Q: We are gluing two or three pieces of wood together to form a panel. Our joints seem very strong, but sometimes they are very weak. We are looking for a way to test our joints so that we can identify the factors that result in a weak joint. One person has recommended a shear test block and sent us some info on how to run that test. Would this be a good way to proceed?
A: When a glue joint is formed, there are two components to its ultimate strength. One component is the mechanical strength that results because the two pieces of wood are full of nooks and crannies. The glue fills these spaces and then solidifies. (Analogy: Think of a puzzle that has interlocking pieces. This interlocking provides some strength.) If you try to slip the one piece that has been glued past the other, the solid glue that fills the nooks and crannies will resist this shearing movement very well.
However, if you try to pull this joint apart (that is, put it under tension), these small nooks and crannies are not very strong in tension and so the joint will fail easily. What builds up tension strength (or pulling apart rather than shear strength) is the chemical bonding between the wood and the adhesive. So, the second component of a strong glue bond is chemical strength.
Unless the surface to be glued is very fuzzy or is damaged, or if there is a lack of glue or adequate pressure, the mechanical bonding will occur 100 percent of the time. The chemical bonding will occur only if the wood is receptive; that is, if the bonding sites are not oily, occupied with dust or moisture, and so on.
(In all the years I have worked in troubleshooting, I have seen a gluing problem related to the adhesive only twice; all the other times, the problem is related to the wood itself.) If I were you, I would be interested in this tension strength of my joint, as this is what is failing when you have a weak joint. The shear test is not what you need. Instead, use a tension test.
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