Primitive Archer
Main Discussion Area => Bows => Topic started by: DC on November 29, 2017, 07:23:47 pm
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Anybody have any idea why Yew is so good in compression? It's such a soft wood you would think it would crush easily.
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Short answer... I dunno ::)
long answer... It has evolved that way so that Yew trees don't buckle, slump and fall over.
I've noticed on some big Willows where they lean, you can see the buckling and thickening round the base where the wood has failed in compression repeatedly and grown over.
It's easy to confuse soft, light, stiff, flexible, strong and other properties, especially when comparing along and across the grain.
Del
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It can be soft but I have occasionally worked some that was as hard as Maple.
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It has a higher elasticity than most woods and decent stiffness. This equates to a lower mass bow that won't take as much set.
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It has a higher elasticity than most woods and decent stiffness. This equates to a lower mass bow that won't take as much set.
So you're thinking that it compresses more than say, Ipe, but the elasticity allows it to recover?
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It can be soft but I have occasionally worked some that was as hard as Maple.
Yeah, I like working that stuff. Feels like it's already been heat treated.
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Short answer... I dunno ::)
long answer... It has evolved that way so that Yew trees don't buckle, slump and fall over.
I've noticed on some big Willows where they lean, you can see the buckling and thickening round the base where the wood has failed in compression repeatedly and grown over.
It's easy to confuse soft, light, stiff, flexible, strong and other properties, especially when comparing along and across the grain.
Del
I realised after I posted this that, "I dunno." was probably the correct answer but I'm liking Badger's answer.
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Because Yew is magical. :OK
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http://www.primitivearcher.com/smf/index.php?topic=55682.0
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So Willie, you're thinkin' that Yew has a greater fibril angle even in more mature wood?
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i will take their word for it....
in post 33 of that thread, Joachim posted a link to a paper he uploaded to google drive.
Ingela Bjurhager*, E. Kristofer Gamstedt, Daniel Keunecke, Peter Niemz and Lars A. Berglund
Mechanical performance of yew (Taxus baccata L.)
from a longbow perspective
Conclusions
Tensile, compression, and 4PB-tests tests were performed
on yew, pine, and juniper, and the stiffness and strength
were determined. Whereas tensile strength did not differ
between the three species, yield stress in compression was
higher for yew. Moreover, yew displayed a surprisingly low
stiffness, considering its high density. This can probably
be ascribed to its unusually high MFA. The experimental
data in combination with a simple bow model showed
that yew has a high toughness, which means that a large
amount of elastic energy can be stored in a yew bow and
transferred to the arrow. Consequently, a bow made from
yew is likely to have a larger range, compared with bows
from other wood species. The 4PB-tests in this study also
showed that yew could withstand high strains in the
plastic region before failure. Bows are often subjected
to bending beyond the elastic limit, and therefore this is
another desirable trait. The remarkable ability of yew to
withstand large deformations and resist crack formation
has been ascribed to the high MFA and a high amount of
rays in the radial direction, where the later results in a
fiber bridging behavior.
It was found that heartwood performed better in terms
of yield stress in compression than sapwood. However,
the difference was not related to the extractives. Instead,
the difference is probably related to morphologic traits
because yew heartwood is likely to be composed mainly of
juvenile wood, which differs from mature wood in terms
of MFA and density. Based on our experimental and model
results, the guideline for making a bow with maximum
performance is to include approximately 30–50% juvenile
wood on the belly side. Moreover, the stiffness and yield
stress in compression for the juvenile wood should be as
high as possible compared with the mature wood on the
backside of the bow.
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Although wood properties do make a difference in suitability as a bow wood, there are surprises from an engineering standpoint. Between two woods with the same modules of elasticity, the amount of stress required to produce a unit of bending response, the wood with the higher elastic limit is the better species for archery. But wait, there's more! Between two woods with the same MOE and the same EL, the less dense wood is better because it will be lighter. Most of the common hardwoods in North America have similar ratios. However, yew is lighter for its MOE than average and still has a high MOR. Garyanna Oak is denser than Yew but has a better MOE/MOR ratio. Osage Orange also stands out. I think I posted a table. I will see if I can find it. Ultimately, within the group of woods that are good or excellent for archery, individual quality of stave and workmanship outweigh species in my opinion.
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I guess I didn't post a table but here is a discussion about the mysteries of bow wood that might give some perspective. There is an essay mentioned that is enlightening.
Oops:
http://www.primitivearcher.com/smf/index.php/topic,60200.0.html
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I wonder how much of a yew log is "crown wood" or "juvenile" wood. That could have a huge impact especially on fibril angle.
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I don't have too much to add, but I've for sure noticed on bows with juvenile belly wood (pith on the belly or close by), they turn out suuuuuuper good. Very little set.
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Note that also compression wood has a higher microfibril angle (at least in some conifers), which in turn reduces its modulus of elasticity (making it more supple, or rubber-band like if you want), and increases its strain tolerance.
As for compression tolerance, don't forget the importance of other wood components: high lignin content also confers better compression properties. Incidentally, this is also higher in compression wood.
There's been a lot of research on these topics, as high MFA also leads to less stable wood (torsion and other distortions), which isn't desired in timber.
Google "compression wood microfibril angle"
From a 2004 paper: "The large MFA in juvenile wood confers low stiffness and gives the sapling the flexibility it needs to survive high winds without breaking. It also means, however, that timber containing a high proportion of juvenile wood is unsuitable for use as high-grade structural timber."
How much juvenile wood there is: in some analyses rings deposited during the first 35 years were still considered juvenile. But I doubt a ring count is a reliable way to assess this. Still, there is a gradual change (in MFA) from juvenile to adult wood, not an abrupt one.
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No set number of rings. I prefer the term crown wood, even though most use juvenile wood, because it doesn't really mean the wood is juvenile. You could have 100 years of juvenile wood if a tree had 100% crown. Much more to do with hormones associated with the crown, so it just makes more sense to me.
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From a 2004 paper: "The large MFA in juvenile wood confers low stiffness and gives the sapling the flexibility it needs to survive high winds without breaking.
I've wondered about this. Yew is an understory wood. At least Pacific Yew is. It doesn't normally get hit by the wind. Actually in a natural situation very few conifer saplings get hit by wind. They would always be protected by the surrounding trees. They may get dumped on as snow slides off the branches of other trees but wind, uh uh. The trees around the edge of the forest would feel the wind but I don't think they would effect the evolution of the species that much.
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I get your point. Saplings don't just get hit by wind, they typically are long and thin. If not very flexible, they would break too easily when bent in any way.
We may not know exactly why its wood behaves in a certain way, but it's been eons of evolution that made it the way it is. If it has certain features, there is probably a good reason for it, we just may not know yet exactly why.
When I see my fruit trees (apples, pears, cherries, peaches and so on), their branches need to be able to bend under the heavy load of the fruit in the summer and fall without breaking. Incidentally, these species make excellent bows as well, because they have similar properties as yew.
I like the way E Jensen put it as crown wood, makes a lot of sense to me: it couples the need for higher elasticity to the functional aspects of being wood that has to bend more.