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The current energy context reveals the need for deep changes in all energy-intensive sectors. These transitions affect the choice of energies and materials, particularly in the field of transport. For aeronautical applications, composites - in particular new materials with a thermoplastic matrix - are widely spread to lighten structures. To guarantee the integrity of structures, materials must be tested in accident situations, especially subjected to a flame. This accidental configuration generates multiphysical phenomena that are very difficult to represent: the heterogeneous material with an intrinsic complex geometry is subjected to thermal or chemical aggression, which modifies its structure and its thermo-physical properties. This attack has a huge impact on its mechanical properties, but mechanical stresses also influence the distribution of heat transfer.
The present PhD is thus part of the scientific transdisciplinary topic of the thermo-mechanical behavior of composites subjected to thermal attack, which has been studied for a dozen years in the research team Mechanics of Materials of the GPM, in collaboration with CORIA, a laboratory specialized in heat transfer, fluid mechanics and combustion. The latest works highlighted the importance of porosity in the mechanisms occurring during a thermal attack. Porosities are generated within the matrix by its degradation (pyrolysis) and by debonding due to internal stresses, in particular expansion stresses induced by strong thermal gradients. These pockets of vacuum or gas, which can connect to each other, will drastically modify the distribution of mechanical stresses and heat transfer within the material. It is therefore essential to be able to represent these porosities in terms of quantity and morphologic distributions.
Among the means of characterization available in the research team, X-ray tomography permits to statistically quantify these porosities depending on the conditions. The distribution of porosities will not only be studied as a function of time, but will also be considered in relation to thermo-physical properties such as thermal diffusivity to verify mixing laws.
In order to represent the application and the real geometry of a fiber/matrix network, these variations in porosity in the matrix will be analyzed to explain its role in matrix/yarn debonding, or even intra-yarn debonding. This analysis of thermo-mechanical couplings will lead to the definition of thermo-mechanical behavioral laws. Finally, these laws will be implemented in FE simulations developed in the laboratory to help better representation of the phenomena.
The doctoral student will hence develop methodologies to conduct in situ X-ray tomography to identify the kinetics of porosity formation/growth/coalescence. This prominent mechanism, greatly influences heat transfer and stress distribution within the material, and acts at several scales. The multi-scale and multiphysics effects of porosity are thus the key factors to be studied to understand the thermo-mechanical coupling of composite materials exposed to fire.
Beginning: October 2024
Contacts: benoit.vieille@insa-rouen.fr, tanguy.davin@insa-rouen.fr
Keywords: composite materials, thermo-mechanical analysis, multi-scales approach, porosity, XR tomography
