Laurent Orgéas

When deformed, ordered fibrous materials such as woven, knitted or braided fabrics, as well as disordered ones such as non-wovens, papers, mats, or soft and fibrous living tissues exhibit mechanical behaviours with strong non-linearities and size effects. We investigate experimentally, theoretically and numerically these behaviours to propose mechanical models with relevant microstructure descriptors.

During their forming processes, short fibre-reinforced composites, hydrogels made of cellulose nanofibrils (CNF) or nanocrystals of cellulose (CNC) behave as complex fluids or soft solids, with evolution of their fibrous structures, inculding fibre network orientation, (de)compaction, (dis)agregation and fluid migration. These features are mainly ascribed to the high connectivities of their fibrous networks, the slenderness and the flexibility of fibres, and to (non)colloidal fibre-fibre interactions. The aim of our research is to better characterise and model these phenomena.

Biosourced fibrous materials, e.g. made of wood fibres (papers, paper boards, polymer composites) or cellulose nanofibrils or crystals (nanopapers, nanocomposites, ice-templated foams), are very sensitive to moisture content. We try to quantify the role of water on the mechanical behaviour of these materials at various scales.

The impregnation and the permeation of fluids through fibrous media occur in many systems, e.g. during the fabrication of fiber-reinforced composites, filtration processes... These phenomena are difficult to characterise and model, due to the rheology of flowing fluids which can exhibit non-Newtonian effects, and to the anistotropy and deformability of fibrous media. Our work is to better understand and model them.

Knowing the internal structures of fibrous materials in 3D and their evolution during deformation is a key to better understand and model the physics and the mechanics of these materials. For that purpose, we often use X-ray microtomography, using either laboratory or synchrotron X-ray sources. The last type allows interesting imaging modes such as phase retrieval and fast image acquisition (0.5 s). We also develop and use image analysis routines to characterise and follow fibrous structures during their deformation.

We study the mechanical behaviour of fibrous materials when subjected to various (combined) mechanical loadings, hygrothermal environments, at various scales (fiber or fibrous media scales) with in situ 2D or 3D imaging techniques in order to analyse microstructure evolutions and deformation micromechanisms.

From the knowledge of the physics and mechanics at the fiber scale, we use upscaling processes to determine the existence and properties of the equivalent continuous media. For that purpose, we use the homogenisation method with multiple scale asymptotic expansions for discrete or continuous microstructures, analytical estimates and numerical simulation on Representative Elementary Volumes using different methods: FVM, FEM, DEM, or MD.

Our work is aimed at better understanding and modelling physical and mechanical phenomena involved during the elaboration and the forming stages of these structural materials with very high specific mechanical properties.