10 Décembre – Thesis defense - Capucine Billard

13 h30 Amphi LCTS - University of Bordeaux (Pessac)

Thermomechanical behavior analysis of a 3D-C/C composite from 25°C to 1000°C: in-situ tests and numerical simulation.

This work deals with the thermomecanical behavior of 3D needle-punched Carbon/Carbon composites (3D-C/C). These high performance materials are widely used for aerospatial applications. Specifically, the investigated material is a multilayer needled-punched carbon prefom densified by a pyrocarbon matrix, that is used for rocket nozzle throat. 3D-C/C display remarkable properties, such as low density, and excellent thermomechanical behavior up to very high temperature. In exchange for this improved properties, 3D-C/C exhibit an intricate multi-scale architecture and a high dependence between its macroscopic mechanical behavior at high temperature, its process parameters and its microstructure.
The objective of this study is to link the micro and meso-structural specificities of such materials to their mechanical properties and response at high temperature. The aim is to better understand how damage appear under mechanical loading, from ambient to high temperature. Our approach is twofold. First, to study the damage onset, in-situ tensile tests under X-ray micro-Computed Tomography (µCT) and scanning electron microscope (SEM) have been carried out on dedicated specimens from 25°C up to 1000°C. The tensile strength of the 3D-C/C was found sensitively enhanced with increasing test temperature due to the closure of initial interfacial debonding by thermal expansion. In-situ observations also showed that further damage mainly occurs at the interfaces between plies and needles, and further highlight the significant link between the material architecture and damage. Digital Volume Correlation analyses allowed us to localize the progressive damage development as a function of the load level and the architecture. Then, based on this analysis, a multi-scale finite element (FE) model was developed to simulate the thermo-mechanical behavior of a 3D C/C with a specific attention to the interfacial phenomena. Interfacial behavior was simulated using cohesive zones models combining interfacial damage and friction. At the meso-scale, representative cells were directly obtained from µCT images. The proposed description allowed us to directly compare the damage predicted with the FE simulations to the experimental observations. Simulations, with or without, damage are presented and compared to experimental test results, in order to conclude on the pertinence of including interface damage to adequately represent the mechanical behavior of these 3D-C/C composites.

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