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Sciences des matériaux

Nombre total de résultats : 330
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We study the spatiotemporal formation and spreading of the high-spin state (HS) during the cooperative relaxation of the photo-induced metastable high spin (HS) state at low temperature of anelastic lattice, in the presence of a defect injected in the center of the lattice. For that, we designed a 2D rectangular-shaped lattice with discrete spins coupled by springs. The distances between the sites are spin-dependent which prevents any analytical resolution of the present problem. The elastic coupling between the spin-crossover (SCO) sites results in a long-range effective interactions between the spin states from which originates the complexity and the richness of this problem. The numerical resolution of the problem is performed using Monte Carlo simulations on the spin states and theatomic positions. The simulations are restricted to a lattice with a hole (simulating the defect) with afixed size. The presence of the defect shows the dynamics of the spin-crossover transformations startsfrom one corner of the rectangular lattice and propagates along the width (shortest distance to thesurface).Then a second regime of longitudinal propagation takes place, whose velocity slows down significantly in the vicinity of the defect. Then the interface leaves the defect, where it was pinned atlow velocity and accelerates when it approaches the border of the lattice. We have also investigated the spatial dependence of the displacement field, from which we derived the spatial distribution of the divergence, which directly connects to the distribution of the internal strain and elastic energy. Valuable information is derived from the time-dependence of the total elastic energy in particular around the defect.Voir les détails

Mots clés : Spin Crossover, phase transition, interface propagation, Defects, elasticity, Monte Carlo Simulations

Measurement of thickness of thin metal sheets by the A0 Lamb mode at low frequency.thickness products


Industrial structures undergo significant stresses during their life and can destroy the environment. This can, in many cases, cause serious damage, often imperceptible. Several methods are used to detect and locate such early stage damage.Lamb waves, which are guided by both surfaces of a planar structure, are good candidates to inspect the plate in a non invasive manner. These waves have the advantage to travel long distances without significant attenuation in the direction of propagation, which allows control quickly, permanently and inreal time, large areas of difficult access. In this work, we present a method to measure the thickness of thin sheets using the propagation of the first anti-symmetric A0 Lamb mode, generated by a piezoelectric transducer. The principle is based on the existing proportionality relationship between the group velocity and thickness of thin metal sheets, at low frequencies. We test this relation on various metallic sheets of thicknesses: 25; 75; 100; 160 and 200 microns.Voir les détails

Mots clés : Lamb waves, group velocity, thickness, Transducer.

Theoretical and numerical study of the reflection ofan ultrasonic pulse radiated by a linear phased array transducer at a fluid-fluid interface


This study is devoted to the calculation, in transient mode, of the ultrasonic field emitted by a linear array and reflected from a fluid-fluid interface thanks to a finite element package widely used in computer simulations for solving partial differential equations describing such physical phenomena. The results obtained show the various waves emerging at the interface: direct and edge waves, specular reflection and the appearance of radiating surface waves at critical angle. The various waves are identified by their arrival times calculated using the ray method.Voir les détails

Mots clés : Transient ultrasonic, liquid-liquid interface, reflection, phased array

Electronic and magnetic properties of Ba2CoWO6: First principal investigation


We report an investigation of the structural; electronic and magnetic properties of a new double perovskite Ba2CoWO6 by means of density functional calculations (DFT); within the generalized gradient approximation (GGA). The lattice constants of Ba2CoWO6 was obtained and found toagree very well with published experimental reports. The total and partial density of state are calculated and discussed. The results reveal that the Ba2CoWO6 has stable antiferromagnetic character. Our results predicts that Ba2CoWO6 have metallic nature.Voir les détails

Mots clés : ab initio, FP-LAPW, Electronic structure, Double perovskite

UV radiations impact on the mechanical and physicochemical properties of polycarbonate


This work is devoted to the experimental study of UV irradiation on the mechanical and microstructural behaviour of polycarbonate (PC). Simple compression tests, X-ray, Scanning Electron microscopy (SEM) and an IRTF analysis were carried out in order to characterize the response of the specimens material after photodegration with a wavelength of 253 nm at room temperature and for several maintained durations of 72, 144 and 216 hours. These investigations showed a decrease of the intrinsic properties of polycarbonate (Young modulus, yield stress, etc.); the superposition of spectra IRTF shows that the intensity of chemical connections are influenced by the duration of UV irradiation; in addition, an increase of diffractogram peaks intensity of the samples after 216 hours of ageing has been noticed. Our objective in this work is to find correlations between the time of maintenance, microstructure and mechanical properties of our materialsVoir les détails

Mots clés : Amorphous polymer, Polycarbonate, UV irradiation, photodégradation

Ultrasonic Time of Flight Estimation Using Wavelet Transforms


The accurate estimation of the time-of-flight (TOF)of the ultrasonic echoes is important in non-destructive testing (NDT). The main interest of the TOF estimation is the flaw detection, localization and coating thickness evaluation. Usually, the backscattered echoes from a thin coating or a flaw located close to the interface overlap in the time domain, so TOF estimation becomes more complicated and requires advanced signal processing methods. In this paper, the wavelet transform was investigated in order to evaluate the TOF. The two applied methods are the continuous wavelet transform based on the scale-averaged power (SAP) and the discrete stationary wavelet transform (DSWT). These methods were applied, firstly, on simulated Gaussian echoes for two practical cases: without andwith partially overlapping echoes. Several numerical tests have been carried out to select a suitable mother wavelet for each method and for each case. The performance was evaluated through the mean square error (MSE) between the estimated and the reference value of the TOF. The numerical tests showed that both methods give a low error for non-overlapping echoes, while a reasonable error was obtained in case of partially overlapping echoes. An experimental validation was performed on a real signal taken from a thermally coated sample in order to evaluate the coating thickness. An overall agreement was observed between simulation and experiment.Voir les détails

Mots clés : Time of Flight estimation, ultrasonic echo, SAP, DSWT

Monitoring of cracking in reinforced concrete beam Under Shear Loadings by using Digital Image Correlation (DIC) and Acoustic Emission (AE)


in this paper, an experimental study is performed on reinforced concrete beams without transverse reinforcement. The kinematics of the diagonal crack was studied in beams, with a constant width of the cross section. The same longitudinal reinforcement ratio and the same shear-span ratio are retained. Three point bending tests are performed in order to obtain the global mechanical behavior. During these tests, strains in longitudinal reinforcement bars were recorded using strain gauges embedded on the steel surface. The Digital Image Correlation (DIC) technique is used to monitor cracking during mechanical loading, in order to measure the intrinsic parameters of the cracking process (crack opening, crack length, slip between lips of cracks) at different stage. In the other face of the beams, sensors of the acoustic emission were placed to record the damage evolution and to locate the movement of the crack during test. Both AE and DIC are efficient techniques to study the failure process of reinforced concrete structures.Voir les détails

Mots clés : reinforced concrete, Digital Images Correlation, acoustic emission, shear cracks.

Ultrasonic TOFD Technique for Cracks Sizing and Locating Based on PSO


Ultrasonic Non-destructive testing has been widely used in industry to detect, characterize and size defects in materials. In this paper, an ultrasonic testing technique and an ultrasonic signal processing method are used to size and locate flaws in materials. The ultrasonic testing technique is based on determination of the time of flight of diffracted echoes from the defect edges (time of flight diffraction TOFD). To improve the arrival time resolution of a TOFD signal, an estimation technique based on Particle Swarm Optimization (PSO) and Matching Pursuit decomposition (MP) is proposed. The finite element method (FEM), using the ABAQUS software package, is employed for modeling the TOFD technique in a two-dimensional geometry. Simulation and experimental results proved the efficiency of the proposed method.Voir les détails

Mots clés : Ultrasonic testing, Time of flight diffraction, Wave propagation, Finite element methode, Particle Swarm Optimization, Crack

Numerical simulations of the propagation of Chirped Vector Soliton in optical fibers with variable coefficients in the presence of third order dispersion and power law nonlinearity


We study in this work, the numerical simulations of propagation of chirped vector solitons in optical fiber systems using the compact split step Padé scheme (CSSPS). This study is done in the case of variable coefficients and in the presence of third order dispersion and power law nonlinearity. A negative chirp makes the soliton broadening, while; a positive chirp leads to a soliton compression. The effect of chirp on the soliton temporal width of an amplification system is greater than that in a loss system. In the presence of third order dispersion, we note an increase of the pulse width with an asymmetric oscillation on the trailing edge. In the same time, we note a shift of the center of the two components of the one managed chirped vector soliton along the propagation distance. It is clearly noted from plot that, the quintic nonlinearity has a marginal role on the propagation characteristics of the two components of managed chirped vector soliton.Voir les détails

Mots clés : vector solitons, chirped solitons, birefringent optical fibers, compact split step Padé scheme, coupled higherorder nonlinear Schrodinger equations with variable coefficients

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


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