Abstract: The present background description concentrates on the structure of wood xylem, on the mechanical strength properties under monotonic external loading, on the degradation of the structure of wood and somewhat also on the influence of thermal treatment (heat treatment) on the structure and properties of wood. The presentation is largely based on a recent publication of the author /1/. In this manner, the description tries to illustrate the status of the present general knowledge in the fields, into which the present papers bring new results and ideas for a better understanding of the structure and behavior of wood.
Abstract: X-ray scattering techniques have been a very useful tool for the non-destructive analysis of the wood structure. X-rays are sensitive to structural parameters such as the composite structure of wood cell walls, the crystal structure of cellulose microfibrils and their helical arrangement in the cell wall, which is usually described by the microfibril angle (MFA). With the availability of synchrotron radiation sources novel experiments on wood have become possible. The increased flux of X-rays makes the in situ and time-resolved investigation of structural changes upon mechanical stress possible. The low-divergence synchrotron radiation X-rays can be focused down to sub-micrometer size, enabling scanning studies of the wood nanostructure with (sub-)microscopic position resolution. This chapter highlights very recent advances in the understanding of wood micro- and nanostructure, which were only possible using synchrotron radiation. Examples include the MFA determination in the individual layers of the secondary cell wall, the imaging of the helical structure of the cellulose microfibrils in the cell wall, lattice strain as induced by applied mechanical stress and the structural changes of different wood types under external tensile stress.
Abstract: The excellent mechanical properties of wood arise from its cellular and cell wall structure. X-ray scattering, ultrasound, and mechanical testing is combined to study the effects of strain on crystalline cellulose in wood. Results for dry and re-moistened softwood samples are reviewed and new results are presented for native, never-dried samples of Silver birch. When softwood is stretched parallel to the cell axis, the mean microfibril angle diminishes significantly in compression wood, but only slightly in clear wood. The cellulose chains in the crystallites elongate and their distance diminishes. In the never-dried Silver birch samples, axial strain caused the mode of the microfibril angle distribution to slightly decrease from the initial value of 14 degrees to 12 degrees. Unlike in softwood, in never-dried birch crystalline cellulose showed auxetic tensile behaviour. The distance of the chains increased and the X-ray Poisson ratio νca was negative, -0.3 ± 0.2. Dehydration of never-dried Silver birch caused no difference to the microfibril angle distribution.
Abstract: Strength values of the sapwood of Siberian yellow pine were measured in a system with orthogonal coordinates along the axial, radial, and tangential directions of the cell structure. Highest strength was the axial normal strength and lowest the tangential normal strength. The difference between these two values was 87-fold. Shear strength values remained between the two normal strength values. The highest shear strength appeared in tangential direction across the reinforcing fibers, i.e., on the plane perpendicular to the axial direction. Lowest shear strength appeared in tangential direction on the plane perpendicular to radial direction. The variations are due to orientation of cells and of fiber reinforcement in the cell wall laminas, especially in the middle layer of the secondary cell wall.
Abstract: Thermal treatments of two wood species were examined. The temperature load at 200 °C on the wood causes characteristic changes in the chemical composition. The determination of specific alterations was carried out by means of suitable methods, both wet chemical and instrumental analyses. The heat treated wood specimens of beech (Fagus sylvatica) and ash (Fraxinus excelsior) were characterized by means of Carbon analysis. The samples were extracted with organic solvents, cold and hot water. The pH-values were determined in the cold water extracts. Sugar analyses were performed after hydrolysis by means of ion-exchange chromatography. Lignin analyses after thioacidolysis were carried out by means of GC and GC/MS.
Abstract: Two man-made wooden objects, called here a “boat” and an “outrigger”, were found above one another in 2004 and 2006 in Kuusamo, Finland. A layer of shore front mud and turf covered them entirely. With partial excavations, samples were taken for age determination and structural studies from both of the items. As measured by the radiocarbon method with a probability of 95.4 %, the items appear to be from periods 5480 – 5300 CalBC and 5025 – 4770 CalBC for the boat and the outrigger, respectively.