By Pentti O. Kettunen
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By local damage due to animals, by growing hinders during the juvenil stage of growing, etc. All these different local changes influence the strength properties of later wood products in a manner resembling the influence of knots. Pentti O. Kettunen D. Orientation of the reinforcing fibers in the middle layer of the secondary cell wall From the microscopic details of the cell structure, the most influential one on the strength properties of wood is the unidirectional fiber reinforcement in the middle layer of the secondary cell wall.
This improves the production of knotless sapwood in the stem and in that way also the subsequent quality of logs obtained from the tree. The method has no influence on the annual growth rings and the possible differences neither in the rings nor on the uniformity of the stem. The bud pruning is also a method for improving the uniformity of stems, if carried out in a proper way. 6. Examples of the monotonic strength values of different species of wood as determined by short-term tests The short-term strength values presented in handbooks are normally the nominal fracture strength values (ultimate strength values); only seldom are the yield strength values or real fracture strength values presented.
Young’s moduli of pine (Pinus sylvestris) as determined at different moisture contents /1/). The values without apostrophe are actual module values and those with apostrophe secant module values. 973 182,5 32 Advances in Materials Science of Wood Table 8. Examples of the axial secant moduli of some Nordic woods at the oven-dry condition /1,6/. Courtesy of Professor Kärkkäinen. Species of wood Pine (Pinus sylvestris ) Spruce (Picea abies) Birch (Betula) Oven-dry density kgm-3 Young’s modulus N/mm2 in tension in compression 490 561 417 450 454 479 620 725 470 In spite of the fact that the elastic moduli values for wood are often determined in the form of more or less inaccurate secant moduli, the values of them depend on many different factors, for example, on the condition at which the wood exists, but also on the methods how the determination is performed.