Glue is not stronger than wood.

[Chuck]

Everyone likes to talk about how a good glue joint is stronger than the wood itself. I think this may be true in a very technical sense, but absolutely false in practice.

Take for example, a panel glue up with two 5" wide oak boards glued edge to edge (no biscuits, etc). There is no way in the world that this panel is stronger than a single 10" wide piece of oak.

If you whack the glued-up panel with a sledgehammer hard enough, it will split along the glue line. It may take a little wood from either side of the joint, but is will definitely break right along the joint.

Conversely, the solid 10" wide piece of oak will certainly withstand a harder blow without breaking than would the glue-up panel.

What am I missing in this 'glue is stronger than the wood itself' argument?

[very_dirty_dave]

How about if you glued two 2.5" oak boards to either side of a 5" oak board and wacked it with a sledgehammer? Would it still break along one or both of the glue lines or right in the middle of the glued up

10" board?

Dave

[J&KCopeland]

for a non-chemical discussion of glue joints. Fact of the matter is that even in your example, while not absolute, a properly done glue joint is likely to be stronger than the wood itself. Even oak. (Although I'd suspect that a hardwood like oak would certainly test the limits of the theory.)

Does seem strange, doesn't it?

James...

[Wes Stewart]

I think Chuck just propped up a tar baby, but I'll take a poke at it anyway. If "it takes a little wood from either side" doesn't that mean that the glue is stronger than at least the part that it was taken from?

[Doug Miller]

Try it. You may be in for a surprise.

I'm guessing you haven't actually attempted the experiment. :-)

Real-world experience.

[Chuck]

That's what I meant by being technically stronger, but not stronger in practice. If the glued-up panel breaks with less force than the solid wood panel, then the glued up panel is weaker. Whether it breaks clean on the glue line, or takes a little wood with it is irrelevant.

[George]

Nothing. Glue is not stronger than the wood. It does, however, seem to bond cellulose stronger than lignin.

Oh yes, your example depends a lot on the orientation of the growth rings.

[toller]

The glue joint is an irregularity, and if you wack it hard enough to break the panel, it will probably break at the irregularity. However, it will not break any easier than the solid panel.

The point of "the glue is stronger than the wood" pretty much means that there is no advantage to using glue that is twice as strong, because the glue is already as strong as possible.

Try your little experiment with epoxy; the result will be the same.

[Larry Bud]

I guess it depends on what you mean by "a little wood".

If it takes 1/8" all the way down on either side of the glue line, I would say the glue like is stronger than the wood itself.

[Larry Bud]

I guess it depends on what you mean by "a little wood".

If it takes 1/8" all the way down on either side of the glue line, I would say the glue line is stronger than the wood itself.

[remove]

A real world application:

My son took Tai Kwan Do a few years back and had to break 1x10 pine boards with various parts of his body. Breaking practice soon became expensive, so I decided to re-cut the wider portions of the boards and edge glue them together with Elmer's carpenter's glue. I clamped them firmly over night and let them set for a couple of days. Of a few dozen practice breaks, only one board broke on the glued joint, leaving "a little wood on either side". All of the other boards broke somewhere other than the glued joint.

Bill

[Guess who]

It's a matter of adhesion: Glue-wood, or wood-wood.

A simple test: Drop some glue directly onto some wood. Let it dry hard. Try to remove the glue without taking along some wood.

[Guess who]

No it's not. It's the key, as I already suggested in another post. If it takes wood with it, then the break is wood-wood. If the glue-wood bond isn't stronger, it will break away from the wood. That is the glue and wood will separate, or the glue itself will separate, with some still adhering to both bits of wood. Try using a *much* lighter glue for example, or bashing itbefroe the glue has completely set, but is adhering. The parts will separate easily at the joint. In fact, this is the reason for failure of some joints.

[Leon]

Reread what you declared in your findings, "It may take a little wood from either side of the joint," think about it a bit and you will realize that the glue is stronger than the wood.

[Pounds on Wood]

Instead of two 5" wide boards, glue up a 3" board and an 7" board. Then whack it with your sledge. I THINK you will find it still splits down the middle, not on the glue line.

--

******** Bill Pounds

[Leon]

NOW you are changing the whole story. In your title you indicate that glue is not stronger than wood. NO ONE has said that glue makes a panel stronger. Most everyone knows that a glued up panel will not break and leave exposed glue, wood is always exposed. Hence the glue is stronger than the wood.

[Leon]

Of a few

Umm if a little wood was left on either side of the glued joint, the joint did not break. I simply broke near the joint. Unless you saw glue down one edge, the wood broke.

[Leon]

Now I have just about heard everything. The break is PROBABLY not going to be at the joint. I may be very close to the joint but not at the joint. Glue is stronger than wood means that the wood will break and the glue joint will not.

[J. Clarke]

Bingo--it breaks in the middle because that's the point of maximum stress. If the OP put the glue line in the middle on his test specimens then he did not consider all the possible variables.

[dadiOH]

Does it? Have you tried? I haven't but I pretty much doubt it, never had a glue joint break.

-- dadiOH ____________________________

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