Micromechanical Modelling of Wood and Fibre Properties
Sammanfattning: Wood is a material with mechanical properties that vary markedly, both within a tree and among trees. Moisture changes lead to shrinkage or swelling and modify the mechanical properties. In the present study both experimental and numerical work concerning the stiffness and the hygroexpansion properties of wood and of fibres and variations in them is presented. The experimental work involves both characterizing the structure of wood at the microstructural level and the testing of clear-wood specimens. The experiments at the microstructural level provide valuable information concerning the cellular structure of wood, information needed for modelling wood on the basis of its microstructure. Deformations in the microstructure due to loading, as characterized by use of a SEM, was also studied. The longitudinal modulus of elasticity, three hygroexpansion coefficients and the density along the radius from pith to bark in the stem were determined by the testing of clear wood specimens. The longitudinal modulus of elasticity and the three shrinkage coefficients were shown to vary considerably along the radial direction of the stem. Models based on the microstructure for determining the stiffness and shrinkage properties of wood are proposed. The models investigated include the chain of modelling from the mechanical properties of the chemical constituents of the cell wall to the average mechanical properties of a growth ring. The models are based mainly on results of the experiments that were performed. Models of the microfibril in the cell wall as well as models of the cellular structure of wood were developed with the aim of determining the stiffness and shrinkage properties of wood from simply a few key parameters. Two models of the cellular structure of wood were investigated. In one of these, the structure was composed of irregular hexagonal cells, whereas in the other the cell structures were obtained from micrographs. Parametric studies performed by use of the hexagonal cell model are presented. The results of these studies showed the parameters governing the stiffness and the hygroexpansion properties of wood to be the microfibril angle of the S2-layer, density and the properties of the chemical constituents. An introductory study of the nonlinear behaviour of cell structures was also carried out. The results of numerical analyses of the deformations in cell structures that occur in compression loading in the radial and tangential directions are presented. The mechancial behaviour of chemically unaltered fibres of simplified geometrical shape was also studied in a preliminary way by means of micromechanical modelling. Three-dimensional finite element models of straight fibres of undeformed and of collapsed cross-sectional shape were involved. Both the force-displacement relationship and the moisture-induced deformations needed for characterization the behaviour of the fibre were determined. The results of simulations of the stiffness behaviour of fibres revealed two unique coupled deformation modes: coupling between extension and twist and coupling between in-plane bending and out-of-plane shear deformation. The deformation modes obtained were shown to be dependent on the value of the microfibril angle in the S2-layer.
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