Q: I'm trying to make very approximate calculations to determine what pull would be necessary to shear bridges from the tops of musical instruments. Different-sized instruments have different area bridges and different sets of string tensions, of course. Do you have such information available to you?
A: The manufacturer can give you the strength of glue when glued properly, but the real issue is how well the bonds are made. Are the two pieces of wood freshly prepared (the technical term is "active")? Are the two pieces within 2 to 6 thousandths of an inch along the entire joint area? Is there enough glue and is the pressure adequate to spread the glue and develop the closeness required? And so on.
Almost every glue we use with wood has the potential to be stronger than the wood itself; the key then is to make the joint strong by following the correct procedures. If indeed the joint is stronger than the wood, then the question you should be asking is "How strong is wood?" The answer is found in the Wood Handbook , which is U.S. Dept. of Agriculture Handbook No. 72 and can be found in many libraries or can be obtained by all libraries through interlibrary loan. The text also can be found as General Technical Report FPL-GTR-113 of the U.S. Dept. of Agriculture, Forest Products Lab. It is found online at www.fpl.fs.fed.us/pubs.htm .
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