Data reduction in flash method thermography

Abstract

An infrared thermography equipment is used to measure the temperature rise at the rear surface of a sample submitted to the pulse irradiation of a flash source, in order to derive simultaneously two thermal properties of the sample. The Levemberg Marquadt technique is used to match the experimental data to the analytical model. Care is taken to tune sample features and the experimental set-up. Since the acquisition time revealed to be a critical parameter, its influence on the final results is investigated too. 1. Introduction The flash method is extensively used for measuring the solids thermal diffusivity. The method essentially consists in measuring the surface temperature rise of a thin cylindrical specimen when the other face is radiated by energy pulse. The experimental data compared with the predictions of the theoretical model allow to determine the thermal diffusivity. In origin the method foresaw the use of one or few points of the temperature-time curve, [1]. Actually, the amount of experimental data can be very large, e.g. thermographic systems are available with a scan rate of 50Hz. Following the idea of the flash method, different models and data reduction techniques have been proposed, [2-8]. The data reduction method seems to play a significant role in the determination of the thermal diffusivity, in particular the non-linear  2 fit based on the Levemberg Marquadt technique is shown to lead to the most precise results, [9]. In this work, using the above mentioned fitting technique, the simultaneous identification of the thermophysical properties, i.e. thermal diffusivity and conductivity, by the classical rear-face flash method is attempted. Particular care was taken about the choice of a proper acquisition time by simulating thermograms with Montecarlo tecnique. The influence of sample features on the results is analysed taking into account infrared thermography equipment features. Finally, in order to validate both the experimental procedure and the identification routine, first tests were carried out on certified Teflon samples. 2. Basic equations and analytical solution The identification of the thermophysical properties from the temperature-time profiles needs a model for the heat diffusion: a thin sample subjected to a flash heating and cooled on both faces by radiative-convective heat transfer is considered. The energy balance equation and the related boundary conditions can be written in dimensionless form as:  * ; * ; (1)