Résumé
Wood is produced in successive cylindrical layers during the radial secondary growth of a tree by a very thin layer of living cells that divide, called cambium, and located under the bark. Wood is composed of different kinds of cells, with a honeycomb-like cellular organisation, and each plays a particular role for the tree. From a mechanical point of view, fibres are the cells that compose the structure (skeleton) of the tree (Thibaut et al, 2001). Figure 1 shows the idealized multi-layered structure of a typical wood fibre (typically around 1 mm long and around 20 μm in diameter). Each wood cell wall layer plays a particular role in the mechanical behaviour of wood and the elastic properties of wood originate mainly in those of its secondary cell wall layers. Each layer, particularly in the secondary wall, can be seen as a unidirectional long fibre composite whose fibres are cristalline (at around 70%) cellulosic microfibrils (as long as the cell and with some nm in diameter) and the matrix is made up of amorphous polymers: hemicelluloses, lignin and extractives (Salmén, 2004). The microfibrils are inclined to the cell axis by the so-called microfibril angle (MFA).