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Effect of resin impregnated core veneer on shear strength of Douglas-fir plywood Chow, Sue-Zone

Abstract

The influence of lathe checks on shear strength of Douglas fir plywood was investigated by means of impregnating lathe checks of rotary-cut veneer to various depths using a phenol-formaldehyde resin. Comparative tensile shear strength tests were conducted on a Table Model Instron machine and photographs taken at various stages of load application to illustrate the varied manner of failure. Strength of rotary-cut veneer plywood was about 60 to 70% that of sawn veneer plywood, but after the lathe checks of core veneer were impregnated by resin there was no significant difference between them. The shear strength (Y) was found to be highly correlated with penetration depth of adhesive into lathe checks (X). The linear relationship between these factors was: Y = 228.22 + 1.28052X (SE[subscript]E = 21.82; r = 0.893). Per cent wood failure estimated by conventional methods failed to relate to shear strength. Rather, the per cent wood failure occurring within 10$ of the initiation of an annual increment was found to be a better indicator of shear strength. Use of photography helped to explain more clearly stress distribution and wood failure in the specimens. It was found that the ultimate strength was reached in conventional plywood when the lathe checks were just opening. Core-impregnated plywood was used in a test to compare the tensile shear resistance when tight-side and loose-side of veneer was next to the glue line. Neither strength nor wood failure were significantly different between the two. Tensile shear strength for plywood made of impregnated core veneer and untreated face veneer was two to three times as high as that of conventional plywood. The per cent wood failure in core veneer and shear strength varied inversely. Results obtained in this study indicated that it is feasible to develop a plywood which has shear strength as high as 500 psi while remaining economical to manufacture.

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