THermO-Mécanique dans les Matériaux cellulaires multi-fonctIonnel – THOMMI

The solid cellular materials present thermal and mechanical properties which makes them very interesting for many applications. One can cite the polyurethane and polystyrene foams with closed pores, largely used in the field of the heat insulation of the building, and metallic foams with open pores which are excellent absorbers of energy. Currently these materials are used respectively, either for their thermal properties, or for their mechanical properties. However the will to reduce the structures make them multipurpose material having a strong potential of application. This aspect is still under exploited in industry. Thus insulating foams are not yet or little used from the point of view of their mechanical properties. The project is based on targeted industrial problem: heat insulation of the cryogenic tanks of the launcher ARIANE V. This function is provided by the foams of low density with closed pores, stuck to the tank. They are strongly requested not only thermically but also mechanically. It is thus necessary to check their mechanical resistance. The knowledge of their mechanical properties is currently insufficient. The choice of an insulator is thus currently validated by a very great number of mechanical tests at the same time very expensive and weak in teaching because the results are of binary type ('that passes or that breaks! »). The objective of the project is thus to understand, model and optimize the thermomechanical behavior of these materials. A new approach of modeling multi-scale, at the same time economic and accurate making it possible to reduce the number of tests, will be developed. Across this objective, the manufacturer has the ambition to exploit the thermomechanical potential of these materials in the dimensioning of future launcher. The solution that we propose is based on an action located at three levels of scale. The knowledge of the properties of the solid phase of materials studied will be improved. The behavior on a mesoscopic scale, it is to say on a restricted number scale of cells (ERV), will be predicts by a modeling by finite elements of the real structure. This real structure will be obtained from a nondestructive and three-dimensional way by tomography with x-rays. The model finite elements on a mesoscopic scale will be thermomecanically loaded in multiaxial way in order to provide the entry parameters necessary to the formulation of a macroscopic model of behavior. Beyond this targeted application, the developed method could be extended to many other fields and will contribute to the use of insulating foams as multipurpose materials.