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Poisson Effect Versus Neutral Plane - A Theory

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tom sawyer:
Justin, I think you are confusing the strength and elasticity terms.  Just because wood is stronger in tension, does not mean the back is the 400lb wrestler.  The wood has one elasticity, also known as stiffness.  This stiffness is the same whether you stretch or compress it.  Maybe I'm not understanding your point though, I'll think on it some more.

Marvin, I didn't say it was all that significant.  But the Poisson Effect and possibly this other business I am describing is there and I'm simply pointing out that there are stresses on a bow limb that we don't always think about.  Its kind of hard to measure things like cross-section of a bow at full draw, so we tend not to think there is anything going on.

Will you make a better bow, knowing these stresses exist?  Heck I don't know.  I do think it changes the argument for rectangular cross-section just a bit.  That subject has been debated pretty heatedly in the recent past.

I agree that "plane" is misleading, in that it implies a flat 3D surface.  Since the limb bends, the plane is curved.  What I am not positive about, is whether the NP (neutral place?) is flat in 2 dimensions or is whatever shape is dictated by the cross-section.

Simon, I see what you're saying, it could be that this is the explanation.  What would you call that?  The edge is obviously moving to the point of least stress, and it is doing so in spite of the Poisson Effect tendency.  So it must be stronger than this effect.  I think it is possible to make a cross-section where there is no net movement.

This must be simple stuff to a physicist who has studied bending and the like.  We aren't discovering some unknown property here.

Mims I felt like I needed to post everywhere, to get a few responses.  I actually got more good input than I expected. 

duffontap:
Lennie,

I'm glad you brought this whole discussion up.  Is this basically what you have in mind?:

If some of the muscle energy used to draw the bow is absorbed by the NP trying to straighten itself out, bows which are less affected by the poisson effect will transfer more stored energy to the arrow.


I wasn't trying to get off-topic by talking about a strict definition of the NP.  I thought a correct understanding was necessary deal logically with your theory. 

                 J. D. Duff

DCM:
Lennie,

Each site has it's on character(s).  ;-)  Not critisizing, just noticing.  But I deplore naming things, "Baker effect", "Perry reflex", etc.  As you've pointed out, there's little or nothing new in this game.  Just a pet-peeve.

Was Paleo the recent heated debate.  I may have missed it.

marvin:
Good, thought provoking stuff Lennie. A refreshing discussion.

"Will you make a better bow, knowing these stresses exist?  Heck I don't know.  I do think it changes the argument for rectangular cross-section just a bit.  That subject has been debated pretty heatedly in the recent past."

I think your right. My experience was that although it may be theoretically a superior cross section shape it did not have as much of a real world effect as other limb design factors. I've made radiused bellied, lenticular like cross section limbs that performed much better then my best rectangular cross section.

"I agree that "plane" is misleading, in that it implies a flat 3D surface.  Since the limb bends, the plane is curved.  What I am not positive about, is whether the NP (neutral place?) is flat in 2 dimensions or is whatever shape is dictated by the cross-section."

Now I understand better what you're talking about and exploring. Very interesting question. I would think the NP would NOT be flat in cross sections where at any given point in the cross sections width there is more wood either side of the physical center regardless of the poisson effect.

marvin:
David, I think it was paleoplanet that he's referring to.

I really appreciate folks like Lennie and David and a few others who discuss/explore the more technical aspects of primitive archery and look forward to seeing more threads like this one.

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