Why does wood split radially?

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

Reply to
deanbrown3d
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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...

Reply to
Dave Balderstone

That's well out of line Dave, it's _not_ obvious why timber splits radially.

Reply to
Andy Dingley

It doesn't split radially from center to perimeter, rather perimeter to center.

These folks have it pretty well worked out.

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Reply to
George

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.

Reply to
Andy Dingley

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

Reply to
deanbrown3d

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.

Reply to
Duane Bozarth

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.

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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.

Reply to
glensmith

Had you read the information in the FPL post, you'd have the answer without 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."

Reply to
George

Apparently attempts at humour aren't obvious, either.

Reply to
Dave Balderstone

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.

Reply to
George

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.

--

******** Bill Pounds
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Reply to
Pounds on Wood

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

Reply to
deanbrown3d

Also a bit backward. As the FPL says, the shrinkage tangentially is greatest. Thus the radial checks - perpendicular to tangential force.

Take a log and measure a chord which does not cross latewood near the center. Let's say it's an inch at best at 20 years. Now go out near the outside and measure a chord of say 5 inches at seventy. With a 10% rate of shrinkage, it means the interior wants to lose 1/10 of an inch, the farther, 5/10. The difference is made up in open air as the radial split opens.

Reply to
George

Sometimes I wonder why I bother posting.

Reply to
Andy Dingley

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.

Reply to
Andy Dingley

Andy Dingley wrote in news: snipped-for-privacy@4ax.com:

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

Reply to
Patriarch

That's an excellent explanation. It also accounts for why the splits are wider, further from the center.

Reply to
Guess who

For the rest of us that did read the interesting information you had?

Reply to
Edwin Pawlowski

Thanks for the post, Andy- particularly useful, as I've been scavaging deadfall for the lathe lately!

Aut inveniam viam aut faciam

Reply to
Prometheus

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