Knifing Technique and Test Method When Used in the Evaluation of Hardboard to Fir Glue Lines

A conventional test of shear strength of glue bonds did not reliably evaluate glue bonds between plywood veneer and hardboard faces. A proposed “knife” test actually tested the hardboard-to-veneer bond while the shear test caused 100 percent hardboard failure. The knife test requires specimens cut to a thickness that includes only 3/32-inch thickness of veneer bonded to the hardboard, and measures 1 inch wide by 9 inches long in the fiber direction of the veneer. A knife is used to pry veneer away from the hardboard across half the width of the specimen, thus causing separation along the glue line in the fiber direction of the veneer. The knife test is a gross test and requires no special equipment but has not yet been applied extensively.

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