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Why does wood split radially?



 
 
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  #1  
Old May 12th 05, 03:58 AM
[email protected]
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Default 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

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  #2  
Old May 12th 05, 04:36 AM
Dave Balderstone
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In article .com,
" wrote:

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


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...

--
~ Stay Calm... Be Brave... Wait for the Signs ~
------------------------------------------------------
One site: http://www.balderstone.ca
The other site, with ww linkshttp://www.woodenwabbits.com
  #3  
Old May 12th 05, 11:37 AM
Andy Dingley
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On Wed, 11 May 2005 21:36:45 -0600, Dave Balderstone
wrote:

Geez. What do *YOU* think? Probably some of that differential drying
rate crap...


That's well out of line Dave, it's _not_ obvious why timber splits
radially.
  #4  
Old May 12th 05, 11:58 AM
George
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wrote in message
oups.com...
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


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/fp.../fplgtr113.htm

Enjoy.


  #5  
Old May 12th 05, 11:59 AM
Andy Dingley
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On 11 May 2005 19:58:09 -0700, "
wrote:

When it dries, why do you think wood splits from the center to the
perimiter?


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.

--
Cats have nine lives, which is why they rarely post to Usenet.
  #6  
Old May 12th 05, 03:28 PM
[email protected]
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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

  #7  
Old May 12th 05, 04:04 PM
Duane Bozarth
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" wrote:

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?


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

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.


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.
  #8  
Old May 12th 05, 04:29 PM
glensmith
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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/publica...mberdrying.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
wrote in message
ups.com...
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



  #9  
Old May 12th 05, 04:57 PM
George
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wrote in message
ups.com...
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?

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."


  #10  
Old May 12th 05, 05:00 PM
Dave Balderstone
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In article , Andy Dingley
wrote:

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


Apparently attempts at humour aren't obvious, either.

--
~ Stay Calm... Be Brave... Wait for the Signs ~
------------------------------------------------------
One site: http://www.balderstone.ca
The other site, with ww linkshttp://www.woodenwabbits.com
 




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