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Poisson Effect Versus Neutral Plane - A Theory

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tom sawyer:
Here's a quote from an engineer guy named Ken, he is a wise person.  "Wise" being defined as someone who agrees with my point of view on the subject.

"The neutral plain may indeed be trying to flatten out and this could be expected with a triangular belly since by flattening of the neutral plane you would have the back and belly both curving down and the effective distance from the neutral axis (not surface) would get smaller and thus reduce the absolute amount of stress (and strain) on the limb. This also has the effect of reducing the Moment of Inertia which slightly raises the Stress level. The equilibrium point you are witnessing is the lowest energy state for the circumstances."

"The thing to remember is that stress and strain go hand in hand. In order to be stressed X amount, the material must strain Y amount. If this can't happen then the material will try to compensate by whatever means necessary so that it reaches the lowest energy state possible for the situation. With materials (solid or laminate), a combination of shear flow and deformation occurs until equilibrium is established or failure occurs. Poisson's ratio is an expression of the relationship between stress and strain and is indicative of how the material will reach the lowest energy state."

Badger:
Lennie, strength in compression or tension only refers to its resistance to bending, has no relation to it's elasticity. You have a modulus of elasticity and a modulus or rupture that refer more to those thngs. Steve

tom sawyer:
OK we don't have to agree on what the terms mean, to think about what is going on in a limb.  Regardless of where the NP lies in a limb, all the wood within that limb is trying to be as close to it as possible so that stress is minimized.  It strains to get there, as evidenced by Baker's observation #2 (the triangular cross-section).

duffontap:

--- Quote from: tom sawyer on June 08, 2007, 05:22:22 pm ---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.

--- End quote ---

Did you even read my last post?  ???   We're talking about the same thing--I'm just (respectfully) disagreeing with you. 

1.  It takes different amounts of energy to stretch a material than it does to compress it.  This is true for every substance on earth.  Why would any substance respond to opposite forces in exactly the same way?
2.  If there is no such thing as elasticity in compression and tension, Perry and Baker are mistaken in much of their thinking.

e-las-tic
adjective
capable of returning to its original length, shape, etc., after being stretched, deformed, compressed, or expanded: an elastic waistband; elastic fiber.

                 J. D. Duff

tom sawyer:
I thought I read it.  And now I've reread it.

Elasticity also equates to stiffness.  If something is less elastic, it is more stiff.  Just words on the ends of the same measurement.

Modulus of elasticity is actually more properly modulus of stiffness, because higher is actually stiffer.  Its a measure of how much energy it takes to bend a board in a particular direction.  More energy means stiffer board.

I'm not trying to get you peeved about this.  We should just agree to disagree.  The observation made isn't dependent on exactly where the NP is in the limb.  You just need to know that the wood strains to get to the lowest energy state which is the closest to the NP.  I think that explains the observation quite nicely.  It and the Poisson Effect, are things we should consider when we discuss the merits of bow limb cross-sectoins.

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