Veneer Characteristics as Related to Failure in Plywood Tension-Shear Specimens

A method was devised for 1) preparing plywood specimens incorporating core veneer having cutting ruptures and high surface roughness, 2) testing the specimens, and 3) recording and analyzing the failure patterns. Green Douglas-fir and hemlock core veneer was collected at random from commercial mills. It was air-dried to room conditions. Sample areas approximately 1-inch square were chosen to represent a variety of characteristics. Included were a sampling of various lathe checks, pullout, compression tearing, exposed earlywood and latewood, and fuzzy grain. To mark the cutting ruptures, the veneer surfaces were momentarily flooded with a dye solution. The dye-marked core veneer was then glued into a 3-ply plywood panel 4 inches square. After aging the bond for at least a week, shear specimens were cut from the panels. A lip which was left on the edge of the test sample was surfaced off with a microtome knife. The specimen was then loaded to failure in a modified shot-type plywood testing machine. A series of photographs was taken of the progressive development of failure during the loading of the specimen. Six failure patterns were recognized and analyzed. Preliminary conclusions are restricted to the effects of lathe checks. Shear testing in a loading mode which causes the lathe checks to pull closed shows that cutting ruptures are not completely involved in failure. The angle of the lathe check to the line of the applied load is significant. Frequent low-angled checks were responsible for an increase in failures. Surface properties of the core could not be related to failure in this work.

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