Liste des publications
Effects of surface texture on journal-bearing characteristics understeady-state operating conditions
Developments in microscopy have a profound effect on the resurgence of tribological applications at themicroscopic level. Using surfaces with controlled micro geometry may prove an effective approach toimproving bearing performance. It is consequently of interest to study the lubrication of journal bearingsystems taking into consideration the effect of surface geometry design. A numerical approach is used inthe analysis of texture effects on bearing characteristics. The results from investigating the performanceof bearing surfaces with spherical dimple textures suggest that contact characteristics such as minimumfilm thickness, maximum pressure, axial oil film flow, and friction torque may be improved through anappropriate surface texture geometry and appropriate textures distribution on the contact surface. Themain purpose of our work is to model and understand the evolution of journal-bearing characteristics withtextures. A rigorous methodology is recommended. The work is divided into two steps. The first oneserves to quantify the evolution of the characteristics with the texture parameters and to deduce theiroptimized values. The second step enhance the performance of the journal bearing by progressivelytaking into account the optimized values of texture parameters, especially the textures disposition. Voir les détails
Mots clés : textured surface, dimples, Hydrodynamic lubrication, journal bearing, Finite difference method
HEAT TREATMENT AND WELDING EFFECTS ON MECHANICAL PROPERTIES AND MICROSTRUCTURE EVOLUTION OF 2024 AND7075 Aluminum ALLOYS
Aluminum alloys are sensitive to hot cracking during the welding operation, both liquidation cracking in the heat affected zone (HAZ) and solidification cracking in the weld can occur. The GTA Welding results of aluminum alloys 2024 T3 and 7075 T6 for 2 mm plate thickness with different parameters of welding are presented in this paper. Before welding, different alloys were heat treated at different temperature to follow the evolution of microstructure and mechanical properties. After welding, the strength of the materials in heat affected zone (HAZ) is reduced. This reduction on properties is due to the different phenomenon that occurs during welding Voir les détails
Mots clés : heat treatment, Aluminium alloys welding, hardness, Precipitation, tensile strength, yield strength, Microstructures
Asymptotic Behavior for Solution of Reaction-Diffusion Systems
The existence, uniqueness, and asymptotic behavior of the solution of a balanced two component reaction-diffusion system have been investigated. It was shown that a global and unique solution existed and its second component can be estimated using the Lyapunov Functional. It was, also, demonstrated that each component of the solution converged, at infinity, to a constant which can be found in terms of the reacting function and the initial data. The results of the current research can be used in several areas of applied mathematics, especially when the system equations originate from mathematical models of real systems such as in Biology, Chemistry, Population Dynamics, and other disciplines. Voir les détails
Mots clés : Asymptotic Behavior, Solution of Reaction-Diffusion
Low-delay 16 kb/s Wideband Speech Coder with Fast Search Methods
In this paper, a low delay 16kb/s wideband speech coder with a buffering delay of 1.25ms is introduced. This coder is basically inspired from the G728 LD-CELP standard for narrowband speech signals. Our main goal is to reduce the implementation complexity of our wideband G728 –like- coder, which is mainly due to the search of the optimal excitation (gain-shape) in codebook. For this reason an algebraic codebook is proposed and an exhaustive optimal search and suboptimal full position and Joint Position and Amplitude search techniques are implemented. Objective performance evaluation on a large corpus of a testing speech database show that the JAPS (Joint Position and Amplitude Search) multistage search technique proposed recently can yield to a very important reduction of coder computational load, with only a slight degradation in the quality of synthesized speech signals compared to other -high complexity- search techniques. Voir les détails
Mots clés : - Low delay CELP, G728, Backward LPC analysis, algebraic codebook
Numerical simulation of a low- and a high-electric-field photocurrent decay in aSi:H
We report a numerical simulation of the photocurrent decay (PCD), from the steady state, in two different structure configurations based on the a-Si:H. The standard DOS of the a-Si:H is used. The high-electric-field PCD is considered in a structure configuration based on a metal/a-Si:H junction. Poisson’s and the two continuity equations are numerically solved in a one-dimensional space to calculate the current density. Two different boundary conditions of the a-Si:H film are considered. The low electric field PCD, which may occurs in a coplanar configuration, is calculated from the solution of a system of two non linear coupled rate equations which govern the free carriers concentrations and the different charges on the localized states in the gap. The calculated PCD versus time curves, for the two configurations, show a shoulder around 1μs which separate two main regions. We can see that the initial current decay is dominated by the electron emission from the conduction-band tail and the recombination via the dangling bonds states. The second current decay is mainly due to the electrons emission from the dangling bonds. We show also that the PCD curve tends towards the PCD of the coplanar configuration when the electric field decreases Voir les détails
Mots clés : a-Si:H film, simulation, photocurrent decay, recombination, Metal/semiconductor contact
Thresholding Techniques and their Performance Evaluation for Weld Defect Detection in Radiographic Testing
In nondestructive testing by radiography, a perfect knowledge of the weld defect shape is an essential step to appreciate the quality of the weld and make decision on its acceptability or rejection. Because of the complex nature of the considered images, and in order that the detected defect region represents the most accurately possible the real defect, the choice of thresholding methods must be done judiciously. In this paper, performance criteria are used to conduct a comparative study of thresholding methods based on gray level histogram, 2D histogram and locally adaptive approach for weld defect detection in radiographic images. Voir les détails
Mots clés : 1D and 2D histogram, locally adaptive approach, performance criteria, Radiographic image, thresholding, weld defect
Probabilistic deformable models for weld defect contour estimation in radiography
This paper describes a novel method for segmentation of weld defect in radiographic images. Contour estimation is formulated as a statistical estimation problem, where both the contour and the observation model parameters are unknown. Our approach can be described as a region-based maximum likelihood formulation of parametric deformable contours. This formulation provides robustness against the poor image quality, and allows simultaneous estimation of the contour parameters together with other parameters of the model. Implementation is performed by a deterministic iterative algorithm with minimal user intervention. Results testify very good performance of such contour estimation approach. Voir les détails
Mots clés : Gaussian and Rayleigh distributions, contour estimation, maximum likelihood, parametric deformable contours
Defects Detection in Welds Using Ultrasonic Analysis
In some austenitic stainless steel welds grains orientation cause deviation and splitting of the ultrasonic beam. It is especially true in the case of multipass welds when the remelting process after each pass causes complex solidification process. With these orientations we define a precise description of the heterogeneous and anisotropic material. Non destructive testing (NDT) is commonly used industrial method to evaluate the integrity ofindividual components. In service induced cracks such as fatigue and stress corrosioncracks can, if they are detected, be sized and monitored in order to postpone repairsor replacements. Voir les détails
Mots clés : Non destructive testing, Ultrasonic Analysis, Weld
Welded micrographic structure determination using ultrasonic analysis
In this paper, the aim of our study is the use of ultrasound to show experimentally the change of the microstructure of a material after a welding operation. Two welded carbon steel plates have been selected to be investigated. Two processes are considered: The flux wire process and manual arc process. Voir les détails
Mots clés : Ultrasonic Analysis, welding, microstructure
Magnetic microwave and absorbing properties of Fe-Co alloy synthesisedby mechanical alloying process
In this paper, the structure and magnetic properties of nanocrystalline Fe(1–x)Coxmixtures are investigated. These structures are prepared using mechanical alloying based onplanetary ball mill under several milling conditions. The structural effects of mechanicalalloying of powders were investigated by scanning electron microscopy, X-ray diffractionanalysis and bench of microwaves. Consequently, alloy powder with an average grain size of10–13 nm was obtained. Maximum saturation magnetisation Ms was obtained at acomposition value of 65%Co. Microwave measurements were performed on the mechanicallymilled Fe(1–x)Cox powder Voir les détails
Mots clés : powder technology; mechanical alloying; microstructure; microwave.