme de génération de cette répartition de grains, basé sur une approche frontale utilisée habituellement dans des méthodes de construction de maillages bidimensionnels ou tridimensionnels, est proposé. En outre, une méthode d'optimisation est introduite afin d'équilibrer les épaisseurs des joints de grains. De fortes améliorations en temps de calcul ainsi qu'en taux de remplissage (densité) sont observées et reportées via des comparaisons avec des méthodes existantes. Par ailleurs, nous montrons que l'on peut transformer les grains circulaires ainsi générés en des grains polygonaux généralement observés dans la réalité. Quelques exemples de modélisations de nanostructures métalliques sont présentés.

International audience; Microindentation (depth of indents = 500 nm) is performed on a longitudinal section of a semi-hard copper sheet, which was broken in a tensile machine [B. Guelorget, M. François, C. Vial-Edwards, G. Montay, L. Daniel, J. Lu, Mater. Sci. Eng., A Struct. Mater.: Prop. Microstruct. Process. 415 (2006) 234.]. As expected, the shorter the distance between the measured point and the fracture, the higher the hardness, due to the work hardening. However, the main goal of our investigation was to observe the variation of Young's modulus, that could be induced by damage evolution. It was found, indeed, that Young's modulus decreases by 36% within a distance of 300 μm from the fracture and is constant beyond. The corresponding evaluated damage is 0.36, with an accuracy better than 0.02. Thus, the measurement of the local variation of Young's modulus through microindentation can be used as a new way of determination of local damage.

International audience; Rainwater rivulets appear on inclined cables of cable-stayed bridges when wind and rain occur simultaneously. In a restricted range of parameters this is known to cause vibrations of high amplitudes on the cable. The mechanism underlying this effect is still under debate but the role of rainwater rivulets is certain. We use a standard lubrication model to analyse the dynamics of a water film on a cylinder under the effect of gravity and wind load. A simple criterion is then proposed for the appearance and position of rivulets, where the Froude number is the control parameter. Experiments with several geometries of cylinder covered with water in a wind tunnel show the evolution of the rivulets with the Froude number. Comparison of the prediction by the model with these experimental data shows that the main mechanism of rivulet formation and positioning is captured. © 2006 Académie des sciences. Published by Elsevier SAS. All rights reserved.

International audience; The aim of this work is to reach a better understanding of the ultrasonic shot-peening process and, in particular, the evolution of the shot speed distribution. A simple 1D modelling of the interaction between the shots and the sonotrode is carried out. The impact is considered as inelastic with an energy absorption that depends on the speed of the shot. It is found that after about 10 interactions (» 1s) the speed distribution in the chamber follows a Maxwell-Boltzmann distribution, which is the distribution found in a perfect gas at equilibrium. The influence of various process parameters such as the sonotrode amplitude, the vibration frequency on the average speed and on the Almen intensity is studied.

International audience