Mechanical model for the deformation of the wood cell wall

The molecular mechanisms responsible for the deformation of wood, as well as the mechanical interaction of cell-wall components, such as cellulose, lignin and hemicelluloses, are not well understood. In a recently published experiment [I], we have shown that wood foils and single cells of compression wood of spruce (Picea abies [L.] Karst.) could deform permanently under tensile load via a stick and slip mechanism at the molecular level occurring during shear of the matrix between cellulose microfibrils. The shear originates from the fact that microfibrils are spiralling around the central lumen of the wood cell which can be considered as a hollow tube. During stretching, the microfibril angle between the cell axis and the direction of the cellulose was found to decrease in synchrotron X-ray diffraction experiments, giving rise to shear deformation of the matrix and to a recovery mechanism after irreversible deformation. The corresponding stick and slip mechanism is treated here in the framework of a simple mechanical model which shows that the cellulose fibril orientation might define slip planes for shear deformation, in analogy to the slip planes in single crystals as described by Schmid's law. Moreover, the effect of fibril reorientation upon straining as well as the possibility of cell torsion are considered in the model and discussed with respect to experimental data.