A micromechanical approach to the behaviour of single wood fibers and wood fracture at cellular level

Mechanical and fracture behaviors of wood are defined by the morphology and mechanical properties of wood fibers and their bonding medium. Parallel orientation of wood fibers makes them the most influential microstructural elements from the mechanical point of view. On the other hand, in wood fracture, the difference between the properties of fiber and bonding medium (which make weak cleavage plates) plays a more important role. Experiments show that the mechanical behavior of a single wood fiber under axial tension is complex, although the cause of this complexity has still not been clearly understood. In this thesis, in order to explain the mechanism underlying the mechanical behavior of wood fibers and the fracture of wood specimens at fiber level, a micromechanical approach has been used. Confocal laser scanning microscopy was used to investigate the pattern of the distribution of microfibrils in different wood fibers. It was shown that the microfibril angle within a single fiber is non-uniform and this non-uniformity in radial walls of earlywood fibers, which contain the bordered pits, is higher than tangential walls of earlywood fibers and also higher than in latewood fibers. Tensile and cyclic tensile tests on single spruce fibers were carried out and their non-linear and force-history dependent behaviors under axial tension were shown. It was found that the fiber behavior is affected by the range of microfibril angle non-uniformities and other defects. After a certain force limit, wood fiber undergoes irreversible strains and the elastic limit of the fiber increases in the tensile loading. To explain these results, a model based on the assumption of helical and non-uniform distribution of cellulose microfibrils in the fiber and damage of the hemicellulose and lignin matrix after yielding, was proposed. The model indicated that multi-damage and evolution of microfibrils in the damages segments are the main governing mechanisms of the tensile behavior of wood fiber. Difficulties of considering the porous and heterogeneous microstructure of wood in a continuum-based fracture model, led us to develop a mixed lattice-continuum model. The three-dimensional geometry of lattice, composed of different beam elements which represent the bonding medium and alternation of earlywood and latewood fibers, enabled us to detect the propagation of cracks in both cross sections and longitudinal sections at the fiber level. Model showed that in Mode I fracture, parallel to the fibers, the location of the developed crack and the resulting stress-strain curves have a good agreement with the experimental evidence.