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Stress and performance

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mmattockx:

--- Quote from: Kidder on June 02, 2022, 01:21:05 am ---Is there any correlation between design stress and performance?

--- End quote ---

Yes. You will get more performance as stresses go up until you go too far and the wood starts taking set or otherwise failing due to the high stresses. As the others have noted, the fundamental problem with wood is its inconsistency in material properties. You never know quite what the limits are for the piece of wood you are working on and you can't tell the peak until you have gone past it.


Mark

Jano:
Firstly I would like to thank Alan Case for publishing his valuable work for other bowmakers and then recommend to read his comments /see the graphs/ here ( and several previous pages too ) :

 http://www.primitivearcher.com/smf/index.php/topic,69529.345.html     .

I ( as not very experienced bowmaker ) do not know other way to do asked task, than to test ( similarly to avcase's method, but preferably using adviced 4 point bending setup ) on appropriate samples of wood from prepared stave, determine the proportional limit and decide what set is still acceptable along different parts of limbs for your bow design. Then use bendmetering at the final part ( at least ) of tillering to not overcome the desired set ( in every stations along the limbs ).
    To resolve the problem mentioned by Mark - "you can't tell the peak until you have gone past it" - I suppose one could make the bow intentionally thicker/stronger/ from the beginning and after overcoming the peak ( evenly ), you will remove damaged belly wood without changing the tillered shape of the bow and by such way achieving desired draw weight and performance.

Selfbowman:
Ok I have some questions?
Is it possible to reduce mass to prevent set in a computer program?
My thinking is if you know the property’s of the wood . Which we can get close on doing bend test and float test. Let’s say the test reveal the wood is 30 percent stronger in tension. Would it make sense to reduce the back of the bow in width 30 percent to let the belly’s compression catch up in strength as not to fail to compression. Trapping the bows back. This would also use the mass to its full potential would it not. Just asking.I would like to see some of the guys playing with computer designs to try this. This may be a good time to figure out perfect diminishing mass using 8 oz on a 28 inch limb. Maybe not. You smart guys please ponder this ! Arvin 

organic_archer:
+1 to what everyone else said. The TBB performance chapters have detailed tests on power stroke as well. The same bow of 45-48 pounds (if I recall correctly) was tested at low 20’s draw and shot 135 fps. It was retillered over and over to 45-48# out to 28” in one inch increments, and it gained 20+/-feet per second in the process. Power stroke is also important.

mmattockx:

--- Quote from: Selfbowman on June 05, 2022, 10:08:03 am ---Ok I have some questions?
Is it possible to reduce mass to prevent set in a computer program?
My thinking is if you know the property’s of the wood . Which we can get close on doing bend test and float test. Let’s say the test reveal the wood is 30 percent stronger in tension. Would it make sense to reduce the back of the bow in width 30 percent to let the belly’s compression catch up in strength as not to fail to compression. Trapping the bows back. This would also use the mass to its full potential would it not. Just asking.I would like to see some of the guys playing with computer designs to try this. This may be a good time to figure out perfect diminishing mass using 8 oz on a 28 inch limb. Maybe not. You smart guys please ponder this ! Arvin

--- End quote ---

Hey Arvin,

To answer your questions in order:

1) A bend test doesn't tell you how much stronger your wood is in tension or compression, it simply gives you the stiffness of the wood in bending (the modulus of elasticity) and the stress level where set starts to occur (assuming you test like Alan Case did in your other thread linked to above). You could figure the tension/compression balance out by trapping bend test samples different amounts and examining how they fail until you find the one that fails nearly simultaneously in tension and compression. You might run out of wood before you find this point, though.

2) The effects of trapping are not linear like that at all. Removing 30% of the back width will not raise the tension stresses by 30%. The change in stresses is based on how much you shift the neutral axis of the limb cross section and that is dependent on the back width along with how much of the side is removed and the final cross sectional shape.

3) I would say the perfect mass distribution is what you get with a quasi-pyramid back profile and constant limb thickness. The constant thickness keeps the strain/stress the same throughout the working limb length and the width changes to optimize the amount of wood used to the minimum required to keep the strain/stress the same everywhere.


Mark

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