2000 年 49 巻 4 号 p. 384-389
To study the mechanism of strength degradation of wood-based panels, outdoor exposure tests and strength tests were conducted. Specimens used were commercial oriented strand boards (OSB) and particle boards (PB) which had been exposed in outdoor condition for 1, 3, and 5 years at a field testing site in Forestry and Forest Products Research Institute. To investigate probabilistic properties of modulus of elasticity (MOE), modulus of rupture (MOR), and internal bond strength (IB) of the boards, sixty or seventy-five specimens in one testing condition were tested. Transitions of the distributions for these strength properties were investigated with considering the degradation of adhesion and spring back of elements.
The mechanism of strength degradation of the boards is explained with a spot adhesion degradation model. In the case of PB, the degradation of spot adhesion occurs mainly in surface layer after one-year exposure, and decreases the MOEs and MORs of the board. But spring back does not take place, because the amount of degradation is not enough to cause the spring back. After 2 or 3 years, the thickness of the board increases because a number of effective spots which are enough to restrict the spring back, do not remain at the surface layer. After 5 years, the effective spots at the surface layer decrease to tear off a part of particles.
Though the rate of the degradation of OSB is lower than that of PB in general, spring back takes place even after one-year exposure because the surface is so rough that some water infiltrates into the board more easily. As the degradation occurs uniformly in the board, relationships between MOEs and MORs do not change. After 5 years, the thickness of the board increases because effective spots enough to restrict the spring back do not remain.