A numerical and experimental study of initial defibration of wood

Abstract: A major drawback of the defibration or refining process in manufacturing pulp is that of its large energy requirements. If the mechanics of the process were better understood, large amounts of energy could probably be saved. The initial defibration seems to be crucial for both the quality of the pulp and the total consumption of energy. Numerical simulations by means of the finite element method represent a powerful tool that can be used to analyse and simulate different defibration processes. The present study was undertaken with the aim of gaining more thorough knowledge both of suitable material modelling approaches for this type of simulation and of the mechanics of the initial defibration. Both experimental and numerical work is presented. The chip shearing tests serve to illustrate the complex behaviour of wood when loaded perpendicular to the grain under conditions resembling those present during refining. In particular, the influence of specimen orientation versus loading direction was investigated. The combined compression and shear tests, in turn, concern the behaviour of wood when loaded by both radial compression and shear. The tensile tests, finally, deal with the effect of the loading rate on fracture mechanical properties. The experiments show that the mechanical behaviour of wood loaded perpendicular to the grain is very complex and is characterized by the development of cracks and, in the case of earlywood subjected to compression, by large volumetric changes. It is also shown that the mechanical properties of earlywood are very different from those of latewood and that the specimen orientation versus loading direction has a strong effect on both the failure process and energy consumption. When numerical analyses of initial defibration processes are to be performed, two main approaches may be applied in the modelling of the wooden material. One approach is to use continuum models based on smeared material properties. The other approach is to develop models of the cellular microstructure involving modelling of the individual fibres. Numerical simulations, related to initial defibration processes, using each of these two approaches, are presented in the thesis. The simulations were performed using the finite element method, taking both the nonlinear material behaviour and geometric nonlinearity into account. The continuum modelling approach involves use of a foam plasticity model and a fictitious crack model. Close agreement between the simulations and the experiments was obtained. The effect of loading rate on the failure process and on energy consumption was investigated, loading rates of up to 50 m/s being considered. At the microstructural level, the consequences of making different constitutive assumptions for the cell wall material, ranging from the basic assumption of linear elasticity to more sophisticated approaches involving both plastic behavior and microcracking, were investigated.