Quantitative infrared detection of thermomechanical heat sources – QIRD_THS

This project concerns the determination of heat source fields arising in connection with material deformation processes. Such sources, to be inferred from surface temperature fields measured by means of infrared (IR) techniques, reflect the dissipative character of deformation processes and indicate the presence of thermomechanical coupling mechanisms. Over the last two decades, the project coordinator team (Partner #1 : team «Chrysochoos», university of Montpellier) has shown the usefulness of calorimetric data for the physical understanding and the modelling of material behaviour. The heat rate created in such processes must be accounted for in the energy balance and helps in assessing the physical relevance of the state variable used for modelling the macroscopical consequences of microstructural evolutions. Applications on various kinds of materials have allowed to study the dissipative effects of localization (metals, polymers), to analyze the propagation of phase change fronts (SMAs), to detect zones in which high-cycle fatigue develops, etc. An overview of the published literature shows that only a few teams have so far developed IR imaging capabilities for the analysis of the constitutive behavior of materials. A growing interest for these techniques in academic and industrial research centres working in the mechanics of materials is nevertheless observed since the early 2000s, especially in France. The four teams constituting the present research group collectively cover the large spectrum of skills necessitated by the multidisciplinary character of this project, and are active in industry-oriented research. The research group features experts in IR metrology (Partner #2 : team «Bissieux», university of Reims), in the characterization of materials by means of nondestructive approaches (Partner #3: team «Batsale», ENSAM, Bordeaux) and in inverse problems (Partner #4: team «Bonnet», Ecole Polytechnique, Palaiseau). The reliability of thermomechanical analyses is highly dependent upon high-quality measurements of temperature fields. Infrared focal plane array (IRFPA) cameras, despite being faster than monodetector cameras, raise significant metrological issues in connection with the heterogeneous response of detector arrays, the lack of (thermal) stability of signals, and distortions induced by interactions between detectors and lenses. These technological limitations, some of which seriously impeding applications, have to be analyzed and pushed back. The present research group (especially Partner #2) maintains close relationships with major IR camera manufacturers, and thus will be in a position to access information allowing to improve the performances of existing systems. The quest of the thermophysical parameters featured in the heat equation is another crucial aspect of this project. Techniques allowing the estimation of heterogeneous specific heat, conductivity and dilatation coefficients will have to be developed (Partner #3, #4). In particular, specifically designed inverse methods will be defined for the purpose of monitoring the evolution undergone by the coefficients as a result of microstructural events such as damage, aging, or phase changes. Estimation of the dilatation coefficient will rely on the adaptation and use of image correlation or microgrid techniques respectively developed by Partner #1 and Partner #4. Accurate identification of the above-mentioned coefficients is also essential for the determination of heat sources. Up to now, Partner #1 has developed direct methods whereby partial differential operators featured in the heat equation are estimated. Such methods are predicated on the measured surface temperature being close to the temperature averaged over the thickness (or cross-section), which makes them suitable for thin samples, even with heterogeneous temperature fields due to the highly regularizing character of diffusion mechanisms. However, these approaches only provide averaged sources. With the help of the significant expertise of Partner #4 in numerical inversion methods applied to the characterisation of heterogeneous elastic media and the identification of defects, new techniques will be developed, geared towards the identification of heat sources in 3-D configurations and thus the detection of localized phenomena inside thick samples.