Effect of Prescribed Defects on Tensile Properties of Douglas-Fir Plywood Strips

The adverse effects of single, centrally located knots on the parallel-to-grain tensile strength of 5-inch-wide Douglas-fir veneer and plywood strips were evaluated in relation to the strengths of side-matched, clear material. Specimens containing 1/2-, 1-, and 1-1/2-inch knots and wood inlays replacing such knots were tested. Strength reductions were based on average values obtained from a minimum of 45 replicate specimens. For veneer, ratios of the strengths of knotty specimens to clear specimens were 0.72, 0.45, and 0.33 for 1/2-, 1-, and 1-1/2-inch knot sizes, respectively. Replacement of these knots with wood inlays of conventional size further reduced these respective ratios to 0.66, 0.41, and 0.25. For plywood, strength reductions due to knots were less pronounced than those determined for veneer. For plywood having a single knot, located in one face only, strength ratios were 0.93, 0.76, and 0.66 for the 1/2-, 1-, and 1-1/2-inch knots, respectively. Corresponding ratios for plywood having similar, opposing knots of these sizes in each outer face were 0.89, 0.64, and 0.48. Comparison of the average tensile moduli of elasticity (MOEt) was not significantly affected by small, 1/2-inch knots. A trend of decreasing MOEt, however, was obtained as knot size increased beyond 1/2-inch.

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