Une approche multiméthode pour la caractérisation dynamique des fibres individuelles destinées aux applications composites

Climate change emphasizes the urgent need for sustainable materials and technologies. Plant fibre composites have emerged as promising alternatives to conventional glass- or carbon-reinforced polymers, offering both a lower environmental footprint and higher intrinsic damping capacity. However, their adoption in engineering remains limited due to the complexity of energy dissipation mechanisms at the fibre scale, which involve the hierarchical structure of the fibres, matrix interactions, interfaces, and porosity.This thesis, conducted within the ANR JCJC MIDIFIC project, focuses on quantifying the specific contribution of fibres to the damping behaviour of composites. The work centres on single-fibre characterisation to provide a physical and quantitative basis for multiscale modelling. A multimethod approach was applied across a wide frequency range. Dynamic Mechanical Analysis (DMA) and nanoindentation were used at low frequencies to probe the local viscoelastic response and microscale mechanical behaviour of individual fibres. Vibration testing under controlled vacuum conditions allowed the measurement of global flexural properties and damping of fibres while minimising aerodynamic effects. Finally, micro-Brillouin light spectroscopy (micro-BLS) provided high-frequency measurements of the fibre modulus and loss factor, capturing phonon interactions and directional mechanical properties at the gigahertz scale. This combination of methods enables a comprehensive characterisation of mechanical and damping properties, covering different spatial scales, frequency ranges, and environmental conditions.The results reveal distinct trends in mechanical and damping behaviour. Inorganic fibres such as glass, basalt, and carbon show the highest stiffness but the lowest damping, reflecting their rigid covalent structures that restrict internal motion. Plant fibres, including flax, hemp, and nettle, exhibit intermediate stiffness with significantly higher damping, attributed to their hierarchical cell-wall organisation and microfibril mobility. Synthetic polymeric fibres present high damping but lower stiffness, whereas aramid fibres achieve a favorable balance of high modulus and considerable damping. These observations demonstrate that hierarchical or partially ordered internal structures promote internal motion and energy dissipation, which is essential for vibration mitigation in structural applications.From an environmental perspective, plant fibres clearly outperform other categories. Their specific modulus is comparable to glass or basalt fibres, yet their production results in dramatically lower CO₂ emissions, making them highly attractive for sustainable composite design.This thesis establishes a comprehensive experimental foundation for understanding fibre-scale damping. Novel experimental setups, including a dedicated vibration test bench and an optical method for anisotropic fibre characterisation, allowed precise evaluation of fibre mechanical and damping properties. The datasets generated cover multiple frequencies, strain levels, and environmental conditions, providing essential information to support multiscale modelling and optimise plant-based composites in terms of stiffness, damping, and environmental impact. Collectively, these findings highlight the potential of plant fibres to replace conventional reinforcements in structural applications while advancing sustainable material solutions.