Glue Joint Myths

[algale]

This really surprised me.

The gist is that while end grain to end grain joints fail along the glue joint when they fail, they actually are stronger than long grain to long grain or end grain to long grain joints.

https://www.youtube.com/watch?v=m7HxBa9WVis

[BuckeyeDennis]

Thanks for sharing this excellent video, Al!

Here’s what I left as a comment on YouTube:

Yet another win for the scientific method!

That said, your results show that long-grain glue joints are stronger, relatively speaking, than solid natural wood. They also show that end-grain joints are much weaker, relatively speaking, than solid natural wood. So in a “relative” sense, you have confirmed the conventional wisdom.

I strongly suspect that conventional wisdom regarding end-grain joints is due to long-term failures in rectangular frames made with semi-end-glued 45-degree miter joints. In that case, the differential long-grain vs. side-grain expansion resulting from humidity changes results in huge internal stresses, with absolutely no external load applied. It stands to reason that glue-joint failures from those humidity-induced stresses could explain the conventional wisdom that end-grain glue joints are very weak.

I subscribed the moment the moment your video ended. Keep up the great work!

[RFGuy]

I also found this video very interesting, but frustrating as well. Thanks Alan for finding and posting this one. He clearly took quite a bit of data but when it came to presenting the data he glossed over all of it. I kind of see Dennis's point, but I was still shocked to learn that end grain to end grain joints are that strong which is contrary to popular opinion in woodworking. It would have been nice to know in that pseudo chart that he presented how all three joint types did relative to each other. I mean he tested end grain to end grain, side grain to side grain and side grain to end grain glue joints. How did each do relative to one another? He also tested the raw strength of a board with NO joints but for some reason called this "long grain" which in my opinion just adds confusion for viewers...I mean this isn't a long grain glue joint that he tested, but yet he called this "long grain" on the pseudo chart. For me what I took away is that end grain to end grain joints are at least as strong as the glue which for many applications may be strong enough. It makes sense for end grain to end grain to only break at the glue joint because breaking across the wood fibers adjacent to it would require significantly more force as he demonstrated with the NO joint board strength test.

P.S. I was waiting for him to say "Myth Busted" (from Mythbusters) at the end of this one because I consider his conclusion here remarkable and against the grain of modern woodworking.

[edma194]

Interesting experiment. I've seen it stated numerous times that PVA glue joints are stronger than than the wood they are joining but never considered the implication of that, since the glue joint is stronger than the wood any break will occur along a weak part of the wood, and that weak part is between the long grains of the wood.

However, many side joints are much longer than end joints and as a result are stronger by spreading any force over a larger area. But I don't think we'll change the way we mill boards to put the end grain on the long side of the board for even stronger glue joints.

[garys]

This all seems like a lot of to-do about nothing. When you are working with wood, just glue it.

[jsburger]

This all seems like a lot of to-do about nothing. When you are working with wood, just glue it.

Why do you say that? All the "wisdom" out there says gluing end grain is useless because it doesn't provide any strength. This person, through what looks like some very scientific measurements, has proven that notion is not only wrong but end grain joints are stronger than others.

[reible]

While I like the approach and the rigger that went into the testing I don't see this changing anything I do.

If I need a longer piece of say 1 x 2 stock I'm not going to end glue it since I know this is never going to be as strong as a longer piece of 1 x 2. If I need a wider piece I'm likely to glue it to make it wider, side by side.

Now I know that if I use pocket holes and glue I can do an end to side grain, like wise it can be doweled or a mortise and tendon or ??? in these cases again on less this is decorative I'm just not going to go with a flat glue joint.

I think we already new a lot of this. If you make a miter frame with just a glue joint and it comes apart it is glue failure and unlikely effects the two pieces of wood. I'm assuming that would be a end grain to end grain joint so...... Likewise when we have other things come apart like a side by side joint it is the wood that fails......... and so on.

I personally don't see anything new here for me.

Ed

[JPG]

Gee i wonder why scarf joints were 'invented'.

[JPG]

This all seems like a lot of to-do about nothing. When you are working with wood, just glue it.

Oh if only it were really that simple!

[algale]

I personally don't see anything new here for me.

Ed

Kudos to you if you knew end grain joints were stronger than end-to-side or side-to-side joints. I'm still trying to wrap my head around it.