Rémi Gilblas

Thermal Modeling in Composite Transmission Laser Welding Process: Light Scattering and Absorption Phenomena Coupling

In previous studies [1, , 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 transparent composites (first component) by introducing light scattering ratio and scattering standard deviation. 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 a 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 valid the analytical model.

Noise effect on the interpolation equation for near infrared thermography

This paper investigates the performance of interpolation equations for a near infrared thermal imager operating over wavelengths from 0.9??m to 1.7??m with various filter bandwidths and a broad temperature range from 300??C to 1000??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.

Automated visual inspection of an airplane exterior

This paper deals with the inspection of an airplane using a Pan-Tilt-Zoom camera mounted on a mobile robot moving around the airplane. We present image processing methods for detection and inspection of four different types of items on the airplane exterior. Our detection approach is focused on the regular shapes such as rounded corner rectangles and ellipses, while inspection relies on clues such as uniformity of isolated image regions, convexity of segmented shapes and periodicity of the image intensity signal. The initial results are promising and demonstrate the feasibility of the envisioned robotic system.

Cost-effective computational method for radiation heat transfer in semi-crystalline polymers

This paper introduces a cost-effective numerical model for infrared (IR) heating of semi-crystalline polymers. For the numerical and experimental studies presented here semi-crystalline polyethylene (PE) was used. The optical properties of PE were experimentally analyzed under varying temperature and the obtained results were used as input in the numerical studies. The model was built based on optically homogeneous medium assumption whereas the strong variation in the thermo-optical properties of semi-crystalline PE under heating was taken into account. Thus, the change in the amount radiative energy absorbed by the PE medium was introduced in the model induced by its temperature-dependent thermo-optical properties. The computational study was carried out considering an iterative closed-loop computation, where the absorbed radiation was computed using an in-house developed radiation heat transfer algorithm -RAYHEAT- and the computed results was transferred into the commercial software -COMSOL Multiphysics- for solving transient heat transfer problem to predict temperature field. The predicted temperature field was used to iterate the thermo-optical properties of PE that varies under heating. In order to analyze the accuracy of the numerical model experimental analyses were carried out performing IR-thermographic measurements during the heating of the PE plate. The applicability of the model in terms of computational cost, number of numerical input and accuracy was highlighted.

A non-invasive experimental approach for surface temperature measurements on semi-crystalline thermoplastics

Most of the thermoforming processes of thermoplastic polymers and their composites are performed adopting a combined heating and forming stages at which a precursor is heated prior to the forming. This step is done in order to improve formability by softening the thermoplastic polymer. Due to low thermal conductivity and semi-transparency of polymers, infrared (IR) heating is widely used for thermoforming of such materials. Predictive radiation heat transfer models for temperature distributions are therefore critical for optimizations of thermoforming process. One of the key challenges is to build a predictive model including the physical background of radiation heat transfer phenomenon in semi-crystalline thermoplastics as their microcrystalline structure introduces an optically heterogeneous medium. In addition, the accuracy of a predictive model is required to be validated experimentally where IR thermography is one of the suitable methods for such a validation as it provides a non-invasive, full-field...

Detection of Micro-Cracks on Highly Specular Reflectors: Dimensioning a Vision Machine Based on Optical Properties

This paper describes the complete dimensioning of a method and apparatus dedicated to the automatic detection of defects on optical mirror-like components called optical solar reflector (OSR), which are used to ensure the thermal control of satellites. First, the requirements of the method are defined according to the properties of the OSR (multilayer and semitransparent) and the defects to be detected. The defect dimensions are then measured and exhibit very dispersed micrometric values. Second, the optical characterization of the OSR and its sublayers is conducted, enabling the dimensioning of the machine with regard to the localization of the defect (surface, interface, or volume). Third, a semi-industrial prototype is developed and evaluated through two kinds of experiments. The first experiment illustrates the production capacity of the prototype and the second the measurement capability. Results showed that the prototype exceeded the initial requirements and could be used to replace the currently used control with confidence.

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

In 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°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.