Main Discussion Area > Bows
Poisson Effect Versus Neutral Plane - A Theory
Justin Snyder:
--- Quote from: tom sawyer on June 08, 2007, 11:56:44 am ---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.
--- End quote ---
If the wood does not stretch, bud does compress the neutral plane will shift toward the back. If it stretches bud don't compress it shifts to the belly. Some woods don't compress better and some stretch better. That is why we use different woods for bellies and backings on a laminated bow. Justin
tom sawyer:
What do you mean by "better"? Stretching and compressing are both aspects of elasticity. Within elastic limits, a particular piece of wood will produce equal amounts of tension and compression work. Your "better", is how much stress it can handle before it breaks.
I'm sure this "bending opposite Poisson effect" is explained by simple engineering terms, but if Baker turns out to have been the first one to apply this to bows and actually explains it to us, then it'd be about like Perry and his reflex method. What'll really tickle me though, is to see him admit that the purely rectangular cross section isn't necessarily such a cut-and-dried best bet. I'm not holding my breath of course.
Badger:
Lennie, I tend to treat all woods as if they are equal in tension and compression when laying out a bow, but the fact is they are not, it becomes evident as you build the bow and get further into the tillering. It is very common for hickory and black locust both to be stronger in tension than compression. I think for general purposes most of the time it is safe to assume they are pretty close to equal. When a stav is pretty well equaly matched in it's tension and compression properties you will be rewarded with a very low set bow with low mass. If you know ahead of time they are not well matched you can compensate in the design. Steve
duffontap:
Lennie,
The following is an attempt to describe how I see it. I could be wrong as I haven't performed a test on anything. I am not as noble or helpful as Tim Baker in that respect. I'm trying to work on that. At any rate:
A rope has high tension resistance and no compression resistance, while a stack of unmortared bricks has high compression resistance but almost no tension resistance. Every piece of wood will have similar qualities to varying degrees.
I don't think elasticity is the same thing as bend strength. Elasticity is the ability of the wood to bend or deform without being damaged. Bend strength is the tension resistance + compression resistance.
Example: Locust can be stretched without damage further than it can be compressed, while juniper can be compressed without damage further than it can be stretched.
If a selfbow finds that it is easier to stretch a little more and compress a little less, this will move the NP.
J. D. Duff
tom sawyer:
l give up trying to convince you guys.
Ok one more time. :P
Yes locust can be stretched farther without breaking than it can be compressed. That is what strength in (compression/tension) tells you.
But it takes exactly as much energy to compress BL 1mm, as it takes to stretch BL 1mm. That is what elasticity tells you. There is no such thing as elasticity in tension or elasticity in compression. Elasticity is another word for stiffness.
I can think of a case where a selfbow doesn't have its NP right in the center of mass of the cros-section. That would be when it shows some set, which is generally due to a slight (you hope) plastic deformation of the belly wood. Now you've damaged your belly wood a bit and its properties are now slightly different. Its a bit compressed, probably a little stiffer now since it has a slightly higher density.
Navigation
[0] Message Index
[#] Next page
[*] Previous page
Go to full version