Testing Glueline Continuity in Standard-Size Hardwood Blanks by Mechanical Methods

Glueline continuity is very critical in standard-size hardwood blanks because the blanks are cut up into small furniture parts, and a small gap in the glueline of a blank may extend across the whole part—or a gap may appear as a crack in the final product. Strength is not a critical factor. The objective of this study was to determine if a gap in a glueline can be detected by mechanical means. Furniture manufacturers want to be sure that any blank that they buy will have continuous gluelines. Every blank should be nondestructively tested, and all blanks with a defective glueline should be rejected from the shipment.Black cherry, red oak, and yellow-poplar standard-size blanks 24 inches wide and 0.75 inches thick were manufactured. Some blanks were crosscut into strips two inches along the grain. The other blanks were cut 24 inches long. Foil defect strips were placed in the middle glueline of some panels to create a gap, thus achieving a range of effective gluelines of 60, 75, 90, and 100%. Static bending perpendicular to the grain, tension perpendicular to the grain, and torsion tests were developed for testing these specimens. Five parameters—modulus of rupture and modulus of elasticity in bending, tensile strength, torsional shear strength, and modulus of rigidity—were measured to determine their sensitivity for detecting defective gluelines.Based on the results of this study, tensile strength perpendicular to the grain is the most satisfactory parameter for detecting defective gluelines. Its combination of high sensitivity and low variability would result in the smallest amount of destruction of good panels, while failing a larger number of defective panels. It was found that mechanical methods are not appropriate for determining glueline continuity since none of the parameters studied is sufficiently sensitive to detect defects of 10% or less