Sciences des matériaux
Investigating Applicability of Discontinuous Galerkin Methods Towards Structural Health Monitoring in Transportation
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.
Driver's drowsiness detection by vision
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
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
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
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
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 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
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 materials Voir les détails
Mots clés : Amorphous polymer, Polycarbonate, UV irradiation, photodégradation
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
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