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International audience; As the world’s population grows, current agricultural activities are putting enormous strain on the climate, including deforestation, excessive water use, and pesticide runoffs. These stresses are expected to increase in the coming future as more food products are needed. One promising method to reduce the adverse impact on the environment is greenhouse-based farming, which can enhance land-use efficiency and water consumption. The major limitation of the greenhouse is the high energy demand in a constrained space. Solar energy has shown a promising approach for integration with agricultural greenhouses in recent years. The developments of greenhouses integrated with various solar energy technologies including photovoltaic (PV), photovoltaic-thermal (PVT), and solar thermal collectors are discussed in this chapter. The results indicated that PV modules installed on greenhouse roofs or walls cause shading and, in some cases, have affected the growth trend of cultivated crops inside. The PVT modules are more efficient at producing both heat and electricity, while higher performance values are reported for greenhouses integrated with solar thermal collectors especially in moderate climate conditions. Additionally, storing thermal energy has shown an improvement in the profitability of solar greenhouses by extending the period of solar heat availability. Also, the most commonly used mathematical models describing the thermal behavior of solar greenhouses as well as their economics are explored and discussed. In the end, various commercial solar greenhouse implemented around the world are introduced.

International audience; This study is dedicated to the effect of surface defects on the fatigue behaviour of high strength steels and the evaluation of their sensitivity. For that purpose, defects were introduced by Electro-Discharge Machining (EDM) on the edge of sheet metal test samples. Different defect depths are tested on high strength steel grades. A stress-based multiaxial fatigue life prediction method is then developed to assess the fatigue life. Input data are cyclic stress tensor states at the notch root that are calculated by a FEM analysis using the open-source Salome-meca software. In fact elastic-plastic numerical simulations were performed for each defect geometry to determine stresses distribution around the defect. A new class of multiaxial fatigue criteria extended from classical formulation to new ones with stress gradient terms related to the normal stress component is formulated. The Fogue integral approach based criterion is used since it is proved to give pertinent prevision under rotating principal stress directions multiaxial loading. For any material plane P the local stress gradient in its normal direction is calculated by parabolic interpolation and integrated to the normal stress component within the multiaxial fatigue criterion. For investigated multiaxial criteria, accounting for stress gradient effect is assessed from the experimental campaign results on two very high strength steel grades with occurence of surface defects

The work presented in this paper deals with the fatigue damage assessment of bolted joints. The influence of the preload is particularly underlined as it is shown that it strongly improves the fatigue resistance of the bolt whatever the geometrical parameters are. A 3D tee-stub model with the bolt submitted to preload force is simulated by using the Salome-Meca FEM software. A parametric study is also carried out to analyse the influence of the bolt location m and the column flange thickness tf onto the bolt loading. In order to analyse the effect of the stress concentration generated by the head-shank transition of the screw, a second 3D finite bolt element model is developed. This allows to exhibit multiaxial stress states in the vicinity of the fillet radius. Finally, a multiaxial fatigue post-treatment tool has been implemented on Matlab software for damage assessment purpose. Two multiaxial fatigue criteria approaches contribute to this tool and may be used for fatigue behaviour prediction. The so-called critical plane approach (Dang Van criterion) and the integral approach (Zenner criterion) may consequently be compared for that analysis.

International audience

Structural integrity of aircraft, nuclear power plants, space rockets, ships, automotive structures, biomedical devices, and many other applications, is a major design purpose and concerns various components subjected to cyclic loads for which fatigue is generally the dominant factor leading to its failure. The fatigue damage cumulation phenomenon is a process which may lead to cracks initiation and possibly to the structure failure under the action of variables stresses cycles. The mechanical components are generally subjected to multiaxial stress states. Taking into account this stress states triaxiality generally deals with multiaxial fatigue criteria. They are suitable tools for assessing the material fatigue resistance against periodical stress states, especially when they are multiaxial. The study carried out in this paper aims to analyze the fatigue behaviour of pre-stressed bolts involved in a so-called tee-stub bolted assembly that is subjected to variable loads. A multiaxial fatigue post-processing tool using two multiaxial fatigue approaches (integral approach and critical plan approach) is developed and implemented on Matlab software in order to assess the bolt fatigue damage and then its fatigue life through an iterative process. The tool is validated by fatigue test results on bolted assemblies found in the literature, which are additionally compared with those obtained by standards (Eurocode, VDI). A parametric study on the tee-stub is then performed.

Candidate materials for thermal insulation combine a solid phase of low thermal conductivity with porosity. Very porous ceramics based on a commercial clay containing 79% kaolinite were made by a direct foaming method using methylcellulose, drying at 70 °C before a consolidation at 1100 °C. The pore volume fraction ( v p ) was varied in the kaolin-based foam by modifying the amount of clay incorporated in the starting mixture. Measurements of the thermal conductivity with the hot disk method revealed a decrease from 0.23 W m−1 K−1 at v p = 0.57 to 0.054 W m−1 K−1 at v p = 0.95 in close agreement to predictions by the Hashin–Shtrikman upper bound, a cubic pore model and numerical simulation of an artificial microstructure representing the polyhedral pore shape with Voronoi mosaics. The effective Young's modulus, obtained from mechanical compression tests, also decreases with pore volume fraction as described by Ashby's relation. However the mechanical strength was sufficient for handling even the more porous kaolin-based foams.

International audience; At the present time, the insertion of radio frequency microelectromechanical switches into real architecture requires reduced actuation voltages, reduced dimensions, and better control of the electrical and electromechanical behavior that gives more importance to surface effects, their understanding, and modeling. The use of such devices requires the development of methods for estimating the contact performances as a function of surface roughness, contact materials, and contact topologies. With increase in computation capabilities, the rough surface topography can be implemented in the finite element model but implies long calculation times or even calculation overloading if a high definition of the roughness is desired. To reduce these limitations, assumptions on the microgeometry are required. This paper treats, by use of finite element modeling, the influence of the definition of roughness of contacting switch members on the electrical contact resistance of resistive switches, and investigates the error introduced by using a minimum defined atomic force microscope sampling interval of 10 nm. The present numerical analysis is implemented for switch test structures.

International audience; Spot-weld joints are commonly used to fasten together metal sheets in automotive industry. The car frame used in Renault vehicles is a representative example of the usefulness of this method. Most of the spot-welds experience fatigue damaging occurrence due to rough roads or driving conditions which apply periodical overloads to the vehicle. Understanding their fatigue behaviour is crucial from the viewpoint of failure prevention in design. In this study, a series of experiments was conducted to study the fatigue failure of spot-welded tensile-shear specimens made of a deep-drawing steel (XES) and High strength low alloy steels (HE360D and XE360D). Two different types of fatigue tests were performed, the first one with a constant-amplitude sinusoidal loading (loading ratio equal to 0.1) and the second one with one incidental overload cycle introduced per 100 cycles. The experimental results show a favourable effect of overloads for HSLA steel specimens, whereas the effect is the opposite for XES steel specimens. A finite element analysis was carried out using the open-source Salome-Meca platform to determine the stress states within the specimens around the weld spot and explains both failure modes observed on the specimens at high and low loads.

New high strength steels are widely used nowadays in many industrial areas as in automotive industry. These steels are more resistant and provide higher fatigue limits than latter ones but they are also more sensible to small defects. Natural defects that outcome from metallurgy (as shrinkage, inclusion, void) are not considered in this study. We focus on small manufacturing defects such as cutting edge defects generated by punching or other surface defects due to stamping. These defects are harmful on the material fatigue behaviour due to high stress concentration at defects root. They also generate stress gradient that is beneficial from the fatigue strength point of view. This study focusses on the stress gradient (it does not account for the size effect) from cylindrical defect on specimen edge. Practically a normal stress gradient is added in multiaxial fatigue criteria formulation. Both critical plane approach and integral approach are involved in the present study. This gradient is calculated from stress states at defects root by using FEM. Criteria fatigue function at N cycles is used to assess the material fatigue strength. Obviously multiaxial fatigue criteria accounting for stress gradient give more precise fatigue functions than criteria that do not consider the gradient influence.

RF MEMS switches with piezoelectric actuation permits us to obtain a reliable switch relative to electrostatic actuated switches since the dielectric charging is reduced and fast respond. The problem here is the deformation induced by the actuation is transmitted to the capacitor plate and by consequences; we lose in the value of capacitance in the down state due to the air gap. A study of the air gap effect on the capacitance value in the down state is done in the first part, showing the importance of flatness in capacitor plate designs. The design that is described in this paper, deals with the point of the application of the force in order to flatten the contact between the electrodes forming the capacitor. An analytical model describing the deformation of the switch was developed. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

International audience; This study was conducted on the both solid and solubilized chitosans to propose an approach for the physico-chemical, thermal and mechanical characterizations of this polysaccharide. The polysaccharide used was a 90% deacetylated chitosan having a molecular weight of 98.4 kDa. The flow property of chitosan solutions was evaluated revealing a shear-thinning behavior. The thermal characterization was carried out by studying heat specific capacity, glass transition temperature, and thermal conductivity on chitosan dried specimens (solid state). Their Tg were measured by DSC and confirmed by DMA at 102 and 122°C depending on concentrations of initial chitosan solutions. The mechanical characterization was conducted by analyzing Young modulus, tensile strength, and elongation at break of chitosan specimens. They exhibited a higher elongation at break and a lower tensile strength when made from high concentrated chitosan solution (9% w/v). Differences in mechanical behavior of specimens were explained by differences of crystallinity.

International audience; A clay-based material exhibiting high pore volume fraction andlow thermal conductivity suitable for thermal insulation isdescribed. Starting with a commercial clay containing >75% kaolinite,foams were made by mixing in water and methyl celluloseas a surfactant then beating. After drying at 70°C, the pore volumefraction >94% remains almost constant for treatments upto 1150°C. In contrast, the phases constituting the solid skeletonevolve strongly with removal of surfactant, dehydroxylation ofkaolinite, and formation of mullite. The latter leads to greatermechanical strength but also an increase in thermal conductivity.Thermal treatment of the kaolin foam at 1100°C yields a suitablecompromise between low thermal conductivity of 0.054W.(m.K) – 1 at room temperature with a compressive yield stressof 0.04 MPa. The radiation component in the effective thermalconductivity is 50% at 500°C

International audience; Incorporation of porosity into a monolithic material decreases the effective thermal conductivity. Porous ceramics were prepared by different methods to achieve pore volume fractions from 4 to 95%. A toolbox of analytical relations is proposed to describe the effective thermal conductivity as a function of solid phase thermal conductivity, pore thermal conductivity, and pore volume fraction (νp). For νp < 0.65, the Maxwell–Eucken relation for closed porosity and Landauer relation for open porosity give good agreement to experimental data on tin oxide, alumina, and zirconia ceramics. For νp > 0.65, the thermal conductivity of kaolin-based foams and calcium aluminate foams was well described by the Hashin Shtrikman upper bound and Russell’s relation. Finally, numerical simulation on artificially generated microstructures yields accurate predictions of thermal conductivity when fine detail of the spatial distribution of the phases needs to be accounted for, as demonstrated with a bio-aggregate material.

International audience; The present paper examines the calculation of thermal conductivity of insulating building materials made from plant particles. To determine the more suitable structure and particle size distribution of raw material to optimize the volume proportion between plant particles, binder and air, a tool for calculating the effective thermal conductivity of heterogeneous materials has been developed. The approach is both based on (i) the discrete element method to generate the volume element and (ii) the finite element method to calculate the homogenized properties. A 3D optical scanner has been used to record plant particle shapes and convert some of them into a cluster of discrete elements. These aggregates are initially randomly distributed in the numerical simulation but without any overlap, and then fall down into a container due to gravity and collide with neighboring particles according to a velocity Verlet algorithm. Once the RVE is built, the geometry is exported in the open-source Salome–Meca platform to be meshed. The calculation of the effective thermal conductivity of the heterogeneous volume is then performed using a homogenization technique, based on an energy method. The numerical model has been applied to packed beds of sunflower pith aggregates. The thermal conductivity of pith particles has been first measured to supply reliable data input for the numerical model. The remarkably low value of this material makes sunflower pith packed beds an excellent candidate for thermal insulation in the building industry. However, due to its low packing density, conduction and radiation heat transfer are significant. The first numerical simulations, considering only conductive heat transfer, differ thus from the measured thermal conductivity on packed beds. Finally, a more robust model taking into account the radiation contribution has been satisfactorily compared with the experimental results.

International audience; In the present paper, a reliability analysis of the effective thermal conductivity of almost fully dense stabilized zirconia ceramics is described. The methodology is used to estimate (i) the probability for the material conductivity value to exceed a critical threshold and (ii) the sensitivity of the effective thermal conductivity to the variability of microstructural heterogeneities. A stochastic model enclosing the probability distributions of microscopic random variables coupled to a heat transfer model was thus established. This approach has been applied to a specific random microstructure exhibiting a highly segregated bimodal distribution of grain sizes and a small pore volume fraction. Due to the complexity of the ceramic microstructure, three representative volume elements were made to depict the ceramic material at different scales. Three-dimensional Voronoï mosaics are used to generate artificial microstructures with a very large number of grains. A computational homogenization method is employed to derive the effective thermal properties of the heterogeneous material for one iteration of random variables. Once the number of iterations achieves a representative sampling of the basic variables, the numerical reliability and sensitivity analysis are carried out. The confidence that can be given to the sensitivity and reliability estimates has been successfully quantified through experimental measurements of thermal diffusivity by the laser flash technique on a series of zirconia samples.

Until nowadays, surface roughness effects were ignored in the analysis, due to the difficulty to generate a rough surface model and also to simplify the model in order to reduce calculation time. However, many engineering fields, such as MEMS, seek to improve the behaviour of the system at the surface level or the interface between surfaces. Thus, with the advance of numerical capabilities, the topography of the surface can be included in finite element simulations. This paper presents two methods for generating rough surfaces, one using the real shape with an original reverse engineering method and the other one by using a parametric design language to generate a normally distributed rough surface. As an application to demonstrate the power of these methods, we choose to predict by simulation the electrical contact resistance and the real contact area between rough surfaces as a function of the contact force. This application is a major concern in RF MEMS ohmic switches and shows an original approach to extract a guideline in choosing a design, materials and process flow to minimize the contact resistance. The agreement between the numerical model and an analytical model is very good and validates this novel numeric approach.

This paper outlines the issues related to RF MEMS packaging and low actuation voltage. It is presented an original approach concerning the modeling of the capacitive contact using multi-physics simulation and advanced characterization. A similar approach is used concerning the packaging development where multi-physic simulations are used to optimize the process. A devoted package architecture is proposed featuring very low loss at microwave range.

A specific experimental setup combining nanoindentation and electrical inputs has been developed in order to determine the reliability and the performances of Micro-ElectroMechanical Systems (MEMS) which include thin free-standing elements like micro-switches applications. The evolution of the electrical resistance with respect to a mechanical solicitation applied on the contact, are henceforth available. The description of the set-up goes with a brief overview of the tests performed on a gold ohmic switch. A discussion is developed considering the mechanisms involved in the contact response. This is based on a confrontation among the experimental results, the analytical modeling and also finite-element analysis.

Till nowadays, MEMS design suffers from the lake of efficient and easy-to-use simulation tools considering the complete MEMS design procedure, from individual MEMS component design to complete system simulation. Finite element analysis (FEA) methods offer high efficiency and are widely used to model and simulate the behaviour of MEMS components. However, as MEMS are subject to multiple coupled physical phenomena at process level, such as initial stress, mechanical contact, temperature, thermoelastic, electromagnetic effects, which highly affects the component, we need to integrate these effects in our model. Doing so, finite element models may involve large numbers of degrees of freedom so that full simulation can be prohibitively time consuming. As a consequence, designers must simplify models or concentrate on interesting results in order to obtain accurate but fast solution. Some multiphysics' softwares, such as COMSOL [3], allow Reduced Order Modeling (ROM) or macro models which considers the global behaviour of the device. Thus designers can create automatically, for example, their own Simulink (Matlab copy) library from a multiphysics finite element modelization, in order to develop a global behavioural model of the whole component. This work deals with a Simulink macro model, generated from a three-dimensional multiphysics finite element analysis (FEA) using COMSOL, aiming to investigate the pull-in and pull-out voltage of microswitches.

This work is about the calculation of thermal conductivity of insulating building materials made from plant particles. To determine the type of raw materials, the particle sizes or the volume fractions of plant and binder, a tool dedicated to calculate the thermal conductivity of heterogeneous materials has been developped, using the discrete element method to generate the volume element and the finite element method to calculate the homogenized properties. A 3D optical scanner has been used to capture plant particle shapes and convert them into a cluster of discret elements. These aggregates are initially randomly distributed but without any overlap, and then fall down in a container due to the gravity force and collide with neighbour particles according to a velocity Verlet algorithm. Once the RVE is built, the geometry is exported in the open-source Salome-Meca platform to be meshed. The calculation of the effective thermal conductivity of the heterogeneous volume is then performed using a homogenization technique, based on an energy method. To validate the numerical tool, thermal conductivity measurements have been performed on sunflower pith aggregates and on packed beds of the same particles. The experimental values have been compared satisfactorily with a batch of numerical simulations.

Silicon oxynitride films were deposited by plasma-enhanced chemical vapor deposition at low temperature and frequency using SiH4 + NH3 + N2O gas mixtures. The process is optimized in order to deposit film with low tensile stress and high resistance during KOH etching. By increasing the gas flow of nitrous oxide (N2O), the film tends to be oxygen rich and the usual as-deposited high compressive stress is reduced to its lowest state at O/Si = 0.74. Annealing films above 480 °C generates low tensile stress suitable for membrane fabrication, and further infrared spectroscopy analysis shows that the shrinking of Si–O and Si–N bonds seems to be the cause of reversing the stress's nature. Young's modulus of the optimized layer is characterized by indentation. In application, 75 Ω coplanar waveguides (CPW) were fabricated on the top of an oxynitride membrane and characterized in term of insertion loss and effective permittivity. The results were compared to those obtained with the well-controlled bilayer silicon oxide–nitride membrane technology. The obtained losses are lower than 0.2 dB at 30 GHz with a free-space propagation signal.

International audience; One of the big global, environmental, and socioeconomic challenges of today is to make a transition from fossil fuels to biomass as a sustainable supply of renewable raw materials for industry. Growing public awareness of the negative environmental effects of petrochemical-based products adds to the need for alternative production chains, especially in materials science. One option lies in the value-added upcycling of agricultural by-products, which are increasingly being used for biocomposite materials in transport and building sector applications. Here, sunflower by-product (obtained by grinding the stems) is considered as a source of natural fibers for engineered biocomposite material. Recent results are shown for the main mechanical properties of sunflower-based biocomposites and the socioeconomic impact of their use. This paper demonstrates that sunflower stem makes a good candidate feedstock for material applications. This is due not only to its physical and chemical properties, but also to its socioeconomic and environmental rationales.