Testing glue joints
Bill Tindall, E.Tn.
The discussion on clamps reminded me of something I recently learned about glue testing. I test my panel joints by breaking the end trimmings from panel ends. These tests provide feed back on my joints in the area where there is most likely to be weakness, ie the ends of panels.
I was recently privy to some testing by Franklin on the strength of repaired joints with Titebond, testing the strength of new glue bonded to cured glue. They recorded force to cause joint failure (psi on an Instron) as well as observation of the failed glue line. The observations were reported as a percentage failure by one of three failure modes- fracture of the wood, fracture within the adhesive, failure of the bond between wood and glue.
I thought the measure of joint strength by observation was per cent of wood failure. Not so. In this work there was no bond failure between wood and glue. But for joints that were essentially the same strength as measured by the force necessary to break the assembly there was a wide variation of what fraction of wood fractured and what fraction of glue failed. This result suggests that for maple the glue is about the same strength as the wood and that a well made joint will fracture randomly within the glue or wood when the stress is enough to break the joint.
The conclusion, at least for maple, observing per cent wood failure underestimates how strong the joint was. An equivalently strong joint may just as well have fractured mostly within the glue and have no wood failure.
