Effects of Seven Variables on Properties of Southern Pine Plywood: Part III Maximizing Dry Strength

As part III of a series, this paper discusses effects of seven variables on the strength properties of southern pine plywood evaluated at 11 percent moisture content. At 11 percent moisture content, both loose- and tight-cut veneers were 7.6 percent more dense than the green solid wood from which they were cut. True rolling shear strength of the core veneer averaged 450 psi in 72 out of a total of 576 panels, it was maximized at an average of 578 psi by: 1) peeling tight and hot to minimize depth of lathe checks; 2) using veneer from trees with specific gravity above 0.5; and 3) spreading 75 instead of 65 pounds of glue per 1,000 square feet of core. With other conditions optimum, strength dropped to 500 psi when glue-spread was 65 pounds; to 484 psi for veneer from trees with specific gravity less than 0.5; and to 425 psi for veneer cut cold and loose. Failures were near the loose-to-loose interface rather than the loose-to-tight interface. Panels were pressed two to the opening, but proximity of the hot platen did not affect location of failures. Specimens pushed “closed” developed 14 percent more strength than if pushed “open.” Compression strength parallel to the grain of the face and back veneer averaged 7,930 psi. In 72 panels it was maximized at 8,700 psi by: 1) using veneer of high specific gravity; 2) using veneer from slow-grown trees with more than 6 rings per inch; and 3) peeling veneer tight and hot to minimize depth of checks. With other conditions optimum, strength dropped to 9.125 psi for veneer from fast-grown trees; to 8,808 psi for veneer peeled loose; and to 8,094 psi for veneer from trees of low specific gravity. Proportional limit for face and back veneers in all panels averaged 3,330 psi or 42 percent of maximum stress. Modulus of elasticity of veneer in compression parallel to grain averaged 2,230,000 psi. In 144 panels, it was maximized at 2,720,000 psi by using veneer from slow-growing trees of high specific gravity. Veneer of high specific gravity, but from fast-growing trees, had an MOE of 2,410,000 psi; and veneer from slow-growing trees of low specific gravity had an MOE of only 2,140,000 psi.

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