Thermal modeling in composite transmission laser welding process: light scattering and absorption phenomena coupling

17th Conference of the European-Scientific-Association-on-Material-Forming (ESAFORM), Espoo, FINLAND, MAY 07-09, 2014International audienceIn previous studies [1, 2], we have presented a detailed formulation of a macroscopic analytical model of the optical propagation of laser beams in the case of unidirectional thermoplastic composites materials. This analytical model presented a first step which concerns the estimation of the laser beam intensity at the welding interface. It describes the laser light path in scattering semi-transparent composites (first component) by introducing light scattering ratio (D) and scattering standard deviation (sigma). The absorption was assumed to be negligible in regard to the scattering effect. In this current paper, in order to describe completely the laser welding process in composite materials, we introduce the absorption phenomenon in the model, in the absorbing material (second component), in order to determine the radiative heat source generated at the welding interface. Finally, we will be able to perform a three dimensional temperature field calculation using commercial FEM software. In laser welding process, the temperature distribution inside the irradiated materials is essential in order to optimize the process. Experimental measurements will be performed in order to confirm the analytical model

A fast and versatile method for spectral emissivity measurement at high temperatures

International audienceIn this paper, the development of a new device for high temperature emissivity measurement is described. This device aims at measuring both spectral and total emissivity for a thermal range of 600–1000 °C. The main targeted properties of this device are versatility and simplicity. To achieve this, a rigorous selection of components such as heating systems, heat sources, sample holders, and measuring devices was made. Sample dimensions and the corresponding sample holder were optimized through a ray tracing model computation. Selection of sensors to compute the total emissivity was also discussed. A near-infrared (NIR) spectrometer and two mid-infrared (MIR) cameras equipped with optical filters covering the bandwidth of 3–5 and 7.5–13 μm were chosen for spectral measurements. The major impediment was the separation of the sample signal and various spurious signals emitted by the environment. A specific measurement methodology was then made for each bandwidth to resolve this issue. Platinum was chosen as the reference material for the device validation. Spectral emissivity measurements were then compared to values from a commercial spectrometer. A good agreement was found between NIR and MIR band I measurements, and a higher error rate was seen in MIR band II which is explained by a less favorable signal to noise ratio. Integrated emissivity is then calculated and compared to values found in the literature. A good agreement between these values is found, and similar trends with temperature are observed. The device is then validated for spectral and total emissivity measurements. Device versatility and simplicity allow for an easy adaptation to a large area of applications

Mesure de champs de températures vraies par thermoréflectométrie proche infrarouge

True temperature field measurement is a key parameter for the optimization and the control of industrial processes. Current systems present limitations, especially on heterogeneous surfaces and/or in dynamical conditions involving the surface's variation. These restrictions are due to the ignorance of the surface's emissivity, which is a complex function of many physical quantities (temperature, wavelength, roughness, direction of detection). This thesis presents the complete development of a new method of true temperature field measurement, called Thermoreflectometry, applicable on any kind of opaque material, in the range [300-1000]°C. It allows the on-line measurement of emissivity by mixing a step of classical THERMOGRAPHY with a step of laser REFLECTOMETRY. The approach of this work is, first, the critical analysis of the method and its influence quantities, and then the optimal dimensionment of the components by simulation studies. Thirdly, a prototype is built and its defaults are characterized, following a CAMERA-based point of view, and the possible corrections are implemented. Finally, the experimental performances are estimated on some complex heterogeneous thermal scenes which emphasize the prototype's precision for all the tested samples.La mesure de champs de température sans contact est un paramètre clé pour l'optimisation et le contrôle des procédés. Les systèmes actuels présentent des limitations, particulièrement sur des surfaces hétérogènes et/ou dans des conditions dynamiques pouvant entraîner une altération de la surface. Ces restrictions sont causées par la méconnaissance de l'émissivité de la surface qui est une fonction complexe de nombreuses grandeurs physiques (température, longueur d'onde, rugosité, direction de détection). La thèse présentée propose le développement complet d'une nouvelle méthode de mesure de champs de température vraie, dénommée THERMOREFLECTOMETRIE, applicable sur tout type de matériaux opaques, dans la gamme [300-1000]°C. Elle permet la mesure en ligne de l'émissivité par le couplage d'une étape classique de THERMOGRAPHIE avec une étape de REFLECTOMETRIE laser. La démarche adoptée consiste premièrement en l'analyse critique de la méthode et de ses facteurs d'influence, ainsi que du dimensionnement optimal des éléments par des études en simulations. Ensuite un prototype opérationnel est mis en oeuvre et ses défauts sont caractérisés, du point de vue d'un système de type CAMERA, et les corrections nécessaires sont mises en place. Enfin, les performances expérimentales sont évaluées sur des scènes thermiques complexes et hétérogènes qui mettent en évidence la bonne précision du prototype pour tous les échantillons testés

Noise effect on the interpolation equation for near infrared thermography

International audienceThis paper investigates the performance of interpolation equations for a near infrared thermal imager operating over wavelengths from 0.9 mu m to 1.7 mu m with various filter bandwidths and a broad temperature range from 300 degrees C to 1000 degrees C. The equations are based on a general formulation of the effective wavelength as a function of the temperature. The quality of the interpolation is assessed in relation to the order of the effective wavelength. However, the noise induced by the imperfections of the thermal imager significantly disturbs the signal, and this phenomenon is enhanced as the bandwidth of the filter increases (i.e. for low-temperature applications). The main purpose of this paper is to establish the right choice of the filter bandwidth and the expression and order of the interpolation equation in relation to the noise level on the thermal imager and the desired accuracy. This paper first outlines the background on interpolation equations and then tests them on synthetic data from signals delivered first by an ideal thermal imager (i.e. free from noise) and then from noisy signals. This simulation study provides a framework for users to select an interpolation equation with an adequate order for near infrared thermal imagers. The performances of the selected interpolation equations are finally demonstrated on real images performed by a near infrared thermal imager

Effect and correction of the shift in spectral images for polychromatic thermography

International audienceThis articles investigates the temperature errors due to chromatic aberration in multiwavelength thermography methods. The Chromatic aberration leads to a shift in the perspective projection of a point in the 3D space on the image formed at different wavelengths. This shift causes an error in the temperature field calculated by polychromatic methods, from the fusion for each pixel of radiance temperature images at different wavelengths. The temperature error can reach 40% on a sample with high spatial non uniformities, due to wide variations of emissivity. This paper suggests an approach to correcting the chromatic aberration that is based not on equipment but on software, coupled with a calibration, using Digital Image Correlation (DIC). This experimental technique is a 2D optical method used in mechanical engineering, for inferring a deformation of a plane structure's surface from a displacement field calculated by correlation between pixels of two successive images. This paper applies the technique to achieve the field displacement induced between two images at two wavelengths by chromatic aberration. After applying this correcting displacement field, the temperature error decreases from 40% to 1% for the pixels located at the boundary of two areas with different emissivities. (C) 2016 Elsevier Masson SAS. All rights reserved

Trichromatic thermoreflectometry for an improved accuracy of true temperature field measurement on a multi-material part

International audienceThis article addresses the problem of measuring an accurate temperature field on a multi-material part which exhibits spatial, temporal, spectral and thermal emissivity variations. The article analyses the contribution of trichromatic thermoreflectometry method compared to bichromatic thermoreflectometry method. Thermoreflectometry, an active thermography method, measures in-situ the emissivity, together with the temperature. The emissivity is measured indirectly by measuring the bidirectional reflectivity of the sample and by estimating its diffusion function. The bichromatic thermoreflectometry assumes an independent diffusion function with wavelength. For trichromatic thermoreflectometry, the diffusion function varies linearly with the wavelength. This article demonstrates the benefit of trichromatic thermoreflectometry on both simulated and experimental data. The simulated data come from measurements of emissivity and diffusion function of six different materials (metallic and dielectric) performed with a FTIR (Fourier Transform InfraRed) spectrometer. The addition of noise on these estimated values enables the propagation of uncertainties, which shows that the bias on temperature estimation is lower with trichromatic thermoreflectometry. Finally, an experimental demonstration on three of the six materials confirms a lower temperature measurement error (difference between the measured temperature and a reference temperature) with trichromatic thermoreflectometry

Sélection de méthodes optiques pour la détection de défauts sur les réflecteurs optiques solaires (OSR)

National audienceLa bonne santé et la durée de vie maximale d’un satellite placé dans des conditions spatiales sontrégies principalement par sa régulation thermique. La minimisation des gradients de température entreles faces « nuit » et « jour » du satellite est un souci permanent pour les acteurs du domaine spatial.Parmi les solutions existantes, le recours à des composants optiques appelés OSR (Optical SolarReflector) permet de garantir un satellite « froid ». Ces composants recouvrent donc les satellites àraison de 625/m² et montrent des performances maximales pour un bon état de surface, sans défauts.Au montage du satellite, au sol, une importante phase de tri est donc mise en place par le constructeur.Le projet OSRAS (OSR Automatic Screening) a pour objectif d’automatiser le tri visuel actuelvoire de le rationnaliser en identifiant les défauts critiques. Les OSR étant des composants fragiles,nous privilégierons les méthodes sans contact, et parmi celles-ci, nous ne choisirons pas les méthodesexploitant une excitation énergétique de type différence de potentiel (électroluminescence[1]) ouultrasons [2]. Enfin, le prototype doit être capable de supporter des cadences élevées, donc nous neprendrons pas en compte les méthodes de déflectométrie[3], les techniques à base de scan laser[4] oula tomographie X[5]. Nous nous focaliserons donc sur les méthodes optiques faibes énergie, commedans la référence [6].Dans cet article, nous présentons la pré-étude permettant la sélection d’une méthode optique dedétection incluant une caractérisation optique des OSR et une caractérisation dimensionnelle desdéfauts principaux. Enfin, nous évaluons les performances des méthodes sélectionnées par un premierprototypage

Sélection de méthodes optiques pour la détection de défauts sur les réflecteurs optiques solaires (OSR)

National audienceLa bonne santé et la durée de vie maximale d’un satellite placé dans des conditions spatiales sontrégies principalement par sa régulation thermique. La minimisation des gradients de température entreles faces « nuit » et « jour » du satellite est un souci permanent pour les acteurs du domaine spatial.Parmi les solutions existantes, le recours à des composants optiques appelés OSR (Optical SolarReflector) permet de garantir un satellite « froid ». Ces composants recouvrent donc les satellites àraison de 625/m² et montrent des performances maximales pour un bon état de surface, sans défauts.Au montage du satellite, au sol, une importante phase de tri est donc mise en place par le constructeur.Le projet OSRAS (OSR Automatic Screening) a pour objectif d’automatiser le tri visuel actuelvoire de le rationnaliser en identifiant les défauts critiques. Les OSR étant des composants fragiles,nous privilégierons les méthodes sans contact, et parmi celles-ci, nous ne choisirons pas les méthodesexploitant une excitation énergétique de type différence de potentiel (électroluminescence[1]) ouultrasons [2]. Enfin, le prototype doit être capable de supporter des cadences élevées, donc nous neprendrons pas en compte les méthodes de déflectométrie[3], les techniques à base de scan laser[4] oula tomographie X[5]. Nous nous focaliserons donc sur les méthodes optiques faibes énergie, commedans la référence [6].Dans cet article, nous présentons la pré-étude permettant la sélection d’une méthode optique dedétection incluant une caractérisation optique des OSR et une caractérisation dimensionnelle desdéfauts principaux. Enfin, nous évaluons les performances des méthodes sélectionnées par un premierprototypage

Modélisation du comportement réfléchissant de surfaces rugueuses métalliques : détermination de la réflectivité bidirectionnelle

Le congrès a pour titre "Thermique et sciences de l'information"National audienceLa mesure de champs de températures vraies par thermoréflectométrie repose sur la mesure indirecte de l’´émissivité à partir de la mesure des réflectivités bidirectionnelles. Ces dernières permettent de calculer l’´emissivité par l’introduction d’un modèle de fonction de diffusion. Cet article présente une modélisation de la fonction de diffusion à partir d’un modèle physique des réflectivités bidirectionnelles.Ce modèle est évalué pour un matériau métallique avec une rugosité contrôlée

Thermoreflectometry: a new system to determine the true temperature fields on surface with unknown emissivities

Conference on the Thermosense - Thermal Infrared Applications XXXIII, Orlando, FL, APR 26-28, 2011International audienceIn a context of quantitative thermography, the major problem in determining the true temperature of an object is the knowledge of its emissivity. This problem is very complicated, above all when its value changes during the measurement. This article deals with a new radiative method for measuring true temperature fields with an on-line determination of emissivity. This method, called thermoreflectometry, consists in the indirect emissivity measurement by a reflectometry method in addition to the radiance temperature measurement. It assumes that the shapes of bidirectional reflectivity distribution is homothetic for two wavelengths. This assumption is much less restrictive than the gray body one (emissivity equal for two wavelengths). Finally, those two measurements and the assumption are fused for determining the true temperature field and the diffusion factor field, a key parameter of the method. This parameter provides information on the surface properties (diffuse or specular) ans it is assumed to be independent of the wavelength. The theoretical basis of thermoreflectometry method are explained and a precise description of the apparatus is given. Measurements on instrumented samples, heated at a temperature of 350 degrees C and with non uniform emissivity, are in broad agreement with the theory and show a high accuracy of the method, in reference to thermocouples measurements. The main assumption of the method is also verified by additional measurements of the bidirectional reflectivity distribution function (BRDF). These results demonstrate the relevance of this method, based on a simple embedded sensor, for measuring the true temperature field on samples with non-uniform and unknown emissivity