Factors Affecting the Strength Properties of Douglas-Fir Plywood Normal to the Glue Line

A total of 18 Douglas-fir plywood blocks were cold-pressed with a modified polyvinyl adhesive (Duro-Lok 50); plywood variables were then measured and calculated. The blocks were used to prepare eight tension, four compression, and three rolling (glue) shear specimens. The effect of the experimentally controlled veneer type, veneer thickness, and gluing pressure was studied by the use of a factorial design and appropriate analysis of variance or covariance. Multiple regression equations were used to evaluate the concomitant veneer and plywood variables. A number of basic technical and statistical assumptions were made or observed, and their validity and influence on the results examined. Veneer type (sawn or rotary-cut) was the most important single factor influencing all plywood strength properties normal to glueline and standard plywood glue shear test. The variance analyses of veneer thickness and gluing pressure, using both observed and adjusted strength values, provided almost identical results. Veneer thickness (l/10, l/7 and 1/5-inch) significantly affected all plywood strength properties normal to the glueline of both veneer types. Gluing pressure (50, 200 and 350 psi) significantly influenced compressive and tensile properties of rotary-cut veneers, compressive strength properties of sawn veneer blocks, and rolling shear strength. The interaction of veneer thickness and gluing pressure was also highly significant for all plywood strength properties, except tension strain and wood failure. The average compressive strengths of sawn and rotary-cut ceneer blocks were almost the same (approximately half that for solid Douglas-fir wood). The average tensile strength of sawn veneer blocks was half that of solid wood and exceeded that of rotary-cut veneer blocks by 3.5 times. Average rolling shear stress and moduli of elasticity values of sawn-veneer blocks exceeded those of rotary-cut veneer blocks by factors of 1.5 and 2 respectively. Under experimental conditions, 1/7 inch or 1/10 inch and 200 psi appear to be the optimum levels of veneer thickness and gluing pressure, respectively. Rolling shear strength could be improved by using higher gluing pressures.

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