A New Method for Testing Wood Adhesives–III. Response of the Cross-Lap Specimen to Joint Defects

Glued wood specimens were prepared to include flaws in the joint which might arise through failure to develop a complete bond. Two methods of testing cross-lap specimens (Dillion Tensile Tester – self-aligning, and NAFM Glue Tester – semi-rigid) were studied in relation to flaw configuration and flaw area and to variations in adhesive quality produced by various additions of wheat flour. The design of the experiment permitted statistical evaluation of the data by a three-factor analysis of variance technique. The difference due to methods of test did not reflect the changes in adhesive quality, and both methods reacted very similarly to the presence of flaws in the glue line. There was some indication that the semirigid grip might be more sensitive to flaws in the joint. When different size circular areas were prevented from bonding in the interior of the joint, no change in observed joint strength occurred. When the bond was prevented from forming at the corners and progressively inward, the breaking load was approximately proportional to the bond area. It was projected that breaking load may be more closely related to length of the bond boundary than to area. Thus, the cross-lap specimen might be considered a multiple cleavage specimen, rather than a tensile specimen.

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