Is the early wood the weak link to cause set?

[Badger]

    Allen is horn less stiff because it stretches more or compresses more? Would it have less stretch if glued to a piece of hardwood for example?

[Badger]

   I just did this same test on a hickory back osage and the line barely moved less than 1/8". Thinking I will go back and recheck it.

[PatM]

There is the stiffness versus strength difference so you'd have to show that a selfbow material  does act as stiff in tension as it does in compression.

  Does a wood back 'act" stiffer because wood tends to be twice as strong in tension as it is in compression?   There has to be some truth in that for trapping to work as well as it does.

  For backed bows you could likely do all sorts of extreme combinations in a test scenario.

[avcase]

    Allen is horn less stiff because it stretches more or compresses more? Would it have less stretch if glued to a piece of hardwood for example?

This is a good question. The only data I have comes from 3-point bending tests, which is a product of the combined tensile and compression properties. I don’t know why the tensile modulus would be much different than the compression Modulus. Usually the two are pretty similar to each other.

Similar to wood, the strength of horn in tension may be different than in compression.  I don’t know much about using it in tension.

I think this is what you are suggesting. To verify if there is a difference in stiffness and compression Modulus, you could laminate a thick-ish piece of horn to a thin piece of something very stiff, like unidirectional glass to make a composite beam. With it supported at two ends, place a weight on the center and accurately measure the deflection. Then flip it over and repeat the test. If the deflection is the same, then the tensile and compression Modulus of the horn is the same.

Alan

[sleek]

    Allen is horn less stiff because it stretches more or compresses more? Would it have less stretch if glued to a piece of hardwood for example?

This is a good question. The only data I have comes from 3-point bending tests, which is a product of the combined tensile and compression properties. I don’t know why the tensile modulus would be much different than the compression Modulus. Usually the two are pretty similar to each other.

Similar to wood, the strength of horn in tension may be different than in compression.  I don’t know much about using it in tension.

I think this is what you are suggesting. To verify if there is a difference in stiffness and compression Modulus, you could laminate a thick-ish piece of horn to a thin piece of something very stiff, like unidirectional glass to make a composite beam. With it supported at two ends, place a weight on the center and accurately measure the deflection. Then flip it over and repeat the test. If the deflection is the same, then the tensile and compression Modulus of the horn is the same.

Alan

Brilliant experiment

[PatM]

There are guys who make glass bows with a  horn belly.  They do behave a bit oddly.

[mmattockx]

Does a wood back 'act" stiffer because wood tends to be twice as strong in tension as it is in compression?   There has to be some truth in that for trapping to work as well as it does.

The strength and stiffness are essentially unrelated properties. Strength is the maximum load the material can carry before it fails, stiffness is how much it deflects under a load. Trapping works by balancing the strains so that the weaker side and the stronger side get closer to using the same amount of their maximum strength. It does not change the inherent stiffness of the material.


Mark

[Tuomo]

I don’t know why the tensile modulus would be much different than the compression Modulus. Usually the two are pretty similar to each other.

Alan - there is a small difference. See for example "Conners, T. & Medvecz, P. (1992). Wood As a Bimodular Material. Wood and Fiber Science 24, 413-423".

In wood tensile modulus is about 8 % greater than compression modulus. But, of course there is a lot of variation. For example, in sugar maple the difference can be almost 28 %.

[PatM]

Does a wood back 'act" stiffer because wood tends to be twice as strong in tension as it is in compression?   There has to be some truth in that for trapping to work as well as it does.

The strength and stiffness are essentially unrelated properties. Strength is the maximum load the material can carry before it fails, stiffness is how much it deflects under a load. Trapping works by balancing the strains so that the weaker side and the stronger side get closer to using the same amount of their maximum strength. It does not change the inherent stiffness of the material.


Mark

  But it changes the relative stiffness, no?

[mmattockx]

But it changes the relative stiffness, no?

I think I see what you are asking about. A wood backing doesn't 'act' stiffer as such, it really is stiffer when only loaded in tension. When working with a composite glue up (that is, multiple lams of different materials) you have to do any calculations taking into account the properties of each lam as it will be loaded. The back lam will only see tension, so you would need to use its tension modulus and the belly lam will only see compression so you would use its compression modulus. The core will see both tension and compression, so you would want to use its flexural modulus (the modulus you get when you measure with a bend test, like Alan mentioned a few posts back).

Since I threw another term in there (engineers, eh? ), I think some explanation is in order. There are 3 ways to measure the stiffness (modulus) of the material. You can load it in tension, compression or bending. Tension and compression are 'pure' loading conditions where all the material sees the same load, but bending (or flexural) puts the test piece in both tension and compression at the same time. The end result is that bending gives you an 'average' modulus of sorts, whereas tension and compression tell you purely about that one load direction.

For a selfbow the flexural modulus is the most accurate one to use because that is the exact loading that the limb sees. For lams you would want the actual tension and compression numbers, but those are hard to get without sophisticated lab equipment. Flexural modulus can be measured reasonably well in a home shop with a bit of care.

Did that get it?


Mark

[Selfbowman]

I got some metal strap. 60” long marked the unstrung bow.  Clamp one end at mark after brace camp the other end .  Less friction clamp at other end. Check at brace , 14” 21” 28”. Pretty wild just how much wood can stretch! Arvin

[Selfbowman]

‘This is at 21”

[mmattockx]

Alan - there is a small difference. See for example "Conners, T. & Medvecz, P. (1992). Wood As a Bimodular Material. Wood and Fiber Science 24, 413-423".

In wood tensile modulus is about 8 % greater than compression modulus. But, of course there is a lot of variation. For example, in sugar maple the difference can be almost 28 %.

Thanks for that paper, it is always nice to have some references to actual research. That much difference in sugar maple indicates that I should be heavily trapping every maple bow I make.


Mark

[Selfbowman]

This pic is of metal strap laying on the bow unstrung. That represents the blue tape. The next dark pencil mark is the bow at brace. The green mark is at 21”

[Selfbowman]

I’ll be playing with this on a variety of bows!  Arvin