Résumé
All plant cell walls are composite materials made of cellulose embedded in a matrix of amorphous polymers. A large part of cellulose is present in a crystalline state. Its mechanical behaviour has been extensively studied, and appears to exhibit very high strength and stiffness and to be mostly insensitive to water. The behaviour of plant cell-walls strongly depends on cellulose properties, but also on its organisation in the wall and its interaction with amorphous polymers. The purpose of our micro-mechanical approach is to understand the relation between the macroscopic behaviour, the microstructure, the rheological properties of the constituents and their interactions. The stiffness of wood mainly originates in its high content of crystalline cellulose. The mechanical state of crystalline cellulose can be probed by X-Ray diffraction (XRD) and lattice analysis. This study aims to assess the real contribution of cellulose to wood stiffness at different levels of strain and moisture content. Four-point bending tests were performed on longitudinal spruce specimens in air-dry and wet conditions. The macroscopic strain was measured by strain gages, while the cellulose crystal strains were scanned along the height of the specimen using wide-angle X-ray scattering. Results show that the crystal strain is proportional to macroscopic strain below a threshold that depends on moisture content. In the proportional domain, the ratio between crystal and macroscopic strains is lower than 1, and this value cannot be explained just by the projection effect of microfibril orientation. This evidences that some amorphous components contribute to the wood strain in an additive way. This additive contribution is significantly larger in wet wood than in dry wood. In the compressed side of the specimens, deviation from linearity of the relation between wood strain and crystal strain is observed at a threshold of 0.38% for dry wood, and 0.1% for wet wood. Above this value, the crystalline cellulose does not deform anymore during wood deformation. This demonstrates that the behaviour of amorphous wood constituents and their interaction with cellulose have a major impact on the rheological properties of green wood.