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Poisson Effect Versus Neutral Plane - A Theory
Justin Snyder:
Interesting Lennie. I would be interested to see you put this into practical use.
I think you may be reading into it a little. I think it is wood being crushed between the back and the belly. If the energy is too great for the back it explodes. If the energy is to great for the belly it frets. The energy on the bow that does neither is stored in the wood in the center of the bow, and tries to escape out the sides. Because it is being pushed toward the belly it pushes the belly wood out more. The belly wood being pushed out is still trying to escape out the belly, so it curls toward the belly and outside edge. By rounding it will not have a straight edge where the human eye can see the movement. Just my thoughts. Justin
SimonUK:
For me it's simpler than that. The bow with a v cross section frowns because it has no belly at the sides to stop them curling towards the belly. After all, you're stretching the back and it wants to be as short as possible.
tom sawyer:
JD and Pat, for a selfbow the position of the NP coincides with the center of mass. A single material has only one elasticity, so it stretches and compresses equally well. The "strength in compression" and "strength in tension" numbers (which are different for a given material) have nothing to do with the neutral plane, they only refer to when the back or belly will break. When you say "a wood is stronger in tension than compression", it doesn't mean the back is doing more work than the belly.
With a crowned back, you'd make your belly either flat or slightly roudned but not as much as the back. That would make the cross section "frown", and it would go away and be flat at full draw. Theoretically.
Using different materials in one limb, like a bamboo backing that is less elastic than wood, shifts the NP towards the backjing but it doesn't change the fact that the NP is not planar and wants to get that way. Come to think of it, theres a good reason to try and keep the cross-seciotn from "smilling" or frowning on a bamboo-backed bow, because this could lead to splintering when the backing gets mashed together by the Poisson Effect.
Can't you feel your head getting bigger?
duffontap:
--- Quote from: tom sawyer on June 07, 2007, 04:58:03 pm ---JD and Pat, for a selfbow the position of the NP coincides with the center of mass. A single material has only one elasticity, so it stretches and compresses equally well. The "strength in compression" and "strength in tension" numbers (which are different for a given material) have nothing to do with the neutral plane, they only refer to when the back or belly will break. When you say "a wood is stronger in tension than compression", it doesn't mean the back is doing more work than the belly.
--- End quote ---
As I understand it:
1. Wood is not completely homogeneous (consider Yew with sapwood or Osage with wide variety of latewood ring widths).
2. The same piece of wood does respond differently to compression and tension stresses.
J. D. Duff
Badger:
Lennie, couple of ways you could look at it, the tension or work you are putting into the string and being transferred to the limb is causing this effect, wouldn't the reverse affect just transfer the energy back to the arrow when you rleased the string?
The other possible way to look at it is also the reason I prefer slightly oval shaped limbs, is that when the affect starts to take place it is simply amatter of the wood kind of giving up and folding, using more mass in the limb than you actually need for the same amount of work. I find I can control mass better on oval shaped limbs than flat limbs, the poisen affect may be a reason for this. Steve
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