Strength of Glue Joints
George VondriskaHow strong are the glue joints compared to the wood it’s holding together? Woodworking expert George Vondriska will explain in this video clip. A WoodWorkers Guild of America (WWGOA) original video.
I've got a couple of boards I glued together edge to edge here and I'm gonna use this as an opportunity to prove something to people that I'm often asked about. When I'm teaching classes and I have people gluing parts together, they always wanna add a nail, add a screw. What I tell them is that today's glues are actually stronger than the material that you're gluing together and if we gave that glue a chance to dry and fractured the board later, it would actually be the wood that would fracture, not the glue joint and they're always a little bit incredulous about this so I'm gonna show you that it's true. These red oak boards were jointed, glued together. I let them sit overnight so the glue would be plenty dry.
Now, just to help highlight kind of the before and after, I'm gonna use a felt tip and I'm gonna mark where I can see the seam is right here. All right, there's our glue line, here's the fun part. I'm gonna swing these boards against the corner of the vice, aiming as much as I can right for that glue joint, we'll see what happens, ready? Now, let's have a look at this. Look at the wood fibers remaining on this piece.
It pulled all of those out of the other piece. So, just like I said, it was the adjacent piece of wood that fractured here, there's wood missing from this one. So, it's actually the wood on either side of the joint that gave way, not the glue joint itself. Proof that the glue that we're using is stronger than the material you're putting together.
Glue coverage, and clamping time as well as cure time are the key factor to failed joints. Yes you can take something out of the clamps a few hours after its glued up, but the joint is not cured. You can't put stress on it. I've always liked to leave items in the clamps overnight. Nails and screws are just to hold and item until the glue drys. After that you can remove them.
But you don't show the black marker line to show how close the fracture was. Makes me wonder.
George, you may want to qualify that you are gluing long grain to long grain and how end grain to end grain would act with all other variables' being equal. Jerry
Very interesting video! Can you please mention the used kind of wood and the used glue (there are so many brands and types, can you recommend one or two?
Can't see why it matters if glue is stronger than wood. Glue joints, which invariably degenerate over time, should never be asked to carry a load/resist a stress that requires that characteristic.