Is the early wood the weak link to cause set?

[bjrogg]

So if I’m following you correctly Arvin. The back side of this bow stretches just over 3/8” when drawn only to 21”.  Seems like it would get considerably longer at 28”

Bjrogg

[Selfbowman]

Yes Brian. That is the bow I’m working on and it’s not been to 28 yet. I’m blown away. If no set that would mean it is compressing that much also. I’ve got a all new respect for wood.

[PatM]

Alan - there is a small difference. See for example "Conners, T. & Medvecz, P. (1992). Wood As a Bimodular Material. Wood and Fiber Science 24, 413-423".

In wood tensile modulus is about 8 % greater than compression modulus. But, of course there is a lot of variation. For example, in sugar maple the difference can be almost 28 %.

Thanks for that paper, it is always nice to have some references to actual research. That much difference in sugar maple indicates that I should be heavily trapping every maple bow I make.


Mark

 I was going to quote similar.   True tensile tests are not that common.   I have seen numbers showing about 7500 for compressive in Maple and  about  15000  for tensile.  Very likely Hickory is similar.
 

[avcase]

Alan - there is a small difference. See for example "Conners, T. & Medvecz, P. (1992). Wood As a Bimodular Material. Wood and Fiber Science 24, 413-423".

In wood tensile modulus is about 8 % greater than compression modulus. But, of course there is a lot of variation. For example, in sugar maple the difference can be almost 28 %.

This is good info.  28% for Sugar Maple is quite a bit of difference, but I see that was from a single separate study, so methods and the way samples were prepared might be a little different than the other studies. I am thinking of trying two experiments, one with horn, and one with sugar maple to see how it compares.

Thanks!

[willie]

Mark,
did you have a chance to review the test data from the Canadian Forest Lab, where sugar maple excelled in certain tests?
 
I am thinking those tests might serve as a proxy for tensile testing.

[willie]

Does anyone following Arvins testing method have access to a yew longbow with a sapwood back?  I am curious how much of the elasticity in yew is attributable to the sapwood. A close to "circle  of arc' tiller could be useful to easily make a calculation comparing the back stretch measurement to the whole.

[willie]

Conners and Medcevz  are referencing  Schnieder et al

https://www.cabdirect.org/cabdirect/abstract/19920656871

for the sugar maple data.   

[PatM]

Does anyone following Arvins testing method have access to a yew longbow with a sapwood back?  I am curious how much of the elasticity in yew is attributable to the sapwood. A close to "circle  of arc' tiller could be useful to easily make a calculation comparing the back stretch measurement to the whole.

 Yew bows with no sapwood are still very elastic.  It  likely owes a lot of its attributes from being a very balanced wood naturally.

[mmattockx]

I was going to quote similar.   True tensile tests are not that common.   I have seen numbers showing about 7500 for compressive in Maple and  about  15000  for tensile.  Very likely Hickory is similar.

Those sound like yield/rupture numbers and not modulus. I would expect sugar maple to be closer to 20,000psi for MOR but wood varies so much that you never know unless you measure the piece you are actually using.

Mark,
did you have a chance to review the test data from the Canadian Forest Lab, where sugar maple excelled in certain tests?
 
I am thinking those tests might serve as a proxy for tensile testing.

I have looked at it, but I don't recall the particulars. I will have to review that.

Conners and Medcevz  are referencing  Schnieder et al

https://www.cabdirect.org/cabdirect/abstract/19920656871

for the sugar maple data.   

That link requires a login to access.


Mark

[bjrogg]

I’m also curious about Arvin’s stretch test. I wonder if it would be different if it had a single growth ring instead of a edge grain.

Arvin is that the bow with edge grain and horn on belly?

Bjrogg

[bjrogg]

I’m also curious if it has horn on belly and no set. Maybe the belly isn’t compressing as much as the back is stretching?  Maybe elastic back is better for no set. Maybe why sinew and horn such a good combination? The capability of the back to stretch would help to keep the belly from having to crush.

Bjrogg

[PatM]

I was going to quote similar.   True tensile tests are not that common.   I have seen numbers showing about 7500 for compressive in Maple and  about  15000  for tensile.  Very likely Hickory is similar.

Those sound like yield/rupture numbers and not modulus. I would expect sugar maple to be closer to 20,000psi for MOR but wood varies so much that you never know unless you measure the piece you are actually using.

 Yes, I was just showing that they are different

  I think they do say that tensile strength and modulus of rupture are very similar in numbers.

[mmattockx]

Yes, I was just showing that they are different

  I think they do say that tensile strength and modulus of rupture are very similar in numbers.

Ah, yes. The properties certainly do vary significantly between the two directions. Modulus of rupture is roughly equivalent to the yield strength of a ductile material. Because wood has no ductility it just blows when you pull too hard on it.


Mark

[willie]

That link requires a login to access.


Mark

I was going to see if I could find the the referenced article when I had more time, but it is notable that Conners and Medcevz are only quoting numbers from the Schnieder article, Mechanical properties of polymer-impregnated sugar maple. which may or may not be relevant. from related quotes and abstracts....

The treated sugar maple wood showed increases in MOE and MOR in bending by 20.0 and 28.0 %, respectively, as it was treated with a vinyl-type monomer to 51 % WPG. ...

[avcase]

I’m also curious if it has horn on belly and no set. Maybe the belly isn’t compressing as much as the back is stretching?  Maybe elastic back is better for no set. Maybe why sinew and horn such a good combination? The capability of the back to stretch would help to keep the belly from having to crush.

Bjrogg

Horn had a lower elastic modulus than wood, so it is less stiff and will be compressing more than the back is stretching. Horn is also able to handle much more compression strain than wood, so this isn’t a problem.