Glue is not stronger than wood.

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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?
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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
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very_dirty snipped-for-privacy@yahoo.com wrote:

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.

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http://www.newwoodworker.com/useglue.html 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...
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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?
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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.
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Chuck wrote:

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.
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Chuck wrote:

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.
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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.
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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.
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If glue is stronger than wood and the break is along a wood-wood line, then why use biscuits or dowels? If anything they would be most needed anywhere but the glue line...
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Consider the real-world mechanics of gluing two long strips of wood, long edge to long end. Even the most minor of variations in thickness, or very minor twisting will make the glue joint very hard to get perfect flat, even with clamps. Both dowels and biscuits will allow aligning the flat, wide surfaces, much better.
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Maybe because they are used to help insure _alignment_ of the joint, not primarily to strengthen it? do you suppose? just maybe? *snort*
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"blueman" wrote in message

The strongest glue joints are long grain to long grain, but not all joints are long grain to long grain.
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Umm because the glue line is STRONGER than the WOOD. Use biscuits for butt joints or mitered joints where the end grain of the wood is going to pull out of the end of the board.
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Chuck wrote:

Does it? Have you tried? I haven't but I pretty much doubt it, never had a glue joint break.
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Dat Brer Fox he'd sho be de sly one.
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Chuck wrote:

No. If the joint is well-prepared and a good quality glue is properly applied and cured (even PVA will work), the joint will rarely be what actually fails. A wide panel breaks more easily because there's a larger moment arm...actually, the appearance of "easier" is an illusion as the stress before the breaking point will be (within the variability of the wood itself) the same.
Have you actually done an experiment or just spouting?
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The glue joint is stronger. It is also less flexible. Stresses that would ordinarily fairly evenly spread over the board will concentrate in this hard spot.

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CW wrote:

I think that depends on both the specific wood and the specific glue...pva's, for instance, aren't very rigid. I don't have figures for modulus, though, but I suspect they're close, at best.
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