Sciences des matériaux
Acoustic emission signal denoising to improve damage analysis in glass fibre reinforced composites
In this contribution, continuous wavelet denoising technique is developed to enhancethe signal to noise ratio of acoustic emission (AE) signals. The time– frequency characteristics of the recorded noise are first determined by calculating the noisewavelet coefficients. Then, an algorithm able to proceed denoising by extracting noise features from those corresponding to AE signals is developed. The denoising procedure is applied to get meaningful signals when the detection threshold is decreased from 30 dB to 25 dB. Furthermore, the spectral flatness measure was applied to eliminate signals originating from noise. Experiments were conducted on cross-ply composite specimens [±45°]6S to examine the efficacy of the proposed technique. The denoised signals were analysed using the continuous wavelet transform and were also analysed in terms of the dominant frequency band which was processed by a fast Fouriertransform. Results show that denoised low-amplitude AE hits are well correlated withhigh-amplitude AE hits (A≥30 dB) in terms of damage characterisation. Voir les détails
Mots clés : acoustic emission, polymer-based composite, Denoising, Signal processing, damage characterisation
Numerical simulation for cracks detection using the finite elements method
The means of detection must ensure controls either during initial construction, or at the time of exploitation of all parts. The Non destructive testing (NDT) gathers the most widespread methods for detecting defects of a part or review the integrity of a structure. In the areas of advanced industry (aeronautics, aerospace, nuclear …), assessing the damage of materials is a key point to control durability and reliability of parts and materials in service. In this context, it is necessary to quantify the damage and identify thedifferent mechanisms responsible for the progress of this damage. It is therefore essential to characterize materials and identify the most sensitive indicators attached to damage to prevent their destruction and use them optimally.In this work, simulation by finite elements method is realized with aim to calculate the electromagnetic energy of interaction: probe and piece (with/without defect).From calculated energy; we deduce the real and imaginary components of the impedance which enables to determine the characteristic parameters of a crack in various metallic parts. Voir les détails
Mots clés : Eddy Current, Non-destructive testing (NDT), Cracks, Finite Element Method
Numerical Simulations of Eddy Current Testing For Plated Aluminum Parts
Eddy current testing is a non-destructive inspection method applied to conductive materials. Its effects on aluminum are more effective than other materials. Modeling is a good tool for understanding and analyzing impedance responses due to flaws. In this paper, discretization by finite element method is used to solve the electromagnetic field equations in terms of magnetic vector and electric scalar potentials in order to calculate the probe impedance. Simulation by finite elements method was realized to calculate the electromagnetic energy of the interaction between coil and tested part. The real and imaginary components of the probe impedance were deduced from calculated energy that allows determining the characteristic parameters of a crack in aluminum parts. Voir les détails
Mots clés : Non destructive testing, Eddy Current, Finite Element Method, aluminum parts, numerical simulations
Properties of Co-doped ZnO thin films grown by pulsed laser deposition on glass substrates
Undoped and cobalt-doped zinc oxide (CZO) polycrystalline piezoelectric thin films (Co: 3,5 at.%) using a series of high quality ceramic targets have been deposited at 4501C ontoglass substrates using a pulsed laser deposition method. The used source was a KrFexcimer laser (248 nm, 25 ns, 2 J∕cm2). X-ray diffraction patterns showed that the Codoped ZnO films crystallize in a hexagonal wurtzite type structure with a strong (002) orientation, and the grain sizes calculated from these patterns decrease from 37 to 31 nmby increasing Co doping. The optical waveguiding properties of the films were characterized by using a prism-coupling method. The distinct M-lines of the guided transversemagnetic (TM) and transverse electric (TE) modes of the ZnO films waveguide have beenobserved. With the aim of study the optical properties of the ZnO films, an accuraterefractive index and thickness measurement apparatus was set up, which is called M-linesdevice. An evaluation of experimental uncertainty and calculation of the precision of therefractive index and thickness were developed on ZnO films. The optical transmittancespectra showed a good transparency in the visible region. Calculated optical band gapvarying from 3.23 to 3.37 eV when the content of Co doping increases from 0 to 5 at.%. Voir les détails
Mots clés : Thin films, ZnO, pulsed laser deposition, Piezoelectric, Waveguiding properties, Optical transmittance
Physical and photo-electrochemical characterizations of ZnO thin films deposited by ultrasonic spray method: Application to HCrO4−photoreduction
ZnO thin films, prepared by ultrasonic spray onto glass substrate, crystallize in the wurtzite structure. The XRD pattern shows preferential orientation along the [0 0 2] direction. The films deposited at 350 consist of 60 nm crystallites with an average thickness of ∼150 nm and SEM images show rough surface areas. The gap increases with increasing the temperature of the substrate and a value of 3.25 eV obtained for films deposited at 350?C. ZnO is nominally non-stochiometric and exhibits n-type conduction because of the native defects such as oxygen vacancies (VO) and/or interstitial zinc atom (Zni) which act as donor shallows. The conductivity is thermally activated and obeys to an exponential type law with activation energy of 57 meV and an electron mobility of 7 cm2V−1s−1. The capacitance-voltage (C−2measurement in acid electrolyte (pH ∼ 3) shows a linear behavior with a positive slope, characteristic N-type conduction. A flat band potential of −0.70 VSCE and a donors density of 5.30 × 1016cm−3are deter-mined. The Nyquist plot exhibits two semicircles attributed to a capacitive behavior with a low density of surface states within the gap region. The centre is localized below the real axis with a depletion angle of 16?ascribed to a constant phase element (CPE) due to the roughness of the film. The energy band diagram assesses the potentiality of ZnO films for the photo-electrochemical conversion. As application, 94% of chromate (3.8 × 10−4M) is reduced after 6 h under sunlight (AM 1) with a quantum yield of 0.06%and the oxidation follows a first order kinetic. Voir les détails
Mots clés : ZnO, thin film, Ultrasonic spray, Photo-electrochemical, Chromate, Sunlight
ETUDE DE LA DÉFORMATION PLASTIQUE PAR LAMINAGE D’UN ACIER INOXYDABLE FERRITIQUE
The objective of this work is to propose a step, in order to analyze and to better include/understand, the influence of the plastic deformation on the microstructure, and the mechanical properties of a ferritic stainless steel type 430 “X8Cr17”. The experimental study aims at characterizing material after plastic deformation before and after heat treatment. Mechanical and thermal treatments carried out for various degrees of temperatures and various rates of deformations. Once these test-tubes are request cut out samples and prepare to undergo the metallographic analysis, the analysis by DRX and a measurement of micro hardness. The results show an evolution of the microstructure (the grains lengthening-pieces with the direction of cold rolling deformation), an appearance of an intermetallic phase ( sigma) in the range of temperature to 900°C from 1000°C. The mechanical behavior present a hardening increases with the increase in the rate of deformation, and an important softening when samples deformed and recrystallized at 700°C. Voir les détails
Mots clés : ferritic stainless steel, intermetallic phase, sigma, Plastic deformation, microstructure
Rôle des particules intermétalliques sur la corrosion de l’alliage AA6061 en présence des chlorures et l’effet du metabisulfite
Le comportement de l’alliage d’aluminium 6061 en milieux aqueux avec la concentration 0.05 M NaCl en absence et en présence du metabisulfite de sodium à été étudié par les méthodes électrochimiques. L’étude potentiodynamique cyclique à permis de déterminer plusieurs facteurs, les potentiels de corrosion et de piqûre et le potentiel de protection. Tandis que l’étude potentiostatique à précisé l’effet du metabisulfite sur la passivité de l’alliage d’aluminium. Le pH de la solution influe sur la dégradation de la couche d’oxyde autre que la valeur neutre. L’étude micrographique à révélée l’intensité d’attaque sur les particules intermétalliques riches en Mg and Si. Voir les détails
Mots clés : corrosion, polarisation, piqûres, particules
Eddy current characterization of nanomaterials
NDT Magnetic measurements as impedance in Eddy currents, remanence and corecitif in hysteresis loop are used to study the different stages of mechanical alloying in the Fe-Co system. In this paper, we want to change the electromagnetic properties of Fe-Co, by developing their metallurgical parameters such as grain size. For this we are used a planetary ball mill, we are milled the Fe-Co alloy for different milling times until to obtain nanostructure, the lamellar structure with some small particles embedded in them was observed during the first stage of mechanical alloying. The characterization by X-ray diffraction shows after 10 h of milling the formation of a disordered solid solution having a body-centered cubic (bcc) structure. After 40h of milling, morphological studies indicated that the average crystallites size is around 15 nm. Voir les détails
Mots clés : nanostructure materials powder metallurgy, NDT, Eddy Current, Magnetic measurement.
Sparse Representations of Acoustic Emission Signal:Application to Damage Analysis in Glass Fiber Reinforced Composites
Sparse representation of signals has been subject of many advances during recent years especially in the field ofapplied physics. The problem is to search for the most compact representation of signals in terms of linearcombinations of elementary waveforms in an over-complete dictionary. The underlying hypothesis in most ofsparse representation problems is that if one can associate a sparse representation to a given signal, its structurereveals information directly linked to the physical nature of the addressed problem. From this point of view it hasbeen shown that sparse representation problems have closed connexions with inverse problems and sourceseparation problems. In this contribution, an application of sparse representation techniques to identify damagemechanisms from Acoustic Emission (AE) signals is presented. The sparse representation is which is based onthe continues wavelet transform. All experiments concern investigations realized on glass fibre reinforcedpolymer composites (GFRP). Voir les détails
Mots clés : acoustic emission, Signal processing, Ultrasonic, precursor damage
Un algorithme de seuillage doux modifié pour la transformée en ondelettes: Application au débruitage de signaux d’émission acoustique
Nous présentons une méthode de débruitage par transformée en ondelettes de signaux d’émission acoustique (EA) utilisés pour la caractérisation des processus d’endommagements de matériaux composites sous contraintes mécaniques. L’objectif est d’améliorer la sensibilité de la chaine de traitement afin de pourvoir prendre en compte les signaux d’EA de très faible amplitude, condérérés comme précurseurs, qui apparaissent au début des tests de contrainte menant à la rupture du matériaux. Voir les détails
Mots clés : Transformée en ondelettes, Emission acoustique, débruitage