Chercheur

Teachers Login Form

 

Admin

Pupils Login Form

 



Liste des communications

Nombre total de résultats : 1141
Pertinence Titre A-Z Plus récents Plus anciens
10 25 50
Année de publication
et

Structural, Mechanical, Electronic, Elastic andChemical Bonding Properties of the Complex K2PtCl6-Structure Hydrides Sr2RuH6: First Principles Study

Communication

We report a systematic study of the structural, electronic and elastic properties of the ternary ruthenium-based hydrides Sr2RuH6 within two complementary first-principles approaches. We describe the properties of the Sr2RuH6 systems looking for trends on different properties. Our results are in agreement with experimental ones when the latter areavailable. In particular, our theoretical lattice parameters obtained using the GGA-PBEsol to include the exchange-correlation functional are in good agreement with experiment. Analysis of the calculated electronic band structure diagrams suggests that these hydrides are wide nearly direct band semiconductors, with a very slight deviation from the ideal direct-band gap behaviour and they are expected to have a poor hole-type electrical conductivity. The TB-mBJ potential has been used to correct the deficiency of the standard GGA for predicting the optoelectronic properties. The calculated TB-mBJ fundamental band gap is about 2.99eV.Calculated density of states spectra demonstrate that the topmostvalence bands consist of d orbitals of the Ru atoms, classifying these materials as d-type hydrides. Analysis of charge density maps tells that these systems can be classified as mixed ionic-covalent bonding materials. The single-crystal and polycrystalline elastic moduli and their related properties have been numerically estimated and analysed for the first time.Voir les détails

Mots clés : Ruthenium-based hydrides, First-Principles calculations, Elastic constants, Electronic structure

Driver's drowsiness detection by vision

Communication

In this work we propose an approach for monitoring the physiological state of driver using a pseudo electrooculogram EOG signal, which is generated from the video signal captured by a 60 fps camera. Comparing to existing methods, our approach has the advantage of being as accurate as the approach that uses the physiological EOG, while being easy to implement, since it avoids the use of electrodes. Different features are extracted from the pseudo EOG and used as inputs to a fuzzy logic based classifier to classify the driver's state as awake or drowsy. Our proposed approach has the advantage of being less intrusive, practical and of reasonable cost, compared to some other existing methods, and efficient which is confirmed by the experimental results obtained.Voir les détails

Mots clés : EOG, physiological state classification, Fuzzy logic, Driver drowsiness

Investigating Applicability of Discontinuous Galerkin Methods Towards Structural Health Monitoring in Transportation

Communication

Ultrasonic guided waves offer an efficient means of rapid non-destructive inspection over long distances. Several researches has been conducted to study and modelise efficiently the propagation of these waves in large structures with arbitrary cross-sections like rails, bars, tubes, and plates. Classical numerical methods such as the finite element method (FEM), the semi-analytical finite element method (SAFE), the hybrid method FEM-SAFE, etc. have proved successful in this regard, but still have a major drawback: the high consumption of resources (memory and CPU time). Recently,the discontinuous Galerkin finite element method (DG-FEM) has revolutionised computations in the time domain through its potential in terms of applications and facilities it provides. In this work, the potential benefits of a class of discontinuous methods namely the interior penalty discontinuous Galerkin methods (IPDG) in the frequency domain are investigated by performing a modal analysis of a finite structure. The natural frequencies and vibration modes are searched via the eigenvalue problem derived from the discretisation of the Helmholtz problem with free boundaries, in one dimension of space. In line with the work of literature, it was foundthat the resulting solutions are correct and free of spurious modes. In addition, the discontinuous formulation provides an interesting algebraic system with a block tridiagonal stiffness matrix and a diagonal block mass matrix. Therefore, the eigenproblem can be reduced from a generalised problem to a standard one. For large size problems, this can lead to a significant gain in computation time. Moreover, it retains its block-tridiagonal form for which effective and well suited for parallel implementation block algorithms are developed. Thus, the DG-FEM allows a double gain strategy in computation time/memory consumption and achieving high order accuracy.Voir les détails

Mots clés : Structural health monitoring (SHM), Interior penalty discontinuous Galerkin methods (IPDG), Eigenvalue problem.

The finite element method and the Infrared Thermography principle applied to the control of homogeneity between a weld and the structure to weld

Communication

The main purpose of this work is to investigate by a numerical method the effect of the thermophysical nature of solder, used to connect two pipe elements, on the distribution of the surface temperature of a considered structure. The analysis of the obtained thermograms can reveal the thermophysical quality of the used solder. The finite element method is used to calculate the temperature distribution on the welded region. The thermal response of the structure, which is subjected to a thermal flux step, is presented and analyzed. The simulations results are presented in the form of surface thermographic images, to show the contrast in temperature due to the presence of the weld zone.To better quantify the contrast induced by the presence of welding on temperature distribution we present the spatial variation of the temperature along a surface axis of the pipe.Voir les détails

Mots clés : thermographical images, finite element, Infrared thermography, surface temperature, Pipe, welding

Rectangular Dielectric Resonator Antenna with Stacked BaxSr1-xTiO3 Ceramic film

Communication

In this communication, we propose a new dielectric resonator antenna loaded with rectangular ceramic of the BST material. The high permittivity of the material allows the shifting of the antenna resonant frequency from 10.77 GHz to 8 GHz, achieving a size reduction of the resonator antenna about 67% compared to an ordinary DRA for the same resonant frequency. The numerical analysis was carried out using two electromagnetic simulators including the CST Microwave Studio in the time domain and the HFSS (High-frequency Structure Simulator) in the frequency domain. The numerical results issued from simulations are presented and compared in terms of the resonant frequency, antenna size, reflection coefficient, and radiation patterns.Voir les détails

Mots clés : Dielectric resonator antenna DRA, BST ceramic, Numerical analysis.

The technique enabling to measurement the dispersion influence of the phase velocity Lamb wave A0 mode

Communication

The application of ultrasonic guided waves (UGW) is one of the latest methods of non-destructive testing (NDT) and structural health monitoring (SHM). Nowadays, it is widely used in various industrial fields to inspect and screenm any structures by exploring the properties of its waves which can propagate long distances with high sensitivity to structural changes. In this paper a technique based on the zero crossing approach combined with the spectrum decomposition methodis proposed. In general, according to the spectrum decomposition technique, the frequency spectrum of the signal is multiplied by special filter and the signal is reconstructed using inverse Fourier transform. Using this filtered signal, further signal processing is performed applying the zero crossing method. This technique has been investigated using simulated and modelled signals of the Lamb wave asymmetric A0 mode propagating in the higher dispersion zone. In order to obtain Lamb wave signals for analysis aluminium plates having thickness of 2 mm are used. The excitation signal was 3 periods, 300 kHz burst with the Gaussian envelope. The obtained measurement results are compared with the theoretical dispersion curves obtained by SAFE method. The results have shown that the proposed measurement technique enables not only to reconstruct the phase velocity dispersion curve of the Lamb wave A0 mode, but also to choose in what precision limits and in what extent of the frequency ranges the dispersion curve of the phase velocity can be reconstructed.Voir les détails

Mots clés : Lamb waves, phase velocity, dispersion, Frequency Spectrum

Theoretical investigation of damage detection in composite structures using harmonic response technique

Communication

Nowadays the fiber-reinforced composite materials have used widely in the construction of aircraft structures; these structures are subjected to different excitations which may cause serious failure. For this reason our research presents a technique aimed to identify the damage in fiber-reinforced composite beam subjected to harmonic load using analytical model. The presence of a crack in structural elements leads to an energy concentration near to the crack region and introduce a local flexibility which affects its dynamical characteristic. Due to this fact, the harmonic response technique assumed to be reasonable technique for damage identification in cracked beam through the evaluating of its dynamic response to harmonic excitation. The discrete spring model has been used for modeling the crack, including the stress intensity factors, in which the composite beam is assumed to be divided into two sub-beams at the crack position, connected together by the additional equivalent spring. Euler-Bernoulli beam and the modal expansion theories are used in this study for solving the differential equation related to forced vibration. A parametric study has been carried out in order to investigate the influence, of crack depth and crack location on the transverse displacement of cracked composite beam under harmonic excitations. The obtained results show that the response amplitudes of a cracked beam changes with the varying of crack depth and location, and therefore, it can be used as a crack detection criterion. The dynamical behavior of composite beam with a lower flexural rigidity is more sensitive to presence of a crack. The vibration amplitudes are more sensitive when the crack depth increases. As aconsequence, the evaluation of dynamical response of cracked beam subjected to moving harmonic excitation can be used as an appropriate technique for damage detection in composite structuresVoir les détails

Mots clés : Composite beam, Damage detection, harmonic excitation, Transverse displacement

Testing the setting and hardening of concrete using the prism technique

Communication

In non-destructive testing of concrete, the ultrasonic pulse-echo technique has proven to be a reliable method for locating cracks and other internal defects. An electro-mechanical transducer is used to generate a short pulse of ultrasonic stress waves that propagate into the object being inspected. Reflection of the stress pulse occurs at boundaries separating materials with different densities and elastic properties. The reflected pulse travels back to the transducer that also acts as a receiver. The received signal can give many insights to the properties of materials under test.The question arises how the setting and hardening of cement paste, mortar, and concrete can be measured continuously. Continuous measurement ask sfor non destructive methods. The current testing method such as the vicat needle for cement paste and penetration resistance test for concrete methods measure at intervals.These methods can be applied before the end of setting. All these methods do not allow continuous measurements and are partly destructive. In this paper a special pulse-echo technique called prism technique is used to evaluate the evolution of proprieties of concrete over time. For that, an automatic system has been developed with labview program in order to monitor the time of flight of the reflected p-wave. Two mixtures of mortar are made using different proportions, of water, cement, and sand. The mortar is poured into a mould that has a form of a prism. Measurements are taken every 10 minutes over a period of 24 hours. This enabled us to plot the evolution of p-wave velocities over time for the samples and compare the results.Voir les détails

Mots clés : hardening of concrete, p-wave velocity, prism technique, ultrasonic system, Setting of concrete

Modeling and Simulation of the Defect Form Effect on Mechanical Behaviour of Shrink-Fit Assemblies.

Communication

The calculation methods of shrink fit assemblies remain traditional and have hardly changed for several years. They consider that the contact surfaces are perfectly smooth and do not take into account their geometrical defects (shape,roughness ....). The models created impose too high manufacturing costs to make consistent assumptions of calculations with the operational conditions of realization. To reduce manufacturing costs, the study of the influence of defect form on assemblies resistance characteristics is essential. The objective of this work is to study the influence of the defect form on the strength characteristics of shrink fit assemblies. It is shown that the defect form is not prejudicial to the strength of the assembly: the mean pressures are almost equivalent to the conventional case of surfaces without defects. Various finite element simulations were performed. The influence of the amplitude and of the default period was studied for different types of tightening.Voir les détails

Mots clés : shrink-fit, finite element, modeling of contact, machining, behavior

Ultrasonic attenuation in trabecular bone: Theoretical approach and experimental measurement

Communication

The aim of this paper is to determine how different parameters such as porosity, scatter size and frequency affect the ultrasonic attenuation process during the acoustic wave propagation through the trabecular bone. In this study, a theoretical model combining absorption and scattering is proposed to predict attenuation in trabecular bones. The total theoretical attenuation coefficient is computed as the sum of the contributions of scattering and absorption. The obtained results have been compared with experimental data achieved on bovine cancellous bone samples filled by water. The porosity of the used samples varies between 40 % and 75 %. A transmission technique is used. This method only requires the measurement of the specimens thickness and recording of two pulses: one without and one with the specimen inserted between the transmitting and receiving transducers. The theoretical model used in this paper appears to give appropriate results compared with experimental measurements. The obtained results show that viscous losses are not sufficient to describe the attenuation intrabecular bone. This study indicates that scattering is the primary mechanism responsible for attenuation in trabecular bone and confirms the strong dependence of the attenuation of the size of the scatterer, the bone porosity and of the ultrasound frequency.Voir les détails

Mots clés : Cancellous bone, Ultrasound, Attenuation, Scattering, Propagation modeling