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International audience; The reduction of pollutant emissions from internal combustion engines, and in particular greenhouse gases, must be achieved by optimizing the combustion process and choosing fuels with a low carbon impact. Heterogeneous mixture combustions such as Diesel induce a difficulty in managing of the pollutants formation even if in its principle the output efficiency remains favorable. Therefore, it is necessary to distinguish this last point, which refers to the compression ratio, in order to move towards homogeneous combustion with a high compression ratio. We then speak generically of HCCI combustion (Homogeneous Charge Compression Ignition), whose process takes place in the context of auto ignition.An important challenge of this combustion is the control of the ignition timing. This paper presents a concept to control the initiation of combustion of a reactive gas mixture with low susceptibility to auto-ignition, which meets the expectation of high efficiency, via the introduction of biomass particles. From a schematic point of view, these particles will play the role of spark plugs distributed throughout the combustion chamber, and should have an auto ignition time much lower than that of the fuel.For this purpose, a particle injector was conceived to directly inject a controlled quantity of biomass powder into the reactive mixture directly. This part is a challenging point. The reproducibility and viability of the particle injector were qualitatively validated by image analysis acquired at high frequency and high resolution. Thus, the system was integrated into an RCM (Rapid Compression Machine) chamber, allowing diphasic gas/solid biomass combustion study. This preliminary test was investigated by high-speed video on a lean diluted biomass enriched with hydrogen mixture (CH4/CO2/H2) at low fuel-air equivalence ratio (Φ = 0.6), with different auto-ignition delays in the condition of experimentation. It was shown that the direct injection of a small amount of biomass particles was able to promote its reactivity and initiate the combustion process. This study highlights the promising effects of the injection of powder to control the beginning of homogeneous combustion, by bringing locally a spot of energy, distributed in the whole chamber. The control given by a global contribution of weak energy is distinguished from the process of combustion by auto-ignition of the fuel mixture whose properties of equivalence air-fuel ratio or dilution will complete the optimization of the process.

The densification and grain growth behavior of a ZnS nanopowder during spark plasma sintering (SPS) were investigated under the pressure of 125 MPa and constant heating rate of 25 °C min−1. At a sintering temperature of 550 °C, almost fully dense ZnS ceramics (98.2%) could be obtained with grain size as low as 75 nm. At higher temperature, ZnS is subject of a very important grain growth that is detrimental to the mechanical properties. A maximum Vickers microhardness of 4.2 GPa was measured, which is 70% higher than the current state of the art. Retaining the grain size below 100 nm appears to be the key to developing competitive ZnS functional ceramics.

International audience; In this article, the active vibration control of a smart structure made out of a single functionally graded piezoelectric material layer, equipped with a network of discrete electrodes, is studied. The material properties vary continuously across the direction of thickness, so that top and bottom surfaces consist of pure PZT4 and the mid surface is composed of pure aluminium. The percolation phenomenon is taken into account. A functionally graded piezoelectric material plate finite element based on the first-order shear deformation theory hypothesis and layer-wise approximation for electric potential is implemented. An optimization procedure is considered to define the relevant electrodes for actuators and sensors, based on controllable and observable criteria. An adaptative fuzzy controller system is used, activating with relevance the actuators according to the most excited eigenmodes. Simulations show the effectiveness of this kind of concept.

International audience; A beam model is developed for the static analysis of composite steel-concrete beams and twin-girder decks which takes into account the effects of shear lag phenomenon as well as the interfacial slip between the concrete slab and steel joists. A sinus distribution is considered for the axial displacements of the concrete slab and steel joists. This kinematic represents the transverse shear strain in the concrete slab and steel joists using a cosine function. The considered kinematic satisfies the boundary conditions of transverse shear stress on the top and bottom surfaces and does not need any shear correction factor. The shear lag effects are introduced by prescribing the warping shape of the slab cross-section and taking its intensity as axial unknowns. A conforming three-nodded 25-dof beam element is derived for numerical simulations. The presented model has been validated by comparing the obtained results with elasticity solutions, 3D finite element results and other analytical and numerical models available in the open literature.Keywords: Interfacial slip; Shear lag; Sinus model; Steel-concrete beams; Twin-girder decks

The delayed damage model has been introduced by Allix and Deu [1] as a way to overcome spuriousmesh dependency in failure analysis involving damage and dynamic loading. The damage rate is boundedthrough a time scale which, combined with the wave speed, introduces implicitly a length scale. In this paper,the delayed damage model is analyzed through numerical experiments on three different loading cases of abar: a slow loading leading to a dynamic failure, pulses and impact. We observe and discuss the load levelneeded for failure (and the dependence of this load level with respect to the loading rate), as well as thedissipation and extent of the fully damaged zone at failure. Observations lead to the following conclusions.First, the delayed damage model has no regularization effect for a dynamic failure initiating from rest. Second,for pulse loadings, the loading rate has no influence on the minimal load level needed for failure (even thoughthe delayed damage model is a time-dependent model), and beyond this minimal load level for failure, theextent of the fully damage zone rises, proportionally to the length scale. Third, regarding the impact, thevelocity needed to reach failure depends only the time-independent parameters of the models, and not theones linked to the delayed damage.

International audience; To study detonation products of condensed matter in the post-combustion phase, a preliminary work is carried out on deflagrations. The study of the radiative properties of molecules during this fast phenomenon is not simple in the mid-infrared range. As the flame front of a H 2 /O 2 /N 2 /CO 2 gas mixtures spreads in a few tens of meters per second, a fast infrared detection system is required. Besides, there are no calibrating sources in that spectral range for the intensity and spectral position calibration. The important feature of the experimental setup presented is the record of high-resolution spectra at high frequency, up to 10 kHz. The setup is composed of a deflagration chamber with optical accesses. The pressure evolution is measured by a high speed piezoelectric sensor. The ignition is synchronized with the camera trigger. The radiation is focused into a monochromator and at its exit slit, a camera records the spectra in real time. The spectral intensity is calibrated using a blackbody. The correspondence between the spatial position of a pixel and the wavelength is fitted using an original method based on the application of a third degree polynomial, taking into account optical aberrations. The resulting spectra can be used to determine the temperature and the emissivity of gases.

International audience; Traditionally, equiaxed alpha grains rather than lamellar microstructure (LM) domains in titanium alloys are regarded as potential internal crack origins in high-cycle fatigue (HCF) and very-high-cycle fatigue (VHCF) re-gimes. Here, we found that the fatigue crack is prone to initiate from a large LM domain in a titanium alloy with the composition of LM and equiaxed microstructure (EM) of fine alpha grains. Then, the mechanisms of internal crack initiation and early growth for the cases of HCF and VHCF under stress ratio R =-1, 0.1 and 0.5 were addressed and a mechanism chart was constructed to illustrate the internal cracking behavior, especially showing that the numerous cyclic pressing process dominates the related microstructure evolution with grain size refinement and nanograin formation underneath the fracture surfaces in the region of crack initiation and early growth.

International audience; A numerical model to take into account the effect of the stress state on the bond behavior between steel and concrete in reinforced concrete structures is proposed. It is based on a zero thickness element, adapted to large-scale simulations and the use of 1D elements for steel bars. The proposed model also assumes the definition of a bond stress – slip law which includes the confining pressure around the steel bar as a parameter. The implementation of the model is presented and the calibration of the bond law is discussed. A general equation is especially proposed. This evolution law is validated through the comparison to 28 pullout tests. The model is able to reproduce the evolution of the bond stress (especially the bond strength) as a function of the confinement pressure, whatever the configuration (different concrete cover to steel diameter ratios). Finally, the effects at the structural level are investigated on a reinforced concrete tie. The response for different confining pressures is especially studied. It shows the capability of the model to reproduce the “expected” tendencies with an increase of the initial elastic stiffness with increasing pressures and consequently a higher number of cracks in the stabilized nonlinear regime. The “transfer length” is also shown to decrease with increasing confining pressures.

This brief introduces an innovative unidimensional thermal propagation modeling. It consists in thermal diffusion on conductor bulk structure based on the Kron–Branin (KB) formalism. The proposed KB model is inspired from the transmission line approach by considering the bulk material thermal resistance and capacitance. The KB-based equivalent graph topology of the heated structure is elaborated. The thermal transfer function (TTF) is established from the graph analytical abstraction. The TTF enables to determine the transient and frequency-dependent temperature diffusion along the structure. The KB model effectiveness is illustrated with both frequency and time-domain analyses. Proof-of-concept constituted by copper-based bulk material is considered. The TTF from KB, finite-element-method and Cauer models are compared. With computation from dc to 100 Hz, it was shown that the frequency domain magnitude and phase are in good correction. In addition, the calculated and simulated transient heat flows along the considered structure are in good agreement by considering 120-ms duration pulse width temperature source.

International audience; The reaction of sodium metal with liquid water is a heterogeneous, exothermic reaction known to be prone to runaway with explosive effects when uncontrolled. The occurrence of runaways requires the reactants to be effectively mixed which is not the case initially for sodium and water. The latter are put in contact in two distinct, condensed phases, that are moreover rapidly separated by a gaseous film generated by the reaction itself. The runaway is triggered by a reactant mixing process that is not elucidated. This mixing could occur either in liquid or gas-phase. In this paper, we investigate the mixing process of sodium-water reaction in the small scale configuration of 1–4 g sodium pellets with excess water in a closed vessel, using time-resolved video, optical, pressure and temperature acquisitions. The results show that sodium vaporization is implicated in the reaction and that it is generated in great quantity during runaway. However it appears from temperature measurements that boiling temperature is not reached in the sodium before runaway or only locally. It is therefore possible that reactant mixing occurs in the gas phase if an intensification of sodium vaporization takes place as a fast local transient. The possibility of reactant mixing in liquid phase is not contradicted by these results but was not identified from the available videos and data.

The global buckling and wrinkling behavior of sandwich plates with anisotropic facesheets is investigated by means of a linearized stability analysis. High-order plate models are formulated referring to a sublaminate variable-kinematic approach: an axiomatic through-thickness description of the displacement field is introduced, wherein the selected model is employed for an arbitrarily defined group of plies, i.e., the sublaminate. The two-dimensional governing equations of the plate model are solved in weak form by means of the Ritz method. The modeling approach is applied to sandwich plates with anisotropic laminated facesheets and orthotropic cores. A wide set of configurations is analyzed: critical loads and wrinkling patterns are determined for panels with foam and honeycomb cores, subjected to uniaxial as well as multiaxial loads. The proposed approach is shown to provide accurate, quasi-3D predictions for both long and short wavelength buckling with a reduced computational effort.

International audience; Due to an ultra-low thermal expansion, Zr2O(PO4)2 could find many applications as a thermal shock resistant material. To this end, ceramic processing is a key step in order to reach best properties. In this work, Zr2O(PO4)2 was sintered by conventional sintering and by the spark plasma sintering technique (SPS) with and without additive. Samples made by conventional sintering with ZnO as sintering aid have a maximum relative density of around 92 %. Microstructure is composed of large grains and microcracks can be observed. When doped 2 with 5 wt. % of MgO, samples can be densified by SPS up to 99.6 % of the relative density and the grain size maintained between 0.5 and 1.5 μm. Thermal conductivity and Vickers microhardness were investigated as a function of the microstructure. Best values were obtained for the ceramic doped with 5 wt.% MgO and sintered by SPS, thanks to a fine microstructure and a small amount of residual microcracks.

International audience; The bearable shear stress amplitude in very high cycle fatigue regime (VHCF) is of interest for many applications in mechanics (springs, crankshafts, ball bearings, etc.) when a very high repetition of load cycles occur. This paper proposes to optimise an existing torsion gigacycle fatigue testing system, developed fifteen years ago by the Pr C. Bathias’ research team. The aim is to increase the shear stress amplitude applied to the investigated specimen and, consequently, meet the expectations from industries about fatigue life for high strength materials under shear loading. The system differs from other ultrasonic torsion fatigue machines that can be found in the literature, by the combination of two amplification horns, transforming a translation movement into a rotational one. In this work, there are two optimisation objective functions: (i) maximize the shear stress level in the specimen and (ii) minimize the stress level in the fatigue system and mainly in the pin connecting the two horns. The development of the optimised device is essentially carried out by a parametric study and numerical simulations through modal and harmonic analysis. Numerical results are compared with the analytical 1D solution and with experimental results obtained from the new real experimental set-up. Finally, new results in the gigacycle domain are presented concerning the torsion fatigue strength of high strength steels (50CrV4 and 16MnCr5) and discussed.

The design of bioactive plasters is of major interest for the amelioration of dental and bone cements. In this article, a one pot and environmentally friendly strategy based on the addition of a cheap polyphenol-tannic acid (TA) or the main phenolic constituent of TA, namely pyrogallol (PY)- able to interact with calcium sulfate is proposed. Tannic acid and pyrogallol not only modify the morphology of the obtained plaster+TA/PY composites but a part of it is released and provides strong-up to twenty fold- antibacterial effect against Staphylococcus aureus. It is shown that the higher antibacterial efficiency of PY is related to a greater release compared to TA even if in solution the antibacterial effect of PY is lower than that of TA when reported on the basis of the molar concentration in PY units.

This paper discusses the thermal buckling analysis of composite plates and sandwich panels by means of a Ritz-based variable-kinematic formulation. Main feature of the proposed formulation consists in the representation of the structure by means of sublaminates, i.e. arbitrary groups of plies composing the panel. Each sublaminate is associated with an independent, arbitrary kinematic description, so that the use of refined, high-order theories can be restricted to specific regions, such as the core of sandwich panels. Monolithic plates can be studied as a special case where the structure is modeled using only one sublaminate. Presented are the critical temperatures, with and without accounting for the nonlinear pre-buckling effects, for a set of monolithic and sandwich configurations. When pre-buckling effects are neglected, the problem is solved as a standard eigenvalue problem. On the other hand, the introduction of pre-buckling effects leads to a nonlinear eigenvalue problem, which is solved with an iterative procedure. The results are validated against 3D solutions, and highlight the importance of accounting for pre-buckling deformations, especially in the case of sandwich panels. As demonstrated, high-fidelity predictions are obtained while keeping at minimum the amount of degrees of freedom.

International audience; Laue microdiffraction, available at several synchrotron radiation facilities, is well suited for measuring the intragranular stress field in deformed materials thanks to the achievable submicrometer beam size. The traditional method for extracting elastic strain (and hence stress) and lattice orientation from a microdiffraction image relies on fitting each Laue spot with an analytical function to estimate the peak position on the detector screen. The method is thus limited to spots exhibiting ellipsoidal shapes, thereby impeding the study of specimens plastically deformed. To overcome this difficulty, the so-called Laue-DIC method introduces digital image correlation (DIC) for the evaluation of the relative positions of spots, which can thus be of any shape. This paper is dedicated to evaluating the accuracy of this Laue-DIC method. First, a simple image noise model is established and verified on the data acquired at beamline BM32 of the European Synchrotron Radiation Facility. Then, the effect of image noise on errors on spot displacement measured by DIC is evaluated by Monte Carlo simulation. Finally, the combined effect of the image noise, calibration errors and the number of Laue spots used for data treatment is investigated. Results in terms of the uncertainty of stress measurement are provided, and various error regimes are identified.

This work addresses theoretical and finite element investigations of functionally graded piezoelectric materials (FGPMs) for shunted passive vibration damping of laminated composite beams. The properties of piezoelectric patches are assumed to vary through the thickness direction following the exponent or power law distribution in terms of the volume fractions of the constituent materials. By employing Hamilton’s principle, the governing differential equations of motion are derived. The resulting system of equations of vibration is solved by employing an efficient three-nodded beam element which is based on a refined sinus piezoelectric model. The effects of effective electromechanical coupling coefficients (EEMCCs), different electric shunt circuits and different material compositions on the shunted damping performance are investigated. The optimal values of the electric components belonging to each shunt circuit are numerically determined.

A BCC polycrystalline single phase Armco iron was tested in the Very High Cycle Fatigue (VHCF) regime using plate specimens and a load ratio of R = −1. Throughout testing until fracture, optical observations of the specimen surface (on one side) and temperature measurements and recordings (on the other specimen side) were performed. After failure, the fracture surfaces were observed via SEM. The macroscopic observations showed an early occurrence of Persistent Slips Bands (PSBs). The observations of fracture surfaces clearly revealed an initiation and propagation stage. In the initiation stage, which ranges from about 200 to 550 μm, the grain traces are visible with intergranular, transgranular and mixed fracture. The comparison with the thermal recordings leads to the conclusion that the crack initiation stage is completed near the end of the fatigue life. Interrupted tests show damage in the bulk specimen before the crack initiation.

Metals used in industry for structures and aero-engine components are sometimes subjected to very high cycle fatigue (VHCF) damage during their working service. In this paper, a novel method is presented to determine the size and location of a circular crack located within a metal specimen under 20 kHz VHCF loading conditions. The method is based on an analysis of the temperature rise on the surface of the specimen and correlation of this temperature rise to the energy dissipation in the plastic zone of the crack. The approach taken is to first determine the heat source location and strength using an inverse heat transfer calculation based on the surface temperature measurements. Next, the relationship between the heat and the material hysteresis loop in the plastic zone, which is a function of stress intensity factor and vibration amplitude cyclic loading of the specimen, is found. The calculation of the stress intensity factor under vibrational loading is often an obstacle in VHCF research because there is currently no standard or existing formula. In this paper a general polynomial formula for the stress intensity factory under 20 kHz loading conditions is obtained using a finite element modeling approach as a function of the specimen’s material properties and position and size of the internal crack.

International audience; In this paper, the large amplitude free flexural vibration characteristics of fairly thick and thin functionally graded graphene platelets reinforced porous curved composite beams are investigated using finite element approach. The formulation includes the influence of shear deformation which is represented through trigonometric function and it accounts for in-plane and rotary inertia effects. The geometric non-linearity introducing von Karman’s assumptions is considered. The non-linear governing equations obtained based on Lagrange’s equations of motion are solved employing the direct iteration technique. The variation of non-linear frequency with amplitudes is brought out considering different parameters such as slenderness ratio of the beam, curved beam included angle, distribution pattern of porosity and graphene platelets, graphene platelet geometry and boundary conditions. The present study reveals the redistribution of vibrating mode shape at certain amplitude of vibration depending on geometric and material parameters of the curved composite beam. Also, the degree of hardening behaviour increases with the weight fraction and aspect ratio of graphene platelet. The rate of change of nonlinear behaviour depends on the level of amplitude of vibration, shallowness and slenderness ratio of the curved beam.

In this paper, the bending, vibration and buckling characteristics of functionally graded porous graphene-reinforced nanocomposite curved beams are studied based on a trigonometric shear deformation theory. The effect of various theories deduced from the proposed formulation on the static and dynamic behavior of curved nanocomposite beams is also studied. The governing equilibrium equations are formed by applying Lagrangian equations of motion coupled with the finite element approach employing a 3-noded C1 continuous curved beam element. The methodology developed here is tested for problems having known solutions in the open literature. A detailed investigation involving various parameters such as coefficient of porosity, type of distribution pattern for the porosity and graphene platelets, radius of curvature of curved beam, length-to-thickness ratio, the platelet geometry, and boundary conditions on the static bending, free vibration and elastic stability behavior of nanocomposite curved beams is conducted. New results for certain boundary conditions of graphene reinforced curved beams are presented. Participation of various types of in-plane and transverse bending modes responsible for yielding the lowest critical buckling loads/natural frequencies are also highlighted.

International audience; Steel-concrete-steel (SCS) composite structures are modular structures combining two steel plates in the center of which a core of concrete is poured. This material presents the advantages of reinforced concrete while offering higher tightness, durability and strength under some extreme solicitations [1]. The efficiency of this structure is essentially based on the connection system between steel plates and concrete, which is performed by steel ties or dowels. Usually designed to assume a full composite action, in some cases (connector failure, construction choice, etc.), this connection may not be sufficient, which leads to the apparition of a partial composite action between steel plates and concrete core. This study focuses on the modelling of the behavior of SC structures in the case of a full or a partial composite action. This involves assessing the influence of simulation hypothesis on the global (force-displacement) and local (mechanical degradation, failure mode) behaviors and proposing a simulation methodology adapted in both cases. From the simulation of two three point bending beams [2], it is especially demonstrated that the interface conditions strongly influence the simulation results. If a “perfect” (“no-slip”) relation between steel plates and concrete is sufficient for a high number of connectors, an additional methodology is necessary when a partial composite action is expected. The methodology is presented and validated by a comparison to the experimental results.

This work compares the thermal responses of an α-iron and two carbon steels (C12 and C65 steel) via in-situ infrared thermography under 20 kHz ultrasonic tension-compression fatigue testing. The α-iron has only ferrite phase, and C12 and C65 steels have ferrite-pearlite dual phases. During a series of interrupted self-heating tests, the surface temperature of the three materials increases gradually with stress amplitude, while the relation is non-linear. The increase of carbon content, inducing the pearlite phase percentage increasing, results in a lower temperature increase and higher fatigue strength. For the three materials, at a certain stress amplitude level, the specimen temperature suddenly increases to hundreds of degrees. Moreover, the dissipated energy per cycle is estimated based on the 1D heat diffusion equation and a power function between dissipated energy per cycle and stress amplitude is found. Finally, the mechanisms driving the observed thermal responses are discussed.

A simple 1D nonlinear finite element model is presented for the analysis of composite foam-insulated concrete sandwich panel (FICSP) structures. The material nonlinearities of concrete wythes, steel rebar and insulation layer are considered. By employing a sinus shear deformation beam theory, the effects of transverse shear stress in concrete wythes and insulation layer are incorporated in the presented finite element formulation . The present formulation takes into account the partially composite action of FICSP structures by using the concept of distributed spring layer model. Full-scale experimental data and 3D finite element results are used to validate the developed finite element formulation. The obtained numerical results demonstrate that the presented finite element model of this study is an efficient tool to predict the detailed structural response of FICSPs till collapse stage.

In this work, the modeling of laminated composite and sandwich beams with a generic cross-section is performed through a variable separation approach. For this purpose, the displacement is approximated as a sum of separated functions of the cross-section coordinates y , z and the axial coordinate x. This choice yields to a non-linear problem that can be solved by an iterative process. This latter consists of solving a 2D and 1D Finite Element problem successively at each iteration. Numerical examples involving several representative sandwich and laminated beams are addressed to show the accuracy of the present method. It is shown that it can provide 3D results and capture local effects for various types of cross-section.

(1) Background: This study investigated the effect of the adhesive layer thickness and the length of resin tags on dentin bond strength of five universal adhesives applied in self-etch mode. (2) Methods: One hundred and fifty extracted human third molars were used. Five different universal adhesives were applied in self-etch mode on the dentin surface. Half of the specimens were subjected to an aging procedure for six months. A shear bond strength (SBS) test was performed and the results were statistically analyzed with a t-test and one-way ANOVA test. Scanning electron microscopy (SEM) was executed to measure the adhesive layer thickness and tag depth. (3) Results: No statistical differences were found between the five adhesive systems after a 24 h storage period, regardless of layer thickness and tag depth (p &lt, 0.05). After 6 months of aging in water at 37 °C, Iperbond Max and Scotchbond Universal preserved the bond strength over time (p &lt, 0.05), whilst the SBS of Iperbond Ultra, FuturaBond M+, and Ibond Universal decreased significantly after the aging period. No relation was observed between the adhesive thickness or tags’ length on SBS. (4) Conclusions: Within the limitation of this study, the stability over time of the bond strength of universal adhesives depends on their compositions regardless of the adhesive layer thickness and/or tags’ length.

International audience; In recent years, metasurfaces have shown extremely powerful abilities for manipulation of electromagnetic waves. However, the local electromagnetic response of conventional metasurfaces yields an intrinsic performance limitation in terms of efficiency, minimizing their implementation in real‐life applications. The efficiency of reconfigurable metasurfaces further decreases because of the high density of meta‐atoms, reaching 74 meta‐atoms per λ2 area, incorporating lossy tunable elements. To address these problems, strong electromagnetic non‐local features are implemented in a sparse metasurface composed of electronically reconfigurable meta‐atoms. As a proof‐of‐concept demonstration, a dynamic sparse metasurface having as few as 8 meta‐atoms per λ2 area is experimentally realized in the microwave domain to control 2D wavefronts in both near‐field and far‐field regions for focusing and beam‐forming, respectively. The proposed metasurface with its sparsity not only facilitates design and fabrication, but also opens the door to high‐efficiency real‐time reprogrammable functionalities in beam manipulations, wireless power transfer, and imaging holography.

In this paper, a finite element based on the variables separation is presented for the thermo-mechanical analysis of bi-dimensional laminated beams. Both the temperature and displacement fields are approx-imated as a sum of separated functions of x (axial coordinate) and z (transverse coordinate). This choice yields to an iterative process that consists of computing a product of two one-dimensional functions at each iteration. A fourth-order expansion with respect to the thickness direction are considered. The capa-bility and the behavior of the presented approach are shown on various laminated and sandwich beams submitted to different thermal boundary conditions. Thermal and mechanical responses are assessed by comparing with other approaches available in literature, exact solutions and 2D finite element computation.

International audience; This paper presents the application of a new steel-concrete bond model on a reinforced concrete shear wall, experimentally tested during the French National Project CEOS.FR. The proposed results include both global (evolution of the force as a function of the displacement for example) and local results (crack opening and spacing). A new post-processing method to compute these local properties even in a case of a complex crack pattern (oriented cracks for example) is proposed. It is based on the change in the sign of the bond slip between steel and concrete. The simulated results are in a good agreement with the experiment and validate the developments. Finally, the interest of including a specific steel-concrete bond model in the finite element simulation is highlighted, compared to classical "no-slip" relation.

International audience; In the present study, the nonlinear flexural bending of thick and thin porous curved composite beams reinforced and functionally graded by graphene platelets is carried out using a three-noded C1 continuous curved beam finite element developed introducing an efficient shear deformation theory based on trigonometric function. The nonlinearity through the strain–displacement relationship by introducing von Karman’s assumptions is considered. The nonlinear equilibrium equations resulting from minimum potential energy principle are numerically solved based on the direct iteration technique. The bending nonlinear features through the load–deflection relationship are presented by selecting various design parameters like long and short beams, shallow and deep curved cases, support conditions, the variation of graphene platelets in the metal foam and the existence of porosity. This investigation reveals that the level of nonlinearity gets noticeably affected by the depth coupled with the curved beam slenderness ratio.

International audience; In this study we elaborate on the recent concept of metagratings proposed in Ra'di et al. [Phys. Rev. Lett. 119, 067404 (2017)] for efficient manipulation of reflected waves. Basically, a metagrating is a set of 1D arrays of polarization line currents which are engineered to cancel scattering in undesirable diffraction orders. We consider a general case of metagratings composed of N polarization electric line currents per supercell. This generalization is a necessary step to totally control diffraction patterns. We show that a metagrating having N equal to the number of plane waves scattered in the far-field can be used for controlling the diffraction pattern. To validate the developed theoretical approach, anomalous and multichannel reflections are demonstrated with 3D full-wave simulations in the microwave regime at 10 GHz. The results can be interesting for the metamaterials community as allow one to significantly decrease the number of used elements and simplify the design of wavefront manipulation devices, what is very convenient for optical and infra-red frequency ranges. Our findings also may serve as a way for development of efficient tunable antennas in the microwave domain.

International audience; Two strain localization modes: the Piobert-Lüders band propagation and the development of necking, were investigated in uniaxial tensile tests for a low alloyed and low carbon steel. These two macroscopic localization phenomena were simultaneously monitored by speckle interferometry (ESPI) and acoustic emission (AE). The coupling of these two experimental techniques gives complementary information about strain localization features and mechanisms. For Lüders bands, it was found that the acoustic activity heard during the travel of the Piobert-Lüders band varies in closely correlated to the tensile force fluctuations, the relations between strain rate, band velocity, band width and plastic strain were investigated. Although the strain rate in the wake of the wave front is not always zero, the acoustic activity remains concentrated in the wave front itself. For necking, the acoustic activity is found to decrease regularly through the homogeneous plasticity stage and the diffuse necking stage and then increases when the localized necking starts, while ESPI patterns show a gradual strain concentration.

Article number: 199; International audience; Aromatic molecules are promising candidates for using as a fluorescent tracer for gas-phase scalar parameter diagnostics in a drastic environment like engines. Along with anisole turning out an excellent temperature tracer by Planar Laser-Induced Fluorescence (PLIF) diagnostics in Rapid Compression Machine (RCM), its fluorescence signal evolution versus pressure and temperature variation in a high-pressure and high-temperature cell have been reported in our recent paper on Applied Phys. B by Tran et al. Parallel to this experimental study, a photophysical model to determine anisole Fluorescence Quantum Yield (FQY) is delivered in this paper. The key to development of the model is the identification of pressure, temperature, and ambient gases, where the FQY is dominated by certain processes of the model (quenching effect, vibrational relaxation, etc.). In addition to optimization of the vibrational relaxation energy cascade coefficient and the collision probability with oxygen, the non-radiative pathways are mainly discussed. The common non-radiative rate (intersystem crossing and internal conversion) is simulated in parametric form as a function of excess vibrational energy, derived from the data acquired at different pressures and temperatures from the literature. A new non-radiative rate, namely, the equivalent Intramolecular Vibrational Redistribution or Randomization (IVR) rate, is proposed to characterize anisole deactivated processes. The new model exhibits satisfactory results which are validated against experimental measurements of fluorescence signal induced at a wavelength of 266 nm in a cell with different bath gases (N2, CO2, Ar and O2), a pressure range from 0.2 to 4 MPa, and a temperature range from 473 to 873 K.

International audience; The main contribution of this study is to propose time-resolved measurements to determine temperature with a novel source of continuous excitation for an induced fluorescence technique with laser diagnosis based on tracer-induced fluorescence, which has become a major tool for experimental studies of fluid dynamics in reaction flows. We use a Hg (Xe) arc lamp as a continuous light source that has a wide range of emissions in wavelength. With this setup, one can reach high spatial and temporal resolution (temperature, pressure, species concentration, and velocity) to acquire quantitative data for the control of fluid thermal systems, such as engines, combustion chambers, furnaces, and reactors. A fluorescence study was performed on various tracers and their configurations. We focus on an anisole tracer using a broad wavelength of excitations. We propose a calibration to achieve temperature measurements in the range of 493–773 K and from 0.2 to 3.5 MPa of pressure. The temperature-dependent fluorescence is based on a two-line technique. The results give a better understanding of the influence of temperature and pressure in a nitrogen bath gas on the fluorescence photophysics in the UV domain. High temporal resolution was acquired using a high-speed intensified camera setup. The application of the photomultipliers manages the time-scale evolution of the flow in continuous emission and this eliminates the signal-to-noise ratio impact.

In this work, a method to compute explicit thermal solutions for laminated and sandwich beams with arbitrary heat source location is developed. The temperature is written as a sum of separated functions of the axial coordinate x , the transverse coordinate z and the volumetric heat source location x 0. The derived non-linear problem implies an iterative process in which three 1D problems are solved successively at each iteration. In the thickness direction, a fourth-order expansion in each layer is considered. For the axial description , classical Finite Element method is used. The presented approach is assessed on various laminated and sandwich beams and comparisons with reference solutions with a fixed heat source location are proposed. Based on the accurate results of the thermal analysis , thermo-mechanical response is also addressed using also a separated representation.

Static and dynamic behaviour of a beam made of functionally graded piezoelectric material (FGPM) is studied. The material properties of the PZT4/aluminium/FGPM are graded in the thickness direction according to a fraction volume power law distribution. Top and bottom external surfaces are made of pure PZT4 and the mid plate of pure aluminium. The percolation phenomenon is taken into account. A FGPM beam finite element based on Timoshenko’s hypothesis and layerwise approximation is used. In static simulations both actuator and sensor efficiencies are studied. In dynamic simulations active vibration control is performed using a linear quadratic regulator (LQR) method with an observer. Results show the effectiveness of this smart FGPM beam and also the significant effects of the volume fraction index, the percolation threshold and the electrodes’ location used for actuation and sensing.

International audience; Multiscale models involving crystal plasticity are essential to predict the elastoplastic behavior of structural materials with respect to their microstructure. However, those models are often limited by a poor knowledge of the local constitutive behavior. This article reports a method to measure the mechanical behavior directly, at the micrometre scale, in bulk crystalline materials. Local strain and stress states were evaluated at the surface of a bent stainless steel crystal by combining total strain measurements – performed with the digital image correlation technique on optical images – with elastic strain measurements obtained by Laue microdiffraction. A local constitutive relation was measured, in an efficient nondestructive way, without the need for full-field simulations. The method was validated by a comparison between the measured local behavior and the macroscopic behavior of the single crystal.

Titanium alloy TC4 is widely used in aeronautics applications where it is subjected to high frequency fatigue loads. Tests are performed to investigate the alloy fatigue behavior sustaining ultrasonic fatigue load in Very High Cycle Fatigue (VHCF) regime. Thermal dissipation for the alloy in 20 kHz frequency is studied and a model is proposed to describe the temperature increment in the framework of thermodynamics by estimation of the anelastic and inelastic thermal dissipation at microscopic active sites in the reference element volume. The failure probability prediction method is used to evaluate the VHCF dispersion based on the two scale model and fatigue thermal dissipation analysis.

Localization that occurs at the end of the tensile test of a ductile 316L stainless steel has been followed in detail by in-plane Electronic Speckle Pattern Interferometry (ESPI). A global description of the whole strain-rate field with an analytical function and physical descriptors such as band width, band inclination and maximum strain rate is proposed. The description with two straight bands of constant amplitude along the width of the specimens is valid from the beginning of the diffuse necking to the fracture of the specimens. It allows distinguishing between two localization scenarii which occur for specimens with a different width to thickness ratio, one with a fracture inclined along the width and the other with a fracture inclined in the thickness. For the former, the two bands keep a constant angle while for the latter, the two bands rotate progressively until they become perpendicular to the tensile direction. The bandwidth can be defined and monitored during the whole necking evolution.

International audience; In this study, the first signs of fatigue crack initiation are studied in the HCF domain on a body centered cubic Armco iron (with 80 ppm of carbon content). Some tests are performed on plate specimens through a piezoelectric fatigue machine. During fatigue tests, the deformation evolution is observed by optical microscope, and the temperature recording on the specimen surface is achieved by an infrared focal plane array camera. From the temperature recording, the intrinsic dissipation field is calculated using a local expression of the heat diffusion equation. The results show that above a given stress level, Slips Marks can be clearly observed on the surface specimen, and related to the intrinsic dissipation distribution. Observations through a Scanning Electron Microscope on the specimen surface and the fracture surface are related to stage I and stage II of fatigue damage.

In this paper, we propose the design of multilayer frequency selective surfaces (FSS) waveguide band-pass fllters (WBPF). The WBPFs are designed to operate at two difierent frequency channels, respectively 71{76GHz (Rx) and 81{86GHz (Tx). The cross section surface of the FSS is imposed by the WR12 waveguide rectangular section's dimensions. The WBPFs are inserted symmetrically in a T-junction waveguide to design a compact diplexer. This is a basic component developed for an e-cient integration in the future E-band millimeter-wave transceiver. The multilayer FSS structure uses only non-resonant sub-wavelength unit cell elements; metallic patch and slot. To reach high channel isolation (… 70dB) a seven order fllter was required. Hence, each fllter is composed of 13 capacitive and inductive metallic FSS spaced by 12 ultra-thin dielectric substrate layers. The dielectric material is Rogers Ultralam 3850 (Liquid Crystalline Polymer; LCP circuit material). The fllter's overall thickness is < ‚=4. The numerical studies have been performed using flnite element method simulator (HFSS) and CST Studio Suites Tools. The experimental validation has been also done in the X band frequency by developing a flfth order FSS WBPF. Good agreements between simulated and measured results are obtained.

Effect of stress ratio on fatigue properties of a titanium alloy (TC-17) in the high cycle fatigue (HCF) and very high cycle fatigue (VHCF) were investigated by electromagnetic and ultrasonic fatigue testing. The S – N curves at R = −1, 0.1, 0.5 and 0.7 at 110 Hz and 20 kHz were obtained and discussed. The effects of frequency on fatigue strength was also investigated. It was concluded that the fatigue strength with 50% fatigue failure probability at R = 0.1, 0.5 and 0.7 is lower to the Goodman line and shows a bilinear decreasing trend. Cleavage fracture of primary grains in the surface and interior initiation zone were observed. The formation of the facets induced by the basal or prismatic slips of the H.C.P grains decreased the fatigue strength with variation in mean stress.

The objective of this paper is to study the gigacycle fatigue behavior of an austenitic steel at temperatures of 600 °C and 700 °C under fully reverse loading (R=−1). Numerical simulation by finite element method (FEM) was used to design the specimens and to analyze the effects of the variation in the dynamic Young modulus with temperature from measurements of the ultrasonic resonance frequency. Finally, new stress-life curves for this material are presented for a lifetime range from 105 to 109 cycles at room temperature, 600 °C and 700 °C.; International audience; The objective of this paper is to study the gigacycle fatigue behavior of an austenitic steel at temperatures of 600 °C and 700 °C under fully reverse loading (R=−1). Numerical simulation by finite element method (FEM) was used to design the specimens and to analyze the effects of the variation in the dynamic Young modulus with temperature from measurements of the ultrasonic resonance frequency. Finally, new stress-life curves for this material are presented for a lifetime range from 105 to 109 cycles at room temperature, 600 °C and 700 °C.

The concept of transformation optics (TO) is applied to control the flow of electromagnetic fields between two sections of different dimensions through a tapering device. The broadband performance of the field taper is numerically and experimentally validated. The taper device presents a graded permittivity profile and is fabricated through three-dimensional (3D) polyjet printing technology using low-cost all-dielectric materials. Calculated and measured near-field mappings are presented in order to validate the proposed taper. A good qualitative agreement is obtained between full-wave simulations and experimental tests. Such all-dielectric taper paves the way to novel types of microwave devices that can be easily fabricated through low-cost additive manufacturing processes.

The aim of the present contribution is to show that metasurfaces such as reactive impedance surfaces (RIS) and artificial magnetic conductors can be efficiently used in the design of low-profile circularly polarized L-band antennas. We present the design and simulation of the compact and low-profile antennas. The solution based on RIS will be compared to a circularly polarized microstrip patch antenna using the same materials in order to prove the benefit of metasurfaces. The engineered metasurfaces allow increasing the bandwidths with few modifications on the thickness and the overall antenna size.

International audience; The dynamic strain aging (DSA) phenomenon that occurs in some materials under certain temperature and strain rate conditions can cause plastic strain localization in the form of Portevin-Le Chatelier (PLC) bands. Carbon-manganese steels are used commonly and frequently in construction because of their ductility, low cost and ability to form mechanically. In these steels, the DSA phenomenon occurs for common quasi-static strain rates from 150 to 300 °C, which makes band observation complicated. PLC bands on a carbon-manganese steel that was sensitive to DSA were studied using an infrared camera. Specimen heating was achieved using an induction furnace (with an adapted coil inductor), which allows for temperature recording during tensile tests. Thermography with an infrared camera was used to estimate the band characteristics and increments in band plastic strain, which is an important parameter for material behavior identification necessary for DSA phenomenon modeling. This technique had been developed only for PLC phenomenon observation at ambient temperature on aluminum alloys. Band characteristics on the carbon-manganese steels have been compared with results obtained previously on aluminum alloys.

International audience; For basic observations or for industrial applications it is of interest to use flat specimens at very high frequency in the gigacycle regime. In this work, thin flat sheet, with 1.2 mm thickness of a complex phase ferrite-martensitic steels were considered for carrying out fatigue tests at high frequency (20 kHz) up to the gigacycle regime (>109 cycles). The crack initiation tests were carried out with water cooling, while the crack growth test were carried out in laboratory air at room temperature. All the tests were carried out under loading ratio R=-1. To do that, special designs of specimens were made and computed using FEM for defining the stress amplitude for endurance tests. Special attachments for specimens to the ultrasonic system’s horn were enhanced. A particular FEM computing of the stress intensity range on crack growth specimens was carried out for determining the specimen dimensions and an equation that defines the stress intensity range as a function of the harmonic displacement amplitude, dynamic Young’s modulus, material density and crack length. Detailed procedures and fatigue results are presented in this paper.

Ultrasonic fatigue tests were performed on friction stir welded AA6061 joint to investigate very high cycle fatigue (VHCF) behaviors. As a result, almost all the fatigue cracks are initiated from local plastic slip markings around the boundary between thermo-mechanically affected zone and heat affected zone. The fatigue strength decreases from the top to root of the welded joint, owing to the variation of plastic deformation history and temperature distribution through the thickness. In fractography, the fatigue crack initiation site is surrounded by a semicircular flat zone, of which the formation in VHCF regime accounts for more than 98% of the total fatigue life.

International audience; Determining the accuracy of elastic strain measurements in plastically deformed alloys is an experimental challenge. To develop a novel cross-validation procedure, a controlled elasto-plastic strain gradient was created in a stainless steel single crystal by four point bending deformation. The corresponding elastic strain field was probed, with an intragranular spatial resolution, in-situ by Laue microdiffraction and ex-situ by High Resolution EBSD. Good agreement is found for the two independent measurements and the predictions of a mechanical model, at plastic strains below 0.5 %. The accuracy of the measurements is estimated at 3.2 × 10 − 4 .

In this work, the modeling of laminated composite plates is performed through a variables separation approach based on a Reissner's Variational Mixed Theorem (RMVT). Both the displacement and transverse stress fields are approximated as a sum of separated functions of the in-plane coordinates x , y and the transverse coordinate z . This choice yields to a non-linear problem that can be solved by an iterative process. That consists of solving a 2D and 1D problem successively at each iteration. In the thickness direction, a fourth-order expansion in each layer is considered. For the in-plane description, classical Finite Element method is used. Numerical examples involving several representative laminates are addressed to show the accuracy of the present LayerWise (LW) method. It is shown that it can provide quasi-3D results less costly than classical LW computations. In particular, the estimation of the transverse stresses which are of major importance for damage analysis is very good.