Poisson Effect Versus Neutral Plane - A Theory

[duffontap]

Very nice Lennie.  I need to pick that up.  If you scroll back to the earlier pages, my arguements were that 1) the 'midway point' may correspond with the NP but that is not what 'neutral plane' means and it is a definition with very limited application; 2) wood can respond differently to tension and compression forces.  I spent the whole weekend imagining a way to test this and I have it figured out.  I will try to test a couple samples to try to show that they can respond differently to the opposite forces but I may prove myself to be in error. 

I think you would agree too that 'a thin section located midway' is not exactly explicit.  I am willing to change my mind on the tension/compression elasticity if evidence gets in the way of my bowyer's intuition.  I sure can take a lesson from Tim Baker and start testing theories on wood once in a while.   

          J. D. Duff

[tom sawyer]

I know what you mean.

We can easily test this hypothesis by measuring the back and belly length changes of a drawn bow or just a slat of wood long enough and bent far enough for a difference in length to be measured.  IF the NP is in the middle, both changes will be equal.

By "we", I mean you of course.

And if you read that other thread, there is some data that shows at least some woods have different MOE in tension versus compression, so you are proved right already.  I don't guess I mentioned that earlier.  hehe  And that other thread has some interesting new posts.

[DirtyDan]

I do not know if this is entirely relevant to the discussion, but my best attempt at the Ambush Bow Challenge was a bamboo-backed osage with horn belly that exploded, splitting the osage core EXACTLY DOWN THE MIDDLE of the osage core on the bottom limb.  I do think that this combination at such a short length of 58" and 28" of draw and reflex-deflex design must have caused a tremendous sheer effect on the exact center of the wooden core.  At least that is what my theory is of why it failed.  I know that I was FROWNING, NOT SMILING, when it happened.  Great discussion, though.  I think in TBB they illustrate the Poisson effect by showing a rubber band bending and the edges curling up, do they not?

Dan Spier

[tom sawyer]

Sounds like the combination of horn and boo might be too much for a core, especially with a stress-concentrating design like a short R/D.  I don't know if horn stretches more or less than boo, but it sounds like there was more shear than normal.  Those things don't usually split down the center like that.  I suppose you could determine that your NP was right in the center, thats doing it the hard way though.

A sinew-backed hornbow puts the NP much closer to the belly than bamboo would.  It probably constrains the core much less too.  The ancient bowyers probably tried the combination you used, and had the same results.  You might have re-created an actual scene in an ancient bowyer's shop, complete with cursing and crying.

I'll look in the TBB, didn't remember Poisson being mentioned but its been a long time since I read those.

[Badger]

bamboo is just the opposite from sinew which would normaly be used as a backing, bamboo doesnt like to stretch that much and horn doesnt like to compress that much, i guess it figured the tug of war was a draw so they split down the middle. Steve

[tom sawyer]

I now horn can take more compression stress than wood, but is it stiffer?

[Justin Snyder]

I suppose you could determine that your NP was right in the center, thats doing it the hard way though.

Once again I think you are reading to much into this result.  The only thing you can determine is that the weak spot was in the middle.  Even if you did assume that the neutral plane was at the break, (which would be a huge assumption) it would not be the center of mass like hypothesized earlier. The horn is significantly heavier than the boo.  Justin

[bobnewboy]

ACS-type section of <whisper>fibreglass limbs
//Bob

I was thinking about an article I read in which fiberglass bows were forced into a hollow 'C' section.  Such bows had far superior performance.  I had thought for a which about hollowing-out a 'C' section bow to see what it acted like but it's too much work.  Is that what you're talking about?

Hi JD,

Yes, it is the curved section limbs that I was thinking about.  The ones mentioned have a convex back, and a concave belly, although with a fair sized radius.  The overall limb section is C-shaped.  The claim is that the limbs are far lower in mass for the same draw weight, and hence they shoot more quickly for the same draw weight.

Considering these limbs with the Poisson effect is what made my brain fade 

//Bob

[tom sawyer]

Justin, I wouldn't assume that a composite would still have its NP in the center of mass.  It would only be the case if the horn and the boo had similar MOEs for their respective positions, correcting for thickness.  The center of mass only equates to NP for single materials.

[SimonUK]

Trying to picture this C shaped bow... If you made it out of wood, you might run the risk of frets on the tips of the C as that's where the compression would be concentrated. However, if the poisson effect flattens out the C as you bend the bow, you could avoid that. I guess it would have to be quite a shallow C shape.

I prefer the sawyer effect bow where the sides are thinner and there is no change in the cross sectional shape as you bend the bow. That way all the energy goes into stretching the back rather than making it skinnier.