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The parameters of mechanical properties considered in this chapter, are received at the tests of small samples of wood named pure and straight-grained, as they do not contain such defects as knots. According to operating terminology concept pure wood assumes absence of any defects.
On trunk cross-section you will see clearly defined parts: 1) The bark consisting from external pith of died out of a layer, which thickness strongly fluctuates at different breeds depending on age of trees, and a thin internal alive layer. 2).
Wood which in the majority of breeds is precisely divided on sap-wood and heartwood.
3) Core, - representing the central cylinder of more dark colour, formed during initial growth in length of a tree trunk and its branches. The majority of branches arises in a core of a trunk; wood of alive branches grows together with wood of a trunk, and thus are formed knited knots.
After dying off of branches, wood of a growing trunk continues to cover their bases.
After branches disappear, their bases acquire wood and in a peripheral zone of the trunk free wood from knots is formed.
During the glueing of the wood, you should pay attention to the glue properties. The choice of corresponding glue depends mainly on tree species and a kind of connection (especially from size of pressure and the intense condition, characteristic for conditions of operation), technical properties of glue, a degree of reliability for the given conditions of operation, cost. Animal glues (collagenic) most for a long time are used for pasting wood. Casein and vegetative albuminous glues among which the basic place is borrowed with glues from soybeans, have started to be applied widely in the industry during the first world war to pasting saw-timbers and shook in products to which rigid demands were not made to water resistance. Glues of a natural origin continue to keep the important place among glues for pasting wood. Glues on the basis of blood fibers are applied independently in a combination to soya fiber or to the small additive phenolic gums for manufactures of the coniferous plywood used in normal temperature- humadity conditions.
For the household purposes and in small industrial workshops use liquid animal or fish glues. In the industry apply also casein glues, delivered in the form of a powder. Synthetic glues have started to apply before the second world war, but on the value they have surpassed all former kinds of glues for pasting wood. Quality of pasting in glued designs for operation in dry conditions check, testing on spading effect of two-layer samples. As criteria of quality consider durability at spading effect and percent of destruction on wood.