19 ноября, 2015 
Pokraskin 8.4.1.2 Strength, Swelling and Shrinking
The radial symmetry of the stem and the arrangement of the cellulose fibrils in longitudinal direction have an important effect on the mechanical and adhesive properties. It has already been mentioned that the parallel and perpendicular tensile strengths differ considerably. The important swelling and shrinking behavior, which depends on the equilibrium moisture content, must be carefully taken into account when wood is joined to wood, or wood to other materials. This behavior shows different patterns depending on the directions within the stem. As a rule of thumb, swelling occurs at a ratio of 1: 10: 20 along the direction of the fibers, radially to the fiber (in the direction of the pith rays) and tangentially to the fiber (in the direction of the annual rings) (Figure 8.18). Within sawn wood, changes in the ambient
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Figure 8.18 The dependence of shrink deformations on the direction of cutting the butt. |
conditions may therefore induce considerable shrink deformations, while very high shear stresses may occur in the interphase between wood and materials which do not swell in the presence of moisture (metals, plastics, glass, stoneware).
8.4.4
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