Geometry as An Independent Variable in Adhesive Joint Studies

The relationship of breaking strength of cross-lap glued specimens to adhesive bond quality was studied extensively in a series of experiments. Block thickness influenced strength values markedly suggesting that rheology of the glue and/or wood may affect the test values. In general, increasing block thickness increased strength and increasing the moisture content of the specimen (up to 13 percent) after bonding but before testing improved strength. When bond quality variables such as wheat flour additions, pregluing wood, moisture change, and rate of hardening were studied over a range of block thicknesses, it suggested that greater specimen thickness was more discriminating with respect to bond quality. The author concludes that a block thickness of 3 inches or more (each side of the glue line) is desirable to evaluate bond quality with a cross-lap specimen. Strength test values had a positive straight line relationship with specific gravity independent of block thickness. Wood moisture content had a more complex effect. Observed strength varied strongly and directly with increased moisture content up to 10 percent. When wood moisture content was decreased from 9 percent to 5 percent after bonding, the test values decreased; when moisture content was increased to 12 percent before testing, strength values increased. The combined effects of specimen thickness and post-gluing moisture content on one cold-setting urea glue gave a four-fold difference in bond strength. This emphasized that a thorough understanding of the interplay between these and associated factors could offer an opportunity for improving joint designs and adhesive properties.

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