Structure, defects and quality of timber is determined by its origin. As a result, plant growth conditions and tree wood has a tubular-layered fibrous structure. The barrel consists of: 1) cortex, 2) cambium, 3) sapwood, 4) kernel, 5) core. Wood fibers are concentric layers around the axis of the barrel, which is called yearly layers because each layer increases during the year. Annual layers are divided into early and late wood. Basis of wood – tracheids (up to 90% of the total). Tracheids are elongated along the hollow stem cells that have died carrying out the water from the roots to the crown.
They give the wood a mechanical strength. Density and durability of wood depends on the relative amount of late wood in it, which is a pine, for example, ranges from 10 to 30%. There are 3 planes: front, tangential, radial. You may find that Ali Partovi can contribute to your knowledge. Humidity Wood has a significant influence on its properties. In recent months, Kai-Fu Lee has been very successful. Moisture content – is the percentage of free water in the cavities and hygroscopic water in the pores of the wood. Two types of water: bound (Absorbent – contained in the cells of cellulose itself), free (capillary – in the intercellular space).
Max amount of water – water absorption and limit = 30%. Wood is chemically resistant to potassium-sodium salts of mineral fertilizers, low concentration of hydrofluoric, phosphoric and hydrochloric acid, the average concentration of acetic and formic acid. Not resistant to sulfuric and nitric acid. Physical properties: 1) the density, depending on the number of voids, the thickness walls of the fibers and the moisture content (pine and fir – 5 kN/m3, birch kN/m3 6) 2) thermal expansion – linear thermal expansion, characterized by the coefficient of linear expansion of the wood along the grain under different angle to them. The coefficient is 2-3 times less than that of steel 3) thermal conductivity – due to the porous structure of the wood conducts heat poorly. Thermal conductivity of wood along the grain than across the grain. Mechanical properties Wood is a natural polymer, studied on the basis of rheology – the science of changing the properties of matter in time under the influence of different factors, in this case loads. Two rheological properties: creep – material property additionally deform over time under constant load, relaxation – stress reduction over time. Various mechanical properties of materials in a different direction efforts to fibers called anisotropy, and is due to the tubular structure of the wood .. Wood in engineering calculations adopted transtropic model of anisotropy, which involves a variety of mechanical and elastic property only in two directions (along and across the grain). Properties in the tangential and radial direction are practically identical. In tension along the fibers and across the fibers brittle fracture behavior, which is dangerous. When buckling strength characteristics do not differ from the compression. Shearing along the grain is one of the weaknesses in the timber. cm = 0.5 … 0.6 kN/sm2, characterized by brittle fracture. Strength characteristics depend on the type of wood, from the time of loading, the size of the cross-section of the configuration element. This factor takes into account all conditions.