Mechanical Properties of Wood for Building Bows and Arrows

[gfugal]

Would willow make good arrow wood then? since it is light yet stiff. It has poor strain but arrows don't need to bend nearly as much as bows.

[gfugal]

Ok so apparently bone composites have successfully been made, which wouldn't surprise me since this was what I was advocating for anyway. The same with steel bows. I just heard they were heavy and break because of metals malleability or something like that. I guess you shouldn't believe everything you hear, and I should have looked at the data.

But why haven't I seen bone composites or steel bows? Either the material isn't suitable at all for a bow despite what the data says, the right ratio hasn't been really established like I'm suggesting,  it's just so difficult to work the material that people just don't attempt it, or it's so obscure that people just don't think about it.

If steel is really a possible bow material, I think it would be cool to blacksmith a bow and may try it one day. PatM you seemed to have known of examples of both bone composites and steel bows. Do you have any links or pictures of such? I would love to see or read about these bows.

[willie]

Stacking in flax: I did some tests with branch bows (as these are better in compression because of more juvenile wood), roughly tillered them to 30#, and then added a flax layer on top, 1 mm thick at most. These bows were a lot stronger, and when drawing them to 15" or so on the tillering tree, they sometimes exploded right away (dry spaghetti-like). My guess is that the flax was so strong that much more wood was forced into compression, leading to sudden hinges that led to over straining of the back fibers.

Thanks for sharing your bow experiment, Joachim. Real life bow breaking often creates more questions than answers.
I had a similar experience with a 42" cable backed/compression spruce bow a few years ago. It was pretillered to about 40# @ 20" before I added the cable. When I got to  80# @ 22" pull, the wood sheared under the cable, (but the bow did not break). I thought it was stacking on account of being overdrawn, but that does not really explain the magnitude of the wall I was pulling against.

[willie]

Greg, a willow atlatl spear was found in a glacier in the yukon, and a bit further north ......
https://www.youtube.com/watch?v=JmfYJBha7SU

[gfugal]

Greg, a willow atlatl spear was found in a glacier in the yukon, and a bit further north ......
https://www.youtube.com/watch?v=JmfYJBha7SU

Yeah, I've seen that video before. It's very interesting, but it isn't a wood composite to my understanding, it's made solely of antler and sinew.

[PatM]

Ok so apparently bone composites have successfully been made, which wouldn't surprise me since this was what I was advocating for anyway. The same with steel bows. I just heard they were heavy and break because of metals malleability or something like that. I guess you shouldn't believe everything you hear, and I should have looked at the data.

But why haven't I seen bone composites or steel bows? Either the material isn't suitable at all for a bow despite what the data says, the right ratio hasn't been really established like I'm suggesting,  it's just so difficult to work the material that people just don't attempt it, or it's so obscure that people just don't think about it.

If steel is really a possible bow material, I think it would be cool to blacksmith a bow and may try it one day. PatM you seemed to have known of examples of both bone composites and steel bows. Do you have any links or pictures of such? I would love to see or read about these bows.

   There was an article on a bone belied composite in the PA mag.  Antler bows of course are well documented.

 Google Seefab steel bows and Indian or Indo-Persian steel bows.  You'll find plenty of examples.

[joachimM]

Greg,
Willow could make arrows just like bows, but still for its MoE and density it's weak in both tension and compression, it has a very low hardness so it is easily dented too.
No reason to use it unless you haven't got better wood.

Why we don't see more bone-bellied bows or steel bows: we have easier materials to work with, and which give at least as good results. But if you're feeling masochistic, have a go at bone for a belly. it's way better in compression than wood (it can take a bit over 1% compression). A thin sliver, say 1.5 mm thick by 1 cm wide, riding along a groove in the belly might do the job of a pre-tillered Molly-design. (use something with short working sections of the limb, so you don't need to have bone all along the length of the bow). 

[gfugal]

Greg,
Willow could make arrows just like bows, but still for its MoE and density it's weak in both tension and compression, it has a very low hardness so it is easily dented too.
No reason to use it unless you haven't got better wood.

For some reason I thought you said it's MoE to density ratio was good. Really stiff but light at the same time. I have a Willow tree in front of my apartment and it's always dropping branches. If it was ideal arrow wood I was going to try and use them, but it sounds like I misunderstood you and it's really poor arrow wood. If that's the case I'll won't waste the effort.

[joachimM]

Yeah sorry my confusing way of telling that you need to take density into account to evaluate stiffness, to get stiffness per mass unit...

[willie]

Greg,
 That tree in front of your apartment may be an ornamental, and there are many hybrids in that market.  Salix alba is the european willow that Joachin has mentioned, it can be a large tree, and is quite different from Salix exigua commonly used by native amreicans.       https://en.wikipedia.org/wiki/Salix_alba         https://en.wikipedia.org/wiki/Salix_exigua
Looking at relative properties like MOE/S.G. can help you choose materiel.  Willow will work for a kids bow, as they are often "head high long", but when you want to draw 50#, the absolute values mean that the bow will need to be warbow long and quite wide.

Generally speakingl ,the wood properties currently published  by the FPL are appropriate for commercial and residential construction applications, such as floors, walls and and roofs.  These tests are designed for evaluating wood to be used with relatively lighter loads acting over durations of months and years. Small deflections typically limit designs, and a slow bend test can gives values useful to the suppliers of that market.  The info can gets us bowyers going in the right directions, but as Marc always points out, it's all about the elasticity, which isn't easily teased out of  the data currently published. The tests are not really designed to quantify values for work performed near the elastic limit.(Knowing MOR is always useful)
 
Jim mentioned WML, or "work to maximum load". Older testing  incorporated impact bending tests. Basically bouncing a weight on a beam from higher and higher heights, until it failed. That test told us more about designing for shock loads or uses working closer to the elastic limit. things like chassis and spokes, mine and bridge and RR timbers, tool handles etc.

Here are some results from a FPL of Canada publication from 1933. The static tests took 7 minutes to reach maximum loads, while the impact test loaded the sample in 1/25 of a second.

[Aussie Yeoman]

Stacking in flax: I did some tests with branch bows (as these are better in compression because of more juvenile wood), roughly tillered them to 30#, and then added a flax layer on top, 1 mm thick at most. These bows were a lot stronger, and when drawing them to 15" or so on the tillering tree, they sometimes exploded right away (dry spaghetti-like). My guess is that the flax was so strong that much more wood was forced into compression, leading to sudden hinges that led to overstraining of the back fibers.

I reckon that’s exactly what happened. Though I would posit that branch bows are better in compression because the back is under so much more stress owing to the high crown.
Did you use thread or raw fibre?

Ok so apparently bone composites have successfully been made, which wouldn't surprise me since this was what I was advocating for anyway. The same with steel bows. I just heard they were heavy and break because of metals malleability or something like that. I guess you shouldn't believe everything you hear, and I should have looked at the data.

But why haven't I seen bone composites or steel bows? Either the material isn't suitable at all for a bow despite what the data says, the right ratio hasn't been really established like I'm suggesting,  it's just so difficult to work the material that people just don't attempt it, or it's so obscure that people just don't think about it.

If steel is really a possible bow material, I think it would be cool to blacksmith a bow and may try it one day. PatM you seemed to have known of examples of both bone composites and steel bows. Do you have any links or pictures of such? I would love to see or read about these bows.

While it is true steel bows were made in India in times past, it is not really a suitable bow material outside the realm of the industrial revolution. The reason is its mass. It weighs far too much for its stiffness/elasticity compared to wood.

Commercially made steel bows, as far as I understand, were 'I-Beam', RHS or hollow round; both of the latter being tapered of course. It would be near impossible to forge such a form.

[joachimM]

Though I would posit that branch bows are better in compression because the back is under so much more stress owing to the high crown.
[\quote]

Look up threads here for juvenile wood, which is basically the wood formed by saplings or young branches. Structurally, juvenile wood differs from later wood by the angle of cellulose microfibrils in the S2-layers of cell walls. The higher the microfibril angle (MFA), the less stiff the wood will be (MoE), and the higher its flexibility (MoR). Interestingly, also compression wood in conifers has ha higher MFA. See also http://cdn.intechopen.com/pdfs/45624/InTech-Cellulose_microfibril_angle_in_wood_and_its_dynamic_mechanical_significance.pdf

Surely, the crowned design relieves stress from the belly as the neutral plane shifts down towards the belly in such designs.

While it is true steel bows were made in India in times past, it is not really a suitable bow material outside the realm of the industrial revolution. The reason is its mass. It weighs far too much for its stiffness/elasticity compared to wood.

Spring steel has about the same relative stiffness as bamboo (MoE of spring steel: 207 Gpa, density: 8 kg/L; relative stiffness: 25.9; Moso bamboo, MoE: 22.7 GPa; density: 0.8 kg/L, relative stiffness: 28.33). Which means that although its density is ten times higher, its energy storage capacity is also ten times higher. Per mass unit, it is about the same (actually, it is 50% larger since it can take 50% more compression and tension than wood).
In theory, you can build a same mass and equally performing 50# bow from steel as from bamboo or other woods.

I wouldn't call it a primitive material.

[Aussie Yeoman]

Look up threads here for juvenile wood, which is basically the wood formed by saplings or young branches. Structurally, juvenile wood differs from later wood by the angle of cellulose microfibrils in the S2-layers of cell walls. The higher the microfibril angle (MFA), the less stiff the wood will be (MoE), and the higher its flexibility (MoR). Interestingly, also compression wood in conifers has ha higher MFA. See also http://cdn.intechopen.com/pdfs/45624/InTech-Cellulose_microfibril_angle_in_wood_and_its_dynamic_mechanical_significance.pdf

Surely, the crowned design relieves stress from the belly as the neutral plane shifts down towards the belly in such designs.

The bit about juvenile wood is fascinating, thank you for the link.

We are sort of dancing around the same point on crowned sapling/branch bows: the crowned back makes for a less stressed belly.

[Badger]

   I have an odd question, if I were building a bow say 80 ft long, intuitively I am thinking I should go to a less dense wood, less dense than what we normally use for bow woods. How would you guys look at this?  Looking at about 18" wide by 6" thick.

[DC]

What draw length are you looking at,- about 35' You building another Da Vinci bow? The hand shock is going to be horrific.