Лесотехнически Университет

Резюме

When the water content changes, the wood undergoes serious changes in its dimensions and volume. The processes of shrinkage and swelling are mutually reciprocal and associated with large energy changes in the different directions of the stem. These changes are due to the fact that wood is formed from polymers that contain hydroxyl groups. Dimensional changes during drying of raw wood begin when the water content falls below the fibre saturation point. Shrinkage is different for different wood species. Shrinkage is also different in different directions of the wood.

The article examines research conducted on different wood species and at different levels of the chemical composition and anatomical structure of the wood, with the aim of explaining the processes of drying and swelling.

Резюме

The Douglas-fir is a perspective species for Europe, introduced in Bulgaria long ago. During this time many studies have been made on the growth and productivity of forests of this tree species. Mechanical properties are an important part in the design of timber structures and the calculation of processing regimes. The values given for the properties depend on the altitude and the location of the wood under study in the stem.

The subject of the research in this study is the Douglas fir wood and the determination of some mechanical properties of the wood, which  has been done for the first time in our country. Standard methods and test bodies were used to establish the mechanical properties in this study.

Резюме

As the water content changes, the wood cells shrink or swell. These are some of the processes characterising the variability of wood. They greatly complicate processing and limit the application of wood materials. These properties depend on many factors, one of which is the scale (dimensions) of the wooden samples. Many observations show that the shrinkage and swelling processes proceed differently when the wooden samples is a few millimeters or a few centimeters. There is an even greater difference in the behavior of earlywood and latewood when they are examined separately.

The article study the processes of shrinkage and swelling during repeated wetting and drying. Samples with a constantly decreasing cross-section were investigated.

Резюме

Upon sorption of water, wood swells, and this swelling can be directly measured at the macroscopic level of the scale. The reverse process is called shrinkage. The two processes are reciprocal and can be studied at different levels of scale. More difficult to observe and measure is the sub-microscopic level, i.e. how the cell wall swells and shrinks. There is a difference in the structure of the radial and tangential cell walls due to the different dynamics of the cambium division. 
The article studies the swelling and shrinkage of radial and tangential cell walls. An attempt has been made to model these processes geometrically at the cell level. Douglas fir early and late wood zones were studied. 

Резюме

The wood of coniferous tree species is mainly made up of tracheids. Abstractly viewed in cross-section, these tracheids can be considered as elements of some structure. In early wood, tracheids are larger in size and have thinner walls. In late wood, they are smaller in size and thicker cell walls. All this implies an uneven structure. 
The paper examines the distribution of tracheids within the annual rings. The Douglas wood is examined in a cross-section. The number of cells in the early and late wood is measured. Tangential stripes of tracheids have been examined. These stripes are arranged in a radial sequence, gradually moving away from the axis of the stem. Dependencies between the anatomical elements in the areas of early and late wood with different widths of the growth rings are sought. A correlation analysis has been made between the structure and some physical properties of the wood. The coefficient of unevenness is determined.