Effect of Wet and Dry Cyclic Exposures Upon Strength and Stiffness of Exterior Grade Southern Pine Plywood

Two types of water repellent were applied to exterior type southern pine plywood specimens. These and untreated specimens were subjected to five cycles of wet-dry exposures and then tested. Results indicate that glueline shear strength of specimens of all groups was not weakened by cycling. The wet-dry cyclic exposures, however, reduced considerably edgewise shear strength of both treated and untreated plywood. The reductions of edgewise shear strength were caused primarily by the severe surface checking developed during cycling. The group treated with an oil-base preservative water repellent developed less face checks and con sequently had the least reduction of shear strength among the tested groups. The wet-dry cycling caused a significant irreversible thickness increase for all plywood specimens. Thickness increases of specimens treated with an oil-base preservative water repellent were less than that of untreated specimens or of specimens treated with a water-base preservative water repellent. Values of moduli of elasticity and moduli of rupture of both treated and untreated plywood specimens were affected considerably by the wet-dry exposures. Reductions in MOE and MOR values, however, were caused by thickness increases of the specimens. It is significant to practical application, that cycling caused little or no reduction of stiffness (EI) or maximum load carrying capacity of specimens of any group. The only strength property reduced seriously by the wet-dry cycling was edgewise shear strength for both treated and untreated plywood.

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