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Factors influencing the strength properties of Douglas fir plywood normal to glueline Palka, Laszlo Cezar

Abstract

The study was designed to evaluate the relative importance of certain factors influencing the strength properties of cold-pressed Douglas fir plywoods normal to glueline. In addition, estimates of strength values were also sought. Rotary-cut veneers were obtained from plywood mills; sawn veneers were prepared from lumber. A 2 x 3 x 3 factorial design was followed using veneer thicknesses of 1/10, 1/7, and 1/5 inch, and gluing pressures of 50, 200, and 350 psi. A cold-setting modified polyvinyl adhesive (Duro-Lok 50) was used in all 18 plywood blocks fabricated. From each of these, 8 tension, 4 compression and 3 glue shear specimens were prepared. Their dimensions were ½ x 1 x 4 1/2 inches, 1 x 1 x 4 1/2 inches and 1 x 3 1/4 x 3/5 inch, respectively. Plywoods of sawn veneers were only half as strong as solid wood in both compression and tension. Solid wood exceeded the compressive strength of rotary-cut veneer blocks by two, and tensile values by seven times. Stiffness of sawn veneers was twice that of rotary cut ones. The ratio of moduli of elasticity in compression to those in tension was found to approximate seven and six for the two veneer types, respectively. The difference between solid wood and sawn-veneer block strength might be attributed mainly to the influence of a suspected acid hydrolysis at the gluelines or possibly to specimen geometry. The much lower strength values of rotary-cut veneers must have resulted from the presence of lathe checks, and the lower quality of veneer surfaces. The functional dependence of all strength properties upon some independent factors, and the ranking of the latter, was established and evaluated by multiple regression analyses. The combination of the 16, 17 or 18 most important veneer and plywood variables accounted for practically all the variation, especially for rotary-cut veneers. In addition, the complete dependence of some plywood variables on independent veneer characteristics and gluing techniques were shown by regression equations. It should be noted that the three experimentally controlled factors, veneer type, veneer thickness and gluing pressure, were not always all included in the six most significant ones. The rank of variables was found to differ for each of the various strength properties observed. Analyses of variance were performed for both observed and adjusted values within each veneer type, both providing almost identical results. The high significance of veneer thickness has been shown for all strength properties, barring shear. This was explained by its strong correlation with a number of independent variables, such as glue content and specific gravity. Gluing pressure exerted a highly significant influence on all strength properties of rotary-cut veneer blocks, and in compressive stress and strain of sawn-veneer plywood construction. Its influence was attributed to the strong correlations indicated between it and other variables, for example, full compression and plastic deformation. Finally, the exploratory nature of the experiment was emphasized.

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