Why does wood split radially?

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When it dries, why do you think wood splits from the center to the perimiter? I think I have finally worked it out, just wanted to know what you all think first:)
Dean
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Because the wood gnomes are tall and thin. They live in the center of the logs and when the tree is cut they need to get out to find a living tree. They can't move lengthwise in the log for physiological reasons, so they force their way out from the center to the perimiter.
Geez. What do *YOU* think? Probably some of that differential drying rate crap...
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On Wed, 11 May 2005 21:36:45 -0600, Dave Balderstone

That's well out of line Dave, it's _not_ obvious why timber splits radially.
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Apparently attempts at humour aren't obvious, either.
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It doesn't split radially from center to perimeter, rather perimeter to center.
These folks have it pretty well worked out. http://www.fpl.fs.fed.us/documnts/fplgtr/fplgtr113/fplgtr113.htm
Enjoy.
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On 11 May 2005 19:58:09 -0700, " snipped-for-privacy@yahoo.com"

If you care about this stuff, find yourself a copy of Hoadley's "Understanding Wood". It's not light reading, but it's well-written and fascinating. You can't be a really skilled woodworker unless you've learned this stuff, and this is perhaps the best book on it (The US Forest Products handbook isn't bad either, and can be read on-line for free).
The reason why it splits isn't obvious. It depends on the fact that tangential shrinkage (around the rings) is about twice radial shrinkage. This ratio varies with species, but "twice" is OK as a rule of thumb. This just isn't obvious without making a scientific study of drying timber, making measurements and taking notes of them (or else reading someone else's notes, like Hoadleys).
If the ratio was 1, then timber would just get smaller as it dried out - no splits. As it is though, the timber turns into a series of rings, each of which is now in tension. Do it on a halved log and the tendency is for the rings to pull themselves straighter, hence cupping of sawn boards. If you do this to a very ring porous timber like sweet chestnut (and probably black ash too) then you might see "ring shakes" developing, where the weak porous timber splits under the shearing force, leaving each ring separate.
Now consider a disk from a log - we've all picked these things up after some chainsaw felling, most of us have thought about making rustic stools or tables from them. Yet it _never_ works. No matter what the species, or how strong it is, you can't get one to dry without splitting. As Hoadley himself admits, he's never dried one bigger than 4" without it going.
The reason why the cracking is so inevitable is related to Hooke's law (the simple law of springs and suchlike). Strain (percentage change in length) is proportional to stress (applied force), by a factor we call the Young's modulus. This modulus varies between species, some much stiffer than others.
Despite our natural tendencies, don't think about forces here. Think instead about strains - the length changes. Timber varies between species in the amount of shrinkage with moisture changes, the initial moisture content and also the strength. However if we look at the total strain from green tree to bone-dry then it's much more consistent and also the strain to break timber in radial tension is consistent, at about 4%. So _any_ strain greater than 4% will cause a crack, no matter whether this is a weak timber where a small force caused this strain, or a strong timber where it took a larger force, but that's also the same size of force to cause that much change in size. Now as "drying" timber (from vaguely green to vaguely dry) can be relied on to generate a 10% strain, we can see that _any_ drying of disks in _any_ timber will cause the cracks.
There are a couple of ways to avoid this.
PEG - a non-volatile glycol used by woodturners to displace water in green timber, rather than evaporating it to dry. This way the timber doesn't shrink.
Cutting a hole in the centre. This allows the disk to shrink as smaller hoops. As the radial force is removed (the centre hole merely shrinks) then the hoops happily shrink without generating the cracking force.
Allowing it to distort. If you saw a _thin_ disk and dry it, it won't crack, but it will twist and buckle into a potato crisp shape (for much the same reason potato crisps do). As a variant of this, a hollow hemispherical bowl turned in green wood can also shrink without cracking after turning, but you'll get distortion instead.
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Angy! Wow great reply!
Ok so why does it strain that way, that is the real question. Why does it shrink on the outside more than the inside?
The reason I think is this. When I look at a round disk of wood, as its drying, you can see the darkness change as it dries. It dries faster on the outside of the disk, than the inside. That's why it splits.
Why does it dry faster on the outside of the disk? Because it has more surfaces through which to evaporate (i.e. the outer surface, where the bark may be). The inside part of the disk can only evaporate through the front of back of the disk. Anyway, the disk does look like its darker on the inside and dry on the edge, when I look at one, its fairly obvious.
Well, anyway, that's my hypothesis anyway.
Comments?
Dean
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" snipped-for-privacy@yahoo.com" wrote:

Because it reduces in direct proportion to the initial size as Andy notes--ergo, larger diameter reduces more than smaller.

The moisture has to diffuse to a surface before it can evaporate and diffusion is driven by concentration differential and controlled by the material properties. In a uniformly porous material, diffusion is more nearly homogenous in all directions where as in wood it is constrained much more to go in the direction parallel to the growth rings. The more porous-diffuse the wood, the more extreme the differential.
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Dean,
As you probably see wood drying/shrinkage is not a simple subject.
There are lots of resources on the web, check the link below and look at the diagram on the 4th page. I find it helpful to understand how a piece of wood will change shape as it dries.
http://www.uvm.edu/extension/publications/nrem/lumberdrying.pdf
Drying the outside of a full round rapidly will contribute to the spliting but that is not the root of the problem. If you dry a full round very slowly in controlled conditions it will still split.
as mentioned the "ratio" of tangential to radial shinkage is the key.
As to your question of WHY is it different, again not a simple answer. The theory that I find easiest to accept and remember is that the wood rays restrict shrinkage in the radial direction.
(it does not shrink more on the outside than inside, it is the direction)
Rays are fibers running radially from the pith to the surface.
Hope this doesn't create more confusion.
Glen

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having to hypothesize. And it's not greater surface. The side of a straw is greater in surface than the walls, but the water runs end to end, anyway.
"With respect to shrinkage characteristics, wood is an anisotropic material. It shrinks most in the direction of the annual growth rings (tangentially), about half as much across the rings (radially), and only slightly along the grain (longitudinally). The combined effects of radial and tangential shrinkage can distort the shape of wood pieces because of the difference in shrinkage and the curvature of annual rings."
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Read before posting, dummy! Sides of straw greater area than the _ends_ is what I meant to say. Structure of wood is designed to move fluid up and down. That's why end grain loses 10-15 times faster than face, which loses faster than quarter.
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On 12 May 2005 07:28:39 -0700, " snipped-for-privacy@yahoo.com"

Sometimes I wonder why I bother posting.
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wrote:

For the rest of us that did read the interesting information you had?
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On Thu, 12 May 2005 11:59:19 +0100, Andy Dingley

<< Snip >>
Thanks for the post, Andy- particularly useful, as I've been scavaging deadfall for the lathe lately!
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I think the easiest way to understand it is this - think of a flat sawn board. As a woodworker, you know the board will shrink most in width, a little less in thickness, and almost not at all in length. It is the difference in shrinkage between width and thickness that matter. Then think of that board within a round log. Because the log is shrinking radially greatly, but in diameter only slightly, the fibers are pulled apart radially around the log.
Not a scientific explanation, but an easy one to grasp.
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Yes, you are describing the effect, but not the cause.
I have heard that wood disks can be dried without splitting, if its done very very slowly over a couple of years. So this to me says something about the rate of drying being important to understand, not just the anisotropic behaviour or wood radially and tangentially.
Which is why I was saying it seems that the drying process is faster on the outer rings of the disk, since its more exposed with additional surfaces (or other properties of those layers of the wood are more prone to drying for some reason).
Dean
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On 12 May 2005 13:08:13 -0700, " snipped-for-privacy@yahoo.com"

I've heard that Elvis is still alive and serving chips, but that doesn't make it true.
You have two choices here. One is to make some disks and spend a few years drying them very carefully indeed. Come back in about 10 years and tell us how you got on. The second is to listen to the explanations, or if you don't believe the peanut gallery here (not entirely unwise!) go look it up for yourself in Hoadley or the Forest Products guide.
There's no _need_ for you to listen to or believe anything you read here. But equally there's no need for you to ever learn anything. That's not our problem.
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<snipped>

It may not have been the motive, but if it had been a troll, I doubt it could have been more successful. Some of the more knowledgable and usually helpful folks responded.
Patriarch
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On 12 May 2005 13:08:13 -0700, " snipped-for-privacy@yahoo.com"

The cause is that the wood cells are very much longer in the vertical [alive and well] direction as compared to the horizontal. Think of a lot of thin tubes lightly glued [bonded] together. Suppose they all shrink equally in all directions. Now think of the structure of the tree, and simplify it. One cell in the center. More around that, more around that, more around that ...ever increasing circles of cells. Now let all shrink in all directions equally, so losing the same percentage per cell. The inner ones will each shrink by the same absolute amount as those individual cells around the outside, but since there are more around the outside, the total absolute amount will be greater there.
If this is correct, then [near as dammit is to swearing], thinking in term of a sequence of circles, the percentage of crack-space in an inner circle should be the same as the percentage of space in the outer edge, and in fact throughout the material: More material, more space, same percentage.
If still confused, think of expansion and contraction of a steel rod due to heating/cooling. A longer rod will expand and contract further than a small one. The measure of wood around the outside of a log is greater than around the center, and the tree grows in definite rings.
Why does it crack in the first place? Bonding. Even steel cracks under pressure since it is uneven throughout, even though it looks uniform outside of a microscope. I'll quit now. My brain is exploding.
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wrote:

That's a bad analogy. Steel is isotropic - it has the same behaviour in all directions. Timber is anisotropic, the shrinkage is different on all three axes. A "steel tree" wouldn't crack as it dried out.
Most carpenters know that timber shrinks "crosswise" but not "lengthwise". That's still not enough to cause cracking. What many carpenters don't realise is that the tangential shrinkage is twice the radial shrinkage. It's this, coupled with the fact that trees are made of nested cylinders, which causes the radial cracking.
The outer _may_ dry out faster than the inner (although this is much overshadowed by the effects of the ends, and the much greater water transport lengthwise). If you bake the outside of a log to dry it you could even "case harden" the timber and cause checking - however this would be the usual honeycomb checking of bad seasoning, not the inevitable radial cracking of drying logs in the round.
A consideration of shrinkage and the geometry will show us that a dry log cracks, no matter how slowly or carefully you do it. Thinking about the strains rather than the stresses will show that all timbers will do this, not just "weak" ones.
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