Poisson Effect Versus Neutral Plane - A Theory

[tom sawyer]

Mims, you never know which site is going to provide the most interesting discussion.  PP tends to have engineers weighing in and they are the ones who would contribute a lot to this discussion, but none really have over there.  And Baker has been oddly silent as well.  He's probably rueing the fact that he posted his results, I suspect he might write about this one day.  I do think my discussins of the Possion Effect got him thinking and caused him to do the experiment in the first place.  thats the great thing about Tim, he goes and measures stuff.  I need to do more of that.  The LW discussion is really good, and I posted a bit of this on a related thread on radiused vs flat bellies and got a stellar reponse from a guy named Ken.  I asked if I could post his reply other places.

This place has had the best overall discussion though.  Very enjoyable.  And I am the first to say that my ideas are not necessariy correct, which is why I try them out on people.

I shouldn't have said "recently" argued, I was referring to Baker and Torges' exchanges several years ago.  But it comes up from time to time and people always have their favorite.  You know they both work pretty well though.

[duffontap]

I shouldn't have said "recently" argued, I was referring to Baker and Torges' exchanges several years ago.  But it comes up from time to time and people always have their favorite.  You know they both work pretty well though.

Is that exchange available to read?

         J. D.

[tom sawyer]

Not that I know of, and to be honest I heard about it second-hand as I wasn't into traditional archery back when it ocurred.

[duffontap]

I know there were some pointed comments in the TBM column that became Hunting the Osage Bow.  I would love to see an actual exchange though. 

          J. D.

[Badger]

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.

David, I used to feel the same way but have grown to like it now, if an individual simply makes something popular, explains it, or invents it, I don't mind seeing their name on it,
   What does bug me is that if the ideas were all ready popularized or somehow credit was robbed from a known individual who deserved some recognition. I have yet to see anything new in bowyering since I have been involved, at best I have seen a better understanding of existing principles start to fall into place. The main reason I like the use of names is for easy identification and uniform understanding of a procedure. I also think it encourages and recognizes bowyers who have gone one step beyond in their homework. I know of only a handful of procedures that would fall into this category. Steve

[Pat B]

Would someone please explain Poisson Effect to me. Maybe that would help. When I build a self bow I let the wood tell me what it wants to be. Usually the theoretically aspects don't even enter my head.  Pat

[duffontap]

Pat,
Take a rectangular pice of foam rubber and bend it in an arc.  The back will cup and narrow and the belly will crown and widen.  I think.

         J. D. Duff

[marvin]

Pat,

The real scientific types will probably cringe at my explanation but essentially the Poisson effect describes the tendency of materials to narrow when stretched or put under tension and conversely widen when compressed.

It's a fancy mathematical way of describing material distorting it's shape when stretched or compressed.

[marvin]

Lennie, if you want some really technical discussion from engineer types you should post this at the flight archery site.

http://www.flightarchery.com/forum

[Pat B]

Thanks Josh and Marvin. That I can understand.     Pat

[Justin Snyder]

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.

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. 

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.