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Main Discussion Area => Bows => Topic started by: pnwarcher on November 08, 2017, 09:40:39 am

Title: Mechanical Properties of Wood for Building Bows and Arrows
Post by: pnwarcher on November 08, 2017, 09:40:39 am
Hi PA Community! I'm new to posting here, but have been stalking these forums for quite awhile. I've gleaned a lot of knowledge from y'all, and I think it's time I gave something back.
Like many of us, I am a total wood nerd and am always eyeing trees and shrubs for potential use making bows and arrows. I'm also a mechanical engineer, so naturally I got to thinking it would be interesting to build a database of wood properties and do some analysis to help identify potential new woods. When I discovered The Wood Database http://www.wood-database.com/ (http://www.wood-database.com/), I knew I'd struck gold. I used this database to calculate some properties relevant to bow and arrow performance, and tabulated everything in this handy Google spreadsheet (also attached as a PDF):

https://docs.google.com/spreadsheets/d/191XdO8Gb0uo2lW5oYLw__SOSh5Mv9Sfk91qr4D6BWAM/edit?usp=sharing (https://docs.google.com/spreadsheets/d/191XdO8Gb0uo2lW5oYLw__SOSh5Mv9Sfk91qr4D6BWAM/edit?usp=sharing)

The header cells have notes describing each mechanical property. Of course, the raw numbers must be taken with a grain of salt since wood is a natural material and there can be wide variations within a species. Maybe someday I'll try to quantify these intraspecies variations for some of our favorite woods.

If you don't see your favorite niche bow wood, it's likely because it is not widely available in the wood market, and therefore hasn't been tested by Eric Meier (the author of the wood database). He does accept wood samples for testing, so I encourage you to send him a specimen! http://www.wood-database.com/donating-wood-samples/ (http://www.wood-database.com/donating-wood-samples/)
Below are notable woods missing from the database, which can likely hold their own with the best. I have access to the first six of these, so testing them is on my to-do list. The other five, I would love to get my hands on someday.
- Ocean Spray
- Vine Maple
- Hazel
- Lilac
- Hawthorn
- Scotch Broom
- Serviceberry
- Red Stopper
- Laburnum
- Mountain Mahogany
- Syringa

Let me know if you have any questions about the spreadsheet, and feel free to give feedback or make suggestions for improvements or additional features.

Happy bowmaking!
Stephen
Title: Re: Mechanical Properties of Wood for Building Bows and Arrows
Post by: pnwarcher on November 08, 2017, 10:06:34 am
If there's one table worth studying in that massive spreadsheet, it's probably this one (see attached pic). This is the list of woods sorted by breaking strain, which is the amount of deformation or bending a wood can withstand before breaking. Woods with a high breaking strain are woods that are strong (high modulus of rupture) and also flexible (low stiffness or elastic modulus). Breaking strain is one of the most important mechanical properties for bow wood. Woods with higher breaking strain allow you to make a bow with shorter NTN length, longer draw length, more extreme recurve/reflex, and narrower, thicker limb cross sections. They also give you more margin for error when using less extreme designs.

Interestingly, while yew and osage orange are near the top of the list, they are not the very best in this metric alone. Makes me really want to try Madagascar rosewood... too bad it's endangered and banned from international trade.
Title: Re: Mechanical Properties of Wood for Building Bows and Arrows
Post by: joachimM on November 08, 2017, 11:43:09 am
pnwarcher,

I did a similar exercise on some 100 species of wood from the wood database data.

https://drive.google.com/open?id=0B3YYA3Sr_3gqb0NpazZqdUFJSTQ
Also with information on backing materials.

Since then, I found out that you need to take these things with a decent grain of salt. Some of this error / variation is due to moisture content; the data in the wood database refer to 12% MC, but we often have lower MC, and heat treatment lowers MC on the belly, not the back (that's why it has such a dramatic effect).
Interesting for bow wood selection and testing, though. Certainly not the definitive answer to all your bow wood questions.
See also earlier threads, like http://www.primitivearcher.com/smf/index.php/topic,50077.msg765904.html#msg765904


Title: Re: Mechanical Properties of Wood for Building Bows and Arrows
Post by: Sir Failalot on November 08, 2017, 02:30:33 pm
This is gold.
Yes I will take it with a bit of salt. Then salty gold  ;)

 Thank you very much!
Title: Re: Mechanical Properties of Wood for Building Bows and Arrows
Post by: willie on November 08, 2017, 10:40:13 pm
Welcome to PA, Stephen. Its nice that you are making a contribution with your first post.
MOE does seem to be a good indicator for usability in bows, but sometimes there are surprises when using woods further down the list. What sets some staves apart from the expected/estimated is something that all of us wish we knew more about.
Title: Re: Mechanical Properties of Wood for Building Bows and Arrows
Post by: Del the cat on November 09, 2017, 02:40:10 am
Interesting that Leyland Cypress(Leylandii) is up near the top. This fast growing tree is prevalent in the UK where it is planted for screening purposes and is often cut because it grows too big too fast. Dunno it it is easy to find clean knot free lengths tho' as it grows bushy (hence it's good for screening)
Del
Title: Re: Mechanical Properties of Wood for Building Bows and Arrows
Post by: Dances with squirrels on November 09, 2017, 04:46:52 am
Yew and osage are up there pretty high. I'm gonna try those  )W(

Welcome pnw!
Title: Re: Mechanical Properties of Wood for Building Bows and Arrows
Post by: gfugal on November 09, 2017, 08:07:44 am
It's funny I started building my own spreadsheet too a while back. JoachimM  then shared his with me too. We had a good discussion on a topic I started here http://www.primitivearcher.com/smf/index.php/topic,59771.0.html
Title: Re: Mechanical Properties of Wood for Building Bows and Arrows
Post by: gfugal on November 09, 2017, 08:25:25 am
If you don't see your favorite niche bow wood, it's likely because it is not widely available in the wood market, and therefore hasn't been tested by Eric Meier (the author of the wood database). He does accept wood samples for testing, so I encourage you to send him a specimen! http://www.wood-database.com/donating-wood-samples/ (http://www.wood-database.com/donating-wood-samples/)
Below are notable woods missing from the database, which can likely hold their own with the best. I have access to the first six of these, so testing them is on my to-do list. The other five, I would love to get my hands on someday.
I think it's awesome we have an engineer in this form. I'm not an engineer, but I'm a science major and love applying the physics I learned to Bowery and archery in general. My brother is a mechanical engineer and I'm always going to him with questions. Do you happen to know how Eric Meier does his wood testing? Is there a way we could do that ourselves on species not listed?
Title: Re: Mechanical Properties of Wood for Building Bows and Arrows
Post by: Jim Davis on November 09, 2017, 08:33:31 am
The factor most needed but not included is YIELD strength not breaking strength (though the two correlate to a degree). If you stay clear of the breaking strength but exceed the yield strength, the wood takes "set."

And I always am slightly annoyed that  Eric Meier is given credit for all that testing when in fact, the information has been lifted from the Forest Products Laboratories' work done nearly a century ago. Unless I am mistaken, which I doubt, Meier has no equipment to test MOR, MOE, or WML. I will admit his arrangement of the information is easier to find and read.

And again, it should be recognized that NOBODY has those moduli numbers for DRY Osage. For some reason, that data was not recorded in the original testing and all we have are the data for green Osage.
Title: Re: Mechanical Properties of Wood for Building Bows and Arrows
Post by: willie on November 09, 2017, 11:46:04 am
Quote
Is there a way we could do that ourselves on species not listed?
yes, a simple bend test. Actually, Tim Baker developed a test for evaluating bow woods, I think there is a description in the bibles, but a simple three or four point bend test similar to spine-ing arrows is an alternative entirely doable by a bowyer who wishes to assess his materiel.
Quote
most needed but not included is YIELD strength
Its a shame that the FPL has not published the graphs that they made to get the data points in their tables. The graph is nothing more than what we would draw when making a F/D curve on a bow.  Wood typically yields in a gradual manner, and a "yield strength" is somebody elses idea of the usefulness the materiel being worked beyond the elastic limit. Being able to see the differences in shape, slope and length of the curves for different species would be much more useful for us bowyers. The image below shows a test where the piece was unloaded and reloaded. What we call set is labeled "plastic deformation"

What would be really sweet is to find a way to quantify the rebound rate of wood near the proportional limit with a test procedure that more closely mimics the draw and release speed of an archer.
Title: Re: Mechanical Properties of Wood for Building Bows and Arrows
Post by: joachimM on November 09, 2017, 12:28:02 pm
Interesting that Leyland Cypress(Leylandii) is up near the top. This fast growing tree is prevalent in the UK where it is planted for screening purposes and is often cut because it grows too big too fast. Dunno it it is easy to find clean knot free lengths tho' as it grows bushy (hence it's good for screening)
Del

I cut and shredded 15 big leyland cypresses two weeks ago, but kept a few logs just in case. Iíll try this shortly!
Title: Re: Mechanical Properties of Wood for Building Bows and Arrows
Post by: Del the cat on November 09, 2017, 01:27:08 pm
Interesting that Leyland Cypress(Leylandii) is up near the top. This fast growing tree is prevalent in the UK where it is planted for screening purposes and is often cut because it grows too big too fast. Dunno it it is easy to find clean knot free lengths tho' as it grows bushy (hence it's good for screening)
Del

I cut and shredded 15 big leyland cypresses two weeks ago, but kept a few logs just in case. Iíll try this shortly!
I'll wait with interest :)
Del
Title: Re: Mechanical Properties of Wood for Building Bows and Arrows
Post by: gfugal on November 09, 2017, 02:09:09 pm
Quote
Is there a way we could do that ourselves on species not listed?
yes, a simple bend test. Actually, Tim Baker developed a test for evaluating bow woods, I think there is a description in the bibles, but a simple three or four point bend test similar to spine-ing arrows is an alternative entirely doable by a bowyer who wishes to assess his materiel.

This is helpful, yes, but this doesn't produce numbers like those displayed in the database. So really there is no way of comparing woods on the database unless you test them all yourself.
Title: Re: Mechanical Properties of Wood for Building Bows and Arrows
Post by: willie on November 09, 2017, 04:29:54 pm
Quote
but this doesn't produce numbers like those displayed in the database.
I am not sure why you think it can't. Could you be more specific as to what useful values in the database can't be produced with a simple bend test?, or why you would need to test them all yourself to get what you need to know about the one you are working with?
Title: Re: Mechanical Properties of Wood for Building Bows and Arrows
Post by: PatM on November 09, 2017, 04:35:28 pm
The database tests do more than just bend a stick and observe what happens. Mind you the bending a stick is probably more useful to us.
Title: Re: Mechanical Properties of Wood for Building Bows and Arrows
Post by: Aussie Yeoman on November 11, 2017, 12:28:27 am
The mechanical properties of wood, vis a vis bowmaking, is a subject near and dear to my heart. I've spent perhaps 2000 hours or more testing wood samples and analysing the results of others' bend tests.

As far as I'm concerned there are only two numbers that define the merit of a wood in bowmaking. One is the wood's working strain under a load that generates a set of 8% total deflection (about 1" for a 28" draw bow). The other is the stiffness of the wood at that point. (the strain is the bending stress divided by the stiffness, so if you're more into bending stress than strain, you can use that figure. I like strain because two woods can have the same strain with wildly different bending stresses, and so can be more easily compared.)

The bending stress I mentioned is the little sister to the MoR. For the sake of bowmaking, MoR is actually quite a useless figure. It is such for a couple of reasons:
* This (MoR) is the bending stress at which the wood fails in some near-cataclysmic fashion, which is seen only in bows that break; and,
* There is little correlation between the working bending stress of a wood and that wood's MoR. Woods I have tested have a working stress from 40% to 66% of its MoR, so it is not a simple matter of scaling the published MoR figure in rating a wood's merit.

The stiffness I mentioned is simply the MoE measured at the aforementioned state of bend, which takes into account the loss of stiffness due to set.

Almost any bend test will do, as long as your measurements are accurate. A four-point bend test is perhaps the best though because the bending moment is the same at all points between the two center supports. This is beneficial in more closely simulating bow-limb conditions in a bend test.

What I ended up making for myself was a spreadsheet that tabulated a huge array of bend tests or different samples of timber from all over the world. There're heaps of columns for different data, but as far as bowmaking goes there are only two that matter: the working strain and the working stiffness. Using these two numbers and a bit of arithmetic, you can calculate dimensions for almost any bow design using any wood. The only limit will be your standards of aesthetics. I also made a couple of bend test stations to test samples, which is far more fun than the number of people who do bend tests would suggest.

As to the reliability of the results: I had some templates laser cut to within a thou. Last weekend I took a stave and had a complete beginner use the template to mark the outlines. After bandsawing and grinding down to the lines, the bow needed only about 25 minutes of guided tillering to get the desired draw force at the desired draw length. The set was as predicted and the tiller shape was wonderful.

If you'd like to read more about my approach to ranking woods for bowmaking, please see a previous write-up I did here:
http://www.ozbow.net/phpBB3/viewtopic.php?f=34&t=13765 (not my website, just 'a' website)

Cheers.
Title: Re: Mechanical Properties of Wood for Building Bows and Arrows
Post by: gfugal on November 11, 2017, 05:40:52 am
Interesting. Yeah working strain will take into account the yeild point which makes it much better than MoR. You also talk about working strain or MoE right? You say these two values are the only ones you really care about. Have you thought about measuring somehow the difference in working stress and strain of a wood in compression vs tension? I'm not sure how you would test that but it could be useful knowledge for making laminate bows or backing bows with things like flax.
Title: Re: Mechanical Properties of Wood for Building Bows and Arrows
Post by: joachimM on November 11, 2017, 08:59:58 am
If you'd like to read more about my approach to ranking woods for bowmaking, please see a previous write-up I did here:
http://www.ozbow.net/phpBB3/viewtopic.php?f=34&t=13765 (not my website, just 'a' website)

Cheers.

Hi Dave, nice explanation there.
I've used Tim Baker's bend test data in the past as well (also through David Dewey, Woodbear), to compare MoR to yield strength, but in the end I was mostly confused by the effect moisture content may have on bend test results. Since Tim's bend tests weren't standardized at 12% MC (like all the data in the wood database), it's difficult to use those data.

More on this issue: https://www.tapatalk.com/groups/paleoplanet69529/viewtopic.php?p=587215&sid=d42191a625efa7d23fe71b90ad600366#p587215
Which, incidentally, you replied to, two years ago :-)

Title: Re: Mechanical Properties of Wood for Building Bows and Arrows
Post by: Aussie Yeoman on November 11, 2017, 11:30:50 am
Interesting. Yeah working strain will take into account the yeild point which makes it much better than MoR. You also talk about working strain or MoE right? You say these two values are the only ones you really care about. Have you thought about measuring somehow the difference in working stress and strain of a wood in compression vs tension? I'm not sure how you would test that but it could be useful knowledge for making laminate bows or backing bows with things like flax.

So, in a bow with a cross section symmetrical about its neutral axis, the stress at back and belly will be exactly the same. But I know what you mean - some woods cope better with one than the other overall.

Doing a bend test to find the working strain/stress/MoE is sort of agnostic about tension/compression strength. But you do get a sort of narrative if, when doing the bend test, you take the sample all the way to failure. For example, if the sample takes its working set early, then takes heaps more and chrysals, then it's weak in compression. If the sample takes its working set very close to its MoR then cracks on its back, it's very strong in compression. If it seems to take very little set overall then explodes in a storm of wood fragments, it's just brittle overall.

I suppose you could take a sample that, say, was found to be strong in tension, and trap the back through various iterations. But then the mathematics becomes a little more complicated.
Title: Re: Mechanical Properties of Wood for Building Bows and Arrows
Post by: willie on November 11, 2017, 02:15:43 pm
Quote
working strain under a load that generates a set of 8% total deflection (about 1" for a 28" draw bow)
Nice point of reference Dave. Would you consider updating or posting a new dropbox link? I can't seem to make the old one work anymore.
Thanks


Greg, I have found bend testing to be more pertinent to compression strengths, as samples tends to yield earlier on the compression side. Actual tensile values for published for wood are not common. Here are some of the few I have found.

TABLE II
RATIO OF STRENGTH OF WOOD IN TENSION AND IN COMPRESSION
(Bul. 10, U. S. Div. of Forestry, p. 44)
NOTE.óMoisture condition not given. A stick 1 square inch in cross section

   
                    Ratio             Pull apart     Crush endwise

Hickory          3.7              32,000            8,500

Elm                3.8              29,000            7,500

Larch             2.3              19,400            8,600

Longleaf        2.2              17,300           7,400
Pine



also from the same publication is a FPL bend test plot

"Stress-strain diagrams of two longleaf pine beams. E.L. = elastic limit. The areas of the triangles 0(EL)A and 0(EL)B represent the elastic resilience of the dry and green beams, respectively."

the green sample could be said to have a yield point at about 1.2 inches deflection, if you equate yield and ultimate to mean the same, however the curve for dry woods don't usually drop off like the green, so I'll go with something like what Aussie has chosen as a reference for bowbuilding.
Title: Re: Mechanical Properties of Wood for Building Bows and Arrows
Post by: Aussie Yeoman on November 11, 2017, 07:44:19 pm
Would you consider updating or posting a new dropbox link? I can't seem to make the old one work anymore.
Thanks


Yep, I will compile a list and post it one way or another.
Title: Re: Mechanical Properties of Wood for Building Bows and Arrows
Post by: Jim Davis on November 12, 2017, 02:38:24 pm
Willie, your information is all I have ever found in the way of tension strength information for wood. Except a note that tension strength is midway between two of the other factors, which I can't recall at the moment and don't recall where I found that.

I have seen that, as exemplified in your short table, almost all woods are 3 to 4 times as strong in tension as compression, so all this talk of bamboo or hickory overpowering the belly wood is balderdash, in that almost any intact backing wood is stronger than almost any belly wood.

Title: Re: Mechanical Properties of Wood for Building Bows and Arrows
Post by: PatM on November 12, 2017, 02:51:46 pm
It comes down to whether the material stretches  or not, no?  It's like comparing sinew to fiberglass or even carbon.  Wood and bamboo also fits on that spectrum with some like say Elm or Yew sapwood at the sinew end and Tonkin bamboo at the fiberglass end.

Title: Re: Mechanical Properties of Wood for Building Bows and Arrows
Post by: willie on November 12, 2017, 04:31:09 pm
Quote
It comes down to whether the material stretches or not, no?
Yes,  I believe both back and belly must each stretch or compress well to make a exceptional bow, and there is more to be learned about what qualities makes a good back.
 
Title: Re: Mechanical Properties of Wood for Building Bows and Arrows
Post by: Badger on November 12, 2017, 05:34:22 pm
   I am more interested in the chart for flight arrows, what is the stiffest wood you have found so far?
Title: Re: Mechanical Properties of Wood for Building Bows and Arrows
Post by: Marc St Louis on November 13, 2017, 03:59:14 am
Willie, your information is all I have ever found in the way of tension strength information for wood. Except a note that tension strength is midway between two of the other factors, which I can't recall at the moment and don't recall where I found that.

I have seen that, as exemplified in your short table, almost all woods are 3 to 4 times as strong in tension as compression, so all this talk of bamboo or hickory overpowering the belly wood is balderdash, in that almost any intact backing wood is stronger than almost any belly wood.

Mechanical strength numbers mean nothing in this respect.  It's all about how elastic the wood is and that is why Bamboo overpowers some wood species, they are not elastic enough to sustain the extra strain the Bamboo puts on the belly

P.S.  To clarify that statement.  Bamboo can be used on species of low elasticity if the bow is made longer to relieve the extra stress
Title: Re: Mechanical Properties of Wood for Building Bows and Arrows
Post by: Jim Davis on November 13, 2017, 04:55:19 am
The stretch/yield numbers I have seen all say total stretch before breaking is about 1% for all wood before rupture.
Title: Re: Mechanical Properties of Wood for Building Bows and Arrows
Post by: PatM on November 13, 2017, 05:12:31 am
Maybe but it takes different amounts of strain to get that stretch to happen.  So  bamboo or Hickory is not going to budge when you try to stretch it over a piece of ERC.
 
 If you don't believe that thick bamboo or Hickory lets weaker woods crush then present an alternative explanation.
Title: Re: Mechanical Properties of Wood for Building Bows and Arrows
Post by: Marc St Louis on November 13, 2017, 05:17:04 am
Maybe I should have clarified even more.  I'm not talking stretch but compression.  If you don't think this is an issue then try backing Black Cherry with some Bamboo, say a 67" bow for a 28" draw, and then let us count the chrysals  :)
Title: Re: Mechanical Properties of Wood for Building Bows and Arrows
Post by: willie on November 13, 2017, 06:21:56 am
Quote
If you don't believe that thick bamboo or Hickory lets weaker woods crush then present an alternative explanation.
Pat, an alternative has already been suggested by Dave.
Quote
I suppose you could take a sample that, say, was found to be strong in tension, and trap the back through various iterations.

Mark,
 bamboo and cherry would not be an easy combination work with, because of the highly differing strengths, but that does not mean bamboo is not a good backer, nor does it mean cherry is not good in compression.
Title: Re: Mechanical Properties of Wood for Building Bows and Arrows
Post by: gfugal on November 13, 2017, 06:47:59 am
Tim Baker showed this with his flax backed pine right? Flax is very stiff (high MoE) pine is not so much. The result was a over stressed belly with tons of chrysals. Anytime you bend a bow the back undergoes tension and the belly compression. If there is tension that means the back is trying to stretch, if the bow actually bends, then it does stretch. But depending on the belly material it may stretch more or less. If the belly is less stiff the flax makes it compress more than it normally would thus reducing the amount it stretches. Vise versa, If the belly was stiffer it would make the flax stretch more. If there was no belly material then I guess you could say it doesn't stretch. But that's only cause flax isn't rigid, and super flexible in compression with practically no compressive stiffness, thus you can loop it or flex it however you want. But if you glue it onto something rigid, then it must stretch if you bend it like a bow, unless the belly material undergoes so much damage that it isn't rigid anymore. Styrofoam might do that, but even the most chrysaled wood still maintains some rigidity.
Title: Re: Mechanical Properties of Wood for Building Bows and Arrows
Post by: gfugal on November 13, 2017, 06:52:12 am
Also I'll add that when we say stretch we don't mean visible stretch, we're talking about microscopic amounts less than a 1% change.
Title: Re: Mechanical Properties of Wood for Building Bows and Arrows
Post by: Marc St Louis on November 13, 2017, 07:00:09 am
Quote
Mark,
 bamboo and cherry would not be an easy combination work with, because of the highly differing strengths, but that does not mean bamboo is not a good backer, nor does it mean cherry is not good in compression.

Bamboo is an excellent backing, for certain types of wood.  Good in compression is an ambiguous statement.  Good for what?

I bought a Bubinga board many years ago, a very strong and fairly heavy wood.  It was fine when backed with Ash or Maple but as soon as I backed it with Bamboo it chrysalled all over the place.
Title: Re: Mechanical Properties of Wood for Building Bows and Arrows
Post by: willie on November 13, 2017, 07:40:48 am
Quote
Good for what?
Good for a lighter draw cherry bow I suppose, at least that what was reported in TBB.

Mark, do you think the Bubinga would have done better with a thinner or narrower piece of bamboo?
Title: Re: Mechanical Properties of Wood for Building Bows and Arrows
Post by: willie on November 13, 2017, 07:42:23 am
Greg,
When Tim put flax on pine, he purportedly proved the combo is mismatched, but was it because he chose High MOE flax to put on low MOE pine or did he just put too much on? What about flax on ipe? or sisal on pine?
Interestingly, Joachim said in an earlier thread that flax "stacks". That might bear further investigation as it indicates some sort of work hardening might be occurring.

Quote
we're talking about microscopic amounts less than a 1% change.
I actually think that smaller amounts of strain means less hysteresis.   .5%  stretch on the back would worth looking into if it could be matched with an elastic wood of comparable strength on the belly.
Title: Re: Mechanical Properties of Wood for Building Bows and Arrows
Post by: gfugal on November 13, 2017, 08:24:07 am
Less amount would help, but will it be enough? maybe. That's something to look into. I've wondered the same thing about bone. Supposedly bone has a similar, if just slightly less, flexiblity as wood. But it's really brittle making it poor in tension. Yet concrete is poor in tension too, but they use it in engineering all the time for it's compressive qualities, even in horizontal load bearing floors multiple stories up. Bridges are another example. If you optimize two materials for the tension and compressive qualities like rebar and concrete (in the right position and ratio mind you) you can manage to construct things that otherwise seem unlikely to work. Bone has a a high MoE and is excellent in compression. Reason tells me if done right it would make a good belly for a bow, but we haven't seen it done very successfully yet. I wonder if that has to do with the ratio's you mention. Maybe there been too much of it? Or maybe it was simply a design flaw. Bone is not nearly as flexible as horn, and maybe people used it as a horn substitute not minding to change the design.
Title: Re: Mechanical Properties of Wood for Building Bows and Arrows
Post by: PatM on November 13, 2017, 08:56:36 am
Bone bellied bows have been done. For all intents and purposes antler is bone.
Title: Re: Mechanical Properties of Wood for Building Bows and Arrows
Post by: willie on November 13, 2017, 09:19:28 am
Quote
like rebar and concrete (in the right position and ratio mind you)

My thought exactly. If we can learn more about tension qualities of the materiel's we work with, I think there is room for improvement. So often the limitations we work to, are on the compression side, and most of what we learn about bow wood is about compression. If we minimize compression damage and consequently, compression hysteresis, by design, I think we can begin to see more about what makes for a better back. I have been watching Badger make better bows with his "no set" tillering method, and think that it may be a practical way to observe back quality differences, and help answer the question, "What natural materiel's are the "Yew and Osage" of the tension side?"
Title: Re: Mechanical Properties of Wood for Building Bows and Arrows
Post by: PatM on November 13, 2017, 09:27:21 am
Don't we already know that?  It's tempting to think something magical  or a combination of some materials will provide radical improvement but I think we are just putting numbers and trying to explain scientifically  what we already know.
Title: Re: Mechanical Properties of Wood for Building Bows and Arrows
Post by: PatM on November 13, 2017, 09:29:51 am
Don't we already know that?  It's tempting to think something magical  or a combination of some materials will provide radical improvement but I think we are just putting numbers and trying to explain scientifically  what we already know.

 This material might be such an example but practically it's not an option.
https://en.wikipedia.org/wiki/Resilin
Title: Re: Mechanical Properties of Wood for Building Bows and Arrows
Post by: DC on November 13, 2017, 09:40:37 am
There has to be more than just tension and compression stats. A steel backed, concrete bellied  bow would have great numbers but it wouldn't last. I think that for bellies we need a wood that will compress substantially and return without damage. How much it will compress we can control by how much wood we use.
Title: Re: Mechanical Properties of Wood for Building Bows and Arrows
Post by: gfugal on November 13, 2017, 09:57:47 am
There has to be more than just tension and compression stats. A steel backed, concrete bellied  bow would have great numbers but it wouldn't last. I think that for bellies we need a wood that will compress substantially and return without damage. How much it will compress we can control by how much wood we use.
Right. Weight is a huge factor. Steel and concrete may be great for stationary construction projects but bows have to propel an arrow. The bone bows that have been done, like the inuit design of sinew over the antler, work for sure but both sinew and antler are heavy materials. They didn't really have wood to work with up in the tundra, but something tells me you could optimize things better if you reduced the amount of the material taking most of the stress (sinew and bone) and have most of the bow be wood still. That way you still get the benefit of the bone and sinew, while reducing weight considerably. But that brings up the point that if you build a wood bow well within it's stress limits then it should be lighter for it's draw weight than a composite bow of sinew, wood, and bone. Therefore a matchup utilizing bone would be most useful if it was incorporating stresses wood normally cannot take. Therefore a heavy war bow would best suited for this material, something with a draw weight over a 100 lbs. Likewise you see that with steel too. It doesn't make a very good bow material for two reasons, it's heavy and repeated bending ultimately leads to the weakening of the metal and it breaks. The few times it has been done successfully is in very heavy weight crossbows where the material can handle that extreems poundage where would couldn't and the draw length is short as to minimize damage done from over bending it.
Title: Re: Mechanical Properties of Wood for Building Bows and Arrows
Post by: willie on November 13, 2017, 10:36:04 am
Quote
something magical................ will provide radical improvement

most would be happy with incremental improvements, I think
 
recombinent resilin, hmmmm. sorta natural, but definitely not primitive. OK for around the campfire, I guess. Be sure and post pics. ;)
Title: Re: Mechanical Properties of Wood for Building Bows and Arrows
Post by: PatM on November 13, 2017, 10:57:04 am
There has to be more than just tension and compression stats. A steel backed, concrete bellied  bow would have great numbers but it wouldn't last. I think that for bellies we need a wood that will compress substantially and return without damage. How much it will compress we can control by how much wood we use.
Right. Weight is a huge factor. Steel and concrete may be great for stationary construction projects but bows have to propel an arrow. The bone bows that have been done, like the inuit design of sinew over the antler, work for sure but both sinew and antler are heavy materials. They didn't really have wood to work with up in the tundra, but something tells me you could optimize things better if you reduced the amount of the material taking most of the stress (sinew and bone) and have most of the bow be wood still. That way you still get the benefit of the bone and sinew, while reducing weight considerably. But that brings up the point that if you build a wood bow well within it's stress limits then it should be lighter for it's draw weight than a composite bow of sinew, wood, and bone. Therefore a matchup utilizing bone would be most useful if it was incorporating stresses wood normally cannot take. Therefore a heavy war bow would best suited for this material, something with a draw weight over a 100 lbs. Likewise you see that with steel too. It doesn't make a very good bow material for two reasons, it's heavy and repeated bending ultimately leads to the weakening of the metal and it breaks. The few times it has been done successfully is in very heavy weight crossbows where the material can handle that extreems poundage where would couldn't and the draw length is short as to minimize damage done from over bending it.

  Bone bows have been built in conventional composite style.  Also steel bows have successfully been done in ancient and modern times.. Not just as crossbow prods.
Title: Re: Mechanical Properties of Wood for Building Bows and Arrows
Post by: joachimM on November 13, 2017, 11:56:32 am
Likewise you see that with steel too. It doesn't make a very good bow material for two reasons, it's heavy and repeated bending ultimately leads to the weakening of the metal and it breaks.

The raw numbers disagree with that statement: spring steel has a relative stiffness (MoE divided by density) equal to that of bamboo (guadua, moso, ...), and will allow a tension strain of 1.5%, more than any wood.
This means that a bow made of spring steel with a mass of 400 g, and designed according to the mass principle, will cast an arrow at the same speed as a bamboo bow, all else being equal. It would just be ten times narrower, which would make it hard to shoot (but there are ways around this issue).


As for the repeated bending leading to breaks:
that's all about the "cycle rate to failure" (or mean cycles between failure), which has to do with design. Say a rope has a guaranteed breaking strength of 10 KN (it can statically bear a load of 1019 kg), that means it can at least sustain this load once.
When loaded statically with a lower mass, say 900 kg, its expected failure rate may only be after 100 loadings, at 800 kg maybe hundreds of times, and at 700 kg its failure rate may reach infinity. (the failure rate distribution is typically exponential)

That's the reason we need to shoot in a bow: if we can shoot it 500 times without breaking, chances are high the bow was built with a reasonable enough safety margin. We see this often in flight shooting, with strings made so thin that they break after a single shot.

So if your bow breaks (down) after some shooting, it just wasn't well made, or at least not made to last. It doesn't matter if it was made of spring steel or of osage...
A bow intended for 50# 28" may shoot thousands of arrows, but drawn once to 60# 31" it may have become junk (if not broken). So the cycle rate to failure may be 1 for 60#, and "infinity" for 50#...
Title: Re: Mechanical Properties of Wood for Building Bows and Arrows
Post by: PatM on November 13, 2017, 12:00:34 pm
The old Seefab steel bows were in fact very slim and painted to look like bamboo!
Title: Re: Mechanical Properties of Wood for Building Bows and Arrows
Post by: Aussie Yeoman on November 13, 2017, 12:03:33 pm
Badger,

I've given little to no thought to the arrow-suitability of wood. In bowmaking, the density of the timber doesn't matter anywhere near so much as with arrows. I suspect there's an ideal ratio of stiffness to density, because a timber that has 60% of the density but 80% of the stiffness of another heavier, stiffer timber is going to make a better arrow.

Nevertheless, here's a screenshot of some of the stiffest timbers. Alas there are no density data.

It seems there's been a lot of discussion since I last got online. There's much to discuss re: strain, stress and suitability of timber for backings, it seems!
Title: Re: Mechanical Properties of Wood for Building Bows and Arrows
Post by: joachimM on November 13, 2017, 01:08:08 pm
Interestingly, Joachim said in an earlier thread that flax "stacks". That might bear further investigation as it indicates some sort of work hardening might be occurring.

Flax stacks indeed a bit, meaning that the stress-strain curve is not a straight line, but a concave one: as the strain increases, the stress (load) increases more than expected.
When loaded slowly (not as in a bow!), it can withstand a strain of 2.4%. The first 1.5% of strain is pretty linear, after that it requires more force than expected from a fixed stress-strain ratio to strain it further.

Irrespective of this, in relation to wood, flax always seems to stack (same as hemp), in that it is about twice as strong in tension as wood, for the same mass of backing added. For the same volume of backing added (say a layer covering the bow's back, of 2 mm thick), it's about five times stronger than wood. Hence a thick enough layer of flax on the back of a bow will force the entire bow's wood into compression. A thin layer, or a narrower crowned strip ("trapped") won't.
When backing a bow with flax, you better opt for a wider than usual bow with thinner than usual limbs, otherwise you'll get excessive fretting. 

The reason for this strength is that flax's cellulose content (per mass unit) is nearly twice as high as in wood (85% compared to 45%), and cellulose is the molecule providing tension strength to wood (while lignin and other compounds provide compression).

Sisal, on the other hand, is much more forgiving than flax. Its MoE is a bit lower than that of bamboo, but it can sustain more elastic strain (>2%) and hence store more energy). It is therefore less likely to overpower a belly (but I have seen it cause a good deal of chrysalling on some bows too).
Title: Re: Mechanical Properties of Wood for Building Bows and Arrows
Post by: Marc St Louis on November 13, 2017, 04:29:16 pm
Quote
Mark, do you think the Bubinga would have done better with a thinner or narrower piece of bamboo?

I tried 2 different styles of bow with Bubinga, longbows with Bamboo and Maple backings and a Bamboo backed RD bow.  The Bamboo was thinned to my usual 1/8" thick in the middle tapering to 1/16" at the tips.  The longbows were perhaps 1 1/8" wide and the RD bow around 1 1/4" wide
Title: Re: Mechanical Properties of Wood for Building Bows and Arrows
Post by: Jim Davis on November 13, 2017, 04:29:32 pm
Aussie, no fault of yours, but the Osage numbers (as always) are the result testing green wood. If you had dry numbers to work with, Osage would be much higher in your list.
Title: Re: Mechanical Properties of Wood for Building Bows and Arrows
Post by: Aussie Yeoman on November 13, 2017, 09:39:42 pm
@ Jim Davis:

Quote
I have seen that, as exemplified in your short table, almost all woods are 3 to 4 times as strong in tension as compression, so all this talk of bamboo or hickory overpowering the belly wood is balderdash, in that almost any intact backing wood is stronger than almost any belly wood.

I would have to heartily disagree here. I backed some Ironbark (similar to Ipe) with reasonably good Elm, and the Elm was torn apart. The reason was the force used to bend the Ironbark produced massive stress, and so the strain packed on early, beyond the strain Elm could handle. Backing choice matters a great deal.

Quote
Aussie, no fault of yours, but the Osage numbers (as always) are the result testing green wood. If you had dry numbers to work with, Osage would be much higher in your list.

Alea culpa. Those numbers are from Tim Bakerís corpus of data. I have a few bits of Osage, which is now well seasoned. I should test it and add it to the database. If I ever got time.

@ PatM:

Sinew has a working strain of around 4%. Exceptional wood of around 1.1%. Many Ďgoodí bow woods around 0.8%, and Willow is down there below 0.6%. No idea where carbon and glass sit.

Quote
Maybe but it takes different amounts of strain to get that stretch to happen.  So bamboo or Hickory is not going to budge when you try to stretch it over a piece of ERC.

Stretch is strain. Strain is the change in length due to compression or tension, given as a percentage.

Quote
but I think we are just putting numbers and trying to explain scientifically  what we already know.

Yes, we are doing exactly that. There is no harm in it, and in fact it can be very helpful. The same things have happened in engineering, medicine and the like and the world is a better place because of it.

@ gfugal:

Quote
Tim Baker showed this with his flax backed pine right? Flax is very Ö.. that it isn't rigid anymore. Styrofoam might do that, but even the most chrysaled wood still maintains some rigidity.

Interesting tidbit: in a beam of homogeneous material, the neutral plane is at half the depth. If you put a backing on with a higher stiffness than the belly, the neutral plane moves toward the back. Given that the surface stress/strain at the surface is equal to the cube of the distance from the neutral plane to the surface, you can quickly see how backing a wood of low stiffness with a material of high stiffness overpowers a belly: it dramatically increases the strain/stress at the belly.

This is why a belly and backing material should have at least vaguely similar stiffness. At the other extreme, see the experience backing Ironbark with Elm, above. Elm is terrific in tension, but only when matched with a belly material of similar stiffness.

Quote
Supposedly bone has a similar, if just slightly less, flexiblity as wood.

Depends on the bone. An adult femur is far less elastic than a rib, for example.

Quote
If we can learn more about tension qualities of the materiel's we work with, I think there is room for improvement.

Itís easy. Pick a timber with a higher working strain and similar stiffness to the belly material, then Perry Reflex the bow.



Title: Re: Mechanical Properties of Wood for Building Bows and Arrowss
Post by: joachimM on November 13, 2017, 10:54:07 pm
greg, Patm,

Strain levels for
Raw glassfiber: 4.9%
Gordon high strength fiber lam (GC 70 ULZ): 3.3%
Carbon (gordon laminate GC 70 UCL): 1.4%, with a MoE of 151 GPa!

See the excel sheet i posted earlier
Title: Re: Mechanical Properties of Wood for Building Bows and Arrows
Post by: PatM on November 14, 2017, 02:44:53 am
So my wording was not refined enough but those points are essentially accurate.
Title: Re: Mechanical Properties of Wood for Building Bows and Arrows
Post by: willie on November 14, 2017, 08:28:03 am
Joachim

I do not question the specs you have posted for various fibers, but wish to know more about your thoughts when using them in bow building applications.

1. The strains  seem to be "safe" elongation, quite useful for comparing the relative qualities of filaments for use in applications where breaking strength is the primary concern. When used in a bow limb however, they seemed to be strained to a much lower degree. The limb thickness on my 60" modern materiel bow is about 6mm, indicating that a very stiff materiel working at lower levels of strain could be what makes the the modern materiel limbs more efficient? Do you think there a correlation between strain levels and hysteresis? Possibly a correlation that could help us when choosing natural materiels for higher efficiency?

2. In what application did you find the flax  to "stack"? Embedded in a matrix of glue? Spun into a yarn? I cannot think of any other natural (animal or vegetable) materiel that exhibits such behavior. Usually work hardening is associated with metals and synthetic crystalline materiel,  but then again, cellulose is said to crystalline also.
Title: Re: Mechanical Properties of Wood for Building Bows and Arrows
Post by: Jim Davis on November 14, 2017, 09:28:48 am
Aussie Yeoman said"

Quote
I would have to heartily disagree here. I backed some Ironbark (similar to Ipe) with reasonably good Elm, and the Elm was torn apart. The reason was the force used to bend the Ironbark produced massive stress, and so the strain packed on early, beyond the strain Elm could handle. Backing choice matters a great deal.

My point was that even in a self bow, the back is going to be stronger than the belly, in all but a few woods, such as cherry.
Title: Re: Mechanical Properties of Wood for Building Bows and Arrows
Post by: Badger on November 14, 2017, 09:43:03 am
   Willie, a few years back I worked out a test to identify hysteresis in wood bows. It proved beyond a doubt there is a direct correlation between hysteresis and strain levels. Once you get into the plastic range on a bow hysteresis starts increasing. I had no need to identify what the number was as long as I could identify at what point this started happening on a bow during the construction phase. As soon as it starts to loose draw weight simply because it was drawn a further distance then you are picking up hysteresis and most likely set. I started using a not set tillering technique which is really just a method of monitoring the condition of the wood as you tiller and my results improved dramatically.
Title: Re: Mechanical Properties of Wood for Building Bows and Arrows
Post by: joachimM on November 14, 2017, 11:08:19 am
Joachim

1. The strains  seem to be "safe" elongation, quite useful for comparing the relative qualities of filaments for use in applications where breaking strength is the primary concern. When used in a bow limb however, they seemed to be strained to a much lower degree. The limb thickness on my 60" modern materiel bow is about 6mm, indicating that a very stiff materiel working at lower levels of strain could be what makes the the modern materiel limbs more efficient?

Do you think there a correlation between strain levels and hysteresis? Possibly a correlation that could help us when choosing natural materiels for higher efficiency?

2. In what application did you find the flax  to "stack"? Embedded in a matrix of glue? Spun into a yarn? I cannot think of any other natural (animal or vegetable) materiel that exhibits such behavior. Usually work hardening is associated with metals and synthetic crystalline materiel,  but then again, cellulose is said to crystalline also.

Excellent observations! Indeed, if the backing is very strong (like in glassfiber, but also in flax), limbs should be made thinner, in order for the belly to be capable of taking the compression. To have enough draw weight, we then need to widen the limbs in flax-wood bows. in GF bows, both the backing and the belly are stronger, while the wood core is merely the matrix on which the GF-epoxy matrix was glued to. It would be a good test to see how thin we can make well-performing flax-wood composites.
As for hysteresis: I'm sure less strain means less hysteresis. Hysteresis being the internal friction between components in a solid matrix. Less strain is less severe fiber friction. Temporary set is IMO nothing else than a slow rebound from hysteresis.

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.
Title: Re: Mechanical Properties of Wood for Building Bows and Arrows
Post by: Aussie Yeoman on November 14, 2017, 11:27:02 am
Aussie, no fault of yours, but the Osage numbers (as always) are the result testing green wood. If you had dry numbers to work with, Osage would be much higher in your list.

Actually, having gone through the data, I don't think the test results were due to green wood. There are four samples in my database, all from either Woodbear or Tim Baker:
Osage 1: 15182 MPa
Osage 2: 14277 MPa
Osage 3: 14015 MPa
Osage 4: 12952 MPa

The results are more consistent than disparate samples within other species. Perhaps you think Osage should have placed higher because it is such an exceptional bow wood. As you will see below, it is not Osage's stiffness that makes it exceptional, but its working strain.

Below I've captured the 30 top woods for stiffness, and also the 30 top woods for working strain. You can see there are very few (red bold) that are common to both lists, but most all of them are favourite bow woods. Does this mean the mathematics/engineering is a fraud? No. It means that the combination of both is not as important. Personally, I think working strain is more important than stiffness, and certainly more important than some sort of ratio. Yew and Willow have a similar stiffness, for example; no one would doubt Yew's suitability as a Top Dog among bow woods.


Title: Re: Mechanical Properties of Wood for Building Bows and Arrows
Post by: joachimM on November 14, 2017, 12:09:27 pm
Aussie, no fault of yours, but the Osage numbers (as always) are the result testing green wood. If you had dry numbers to work with, Osage would be much higher in your list.
Perhaps you think Osage should have placed higher because it is such an exceptional bow wood. As you will see below, it is not Osage's stiffness that makes it exceptional, but its working strain.

Personally, I think working strain is more important than stiffness, and certainly more important than some sort of ratio. Yew and Willow have a similar stiffness, for example; no one would doubt Yew's suitability as a Top Dog among bow woods.

There is actually a trade-off between strain and stiffness: woods with high stiffness (MoE) endure little strain, and vice-versa. For arrows, we use the stiffest woods, which we know are also less ideal as bow woods (larch, port orford cedar, douglas fir, ...).
Bow woods which we generally consider as very good have low (relative) stiffness but sustain high strain levels (osage, yew, plum, ...). Idem with juvenile wood (branch / sapling bows!): this has lower MoE, but higher strain tolerance.

The ideal would be high stiffness and high strain tolerance. Glass fiber is right up there. cough cough. Within all natural materials, bamboo has the best trade-off: high MoE and high strain tolerance.

As for willow, it's unfair to compare it in this way to yew, as its density is only half that of yew. There's just less wood to be strained. That's why it's important to (as per the mass principle) to compare relative stiffness, the MoE divided by density. This allows to compare what a bow of the same mass would perform like. Most willows have a relative stiffness comparable to bamboo (but much lower density).
Still, white willow (Salix alba) is still a very poor bow wood for its density and stiffness, both in tension and compression. 
Title: Re: Mechanical Properties of Wood for Building Bows and Arrows
Post by: gfugal on November 14, 2017, 01:19:43 pm
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.
Title: Re: Mechanical Properties of Wood for Building Bows and Arrows
Post by: gfugal on November 14, 2017, 01:32:21 pm
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.
Title: Re: Mechanical Properties of Wood for Building Bows and Arrows
Post by: willie on November 14, 2017, 01:38:14 pm
Quote
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.

Title: Re: Mechanical Properties of Wood for Building Bows and Arrows
Post by: willie on November 14, 2017, 01:43:42 pm
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
Title: Re: Mechanical Properties of Wood for Building Bows and Arrows
Post by: gfugal on November 14, 2017, 01:57:08 pm
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.
Title: Re: Mechanical Properties of Wood for Building Bows and Arrows
Post by: PatM on November 14, 2017, 01:58:07 pm
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.
Title: Re: Mechanical Properties of Wood for Building Bows and Arrows
Post by: joachimM on November 14, 2017, 11:25:37 pm
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). 

Title: Re: Mechanical Properties of Wood for Building Bows and Arrows
Post by: gfugal on November 15, 2017, 08:04:13 am
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.
Title: Re: Mechanical Properties of Wood for Building Bows and Arrows
Post by: joachimM on November 15, 2017, 09:51:23 am
Yeah sorry my confusing way of telling that you need to take density into account to evaluate stiffness, to get stiffness per mass unit...
Title: Re: Mechanical Properties of Wood for Building Bows and Arrows
Post by: willie on November 15, 2017, 03:27:36 pm
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.
Title: Re: Mechanical Properties of Wood for Building Bows and Arrows
Post by: Aussie Yeoman on November 16, 2017, 12:41:48 am

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 thatís 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.
Title: Re: Mechanical Properties of Wood for Building Bows and Arrows
Post by: joachimM on November 16, 2017, 02:05:23 am
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.
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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.
Title: Re: Mechanical Properties of Wood for Building Bows and Arrows
Post by: Aussie Yeoman on November 16, 2017, 02:51:33 am

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.
Title: Re: Mechanical Properties of Wood for Building Bows and Arrows
Post by: Badger on November 16, 2017, 12:50:48 pm
   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.
Title: Re: Mechanical Properties of Wood for Building Bows and Arrows
Post by: DC on November 16, 2017, 01:28:55 pm
What draw length are you looking at,- about 35' ;D ;D ;D ;D You building another Da Vinci bow? The hand shock is going to be horrific.
Title: Re: Mechanical Properties of Wood for Building Bows and Arrows
Post by: willie on November 16, 2017, 07:30:38 pm
Quote
What draw length are you looking at,-
and how many pounds do you want it to pull?
Title: Re: Mechanical Properties of Wood for Building Bows and Arrows
Post by: Badger on November 16, 2017, 09:10:42 pm
   I don't have my paperwork handy, but I think it was about 15 to 20,000# at 32 feet and about 120,000# stored energy. I may redo the plans for 60 ft total length.
Title: Re: Mechanical Properties of Wood for Building Bows and Arrows
Post by: willie on November 16, 2017, 10:27:13 pm
20,000# pull using doug fir
60 ' long elb drawing 30'

6 and 1/8 in thick at handle, but it's gotta be 96 inches wide.  will store just under 300,000 ft lbs, though



Title: Re: Mechanical Properties of Wood for Building Bows and Arrows
Post by: joachimM on November 17, 2017, 12:58:15 am
   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.

I don't see why they would have to be less dense. Basically, you scale up everything.
Bows of different dimensions but of the same design, shot at 10 gpp and with a draw length scaled to the bow's length (DL = bow length/2.5) will be strained the same amount, and will shoot equally fast when shot in a vacuum (air density is the only thing we cannot scale up or down, so a lighter arrow will experience more drag than a heavier one).
Title: Re: Mechanical Properties of Wood for Building Bows and Arrows
Post by: Badger on November 17, 2017, 05:13:55 am
  You don't really use the grains per pound on a very large bow. You would scale up the grains per pound of stored energy. Just by coincidence a 28" draw bow stores about the same number of units as its peak draw force. So to do this you have to scale up the size of all the units as well.  I am actually planning on using an almost dry fire situation of about 1 grain per pound of stored energy. Maybe 10 grains per pound to demonstrate destructive force.
Title: Re: Mechanical Properties of Wood for Building Bows and Arrows
Post by: willie on November 18, 2017, 11:36:17 am
Interesting project, Steve. One that certainly requires a bit of number crunching to execute.

I hope we have not dissuaded PNWarcher from posting further, with all our meanderings. Seems like the PA way sometimes.

I have found a database that has some values collected from a few different sources. It includes the works of George Garratt from 1931.

Code: [Select]
http://www.matweb.com/search/PropertySearch.aspx
Also if interest might be this discussion from a few years back at PP.
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https://paleoplanet69529.yuku.com/picking-a-wood-for-a-bow-t28706.html#p310687
for those that wish to learn how to make adjustments from the published data for varying moisture contents and densities, this pub from back when airplane were built from wood may be useful. (also other pubs at westcoastpiet.com)

http://westcoastpiet.com/images/Construction/ANC-18%20Part%201%20of%203.pdf

Title: Re: Mechanical Properties of Wood for Building Bows and Arrows
Post by: joachimM on November 19, 2017, 10:28:01 pm
   I don't have my paperwork handy, but I think it was about 15 to 20,000# at 32 feet and about 120,000# stored energy. I may redo the plans for 60 ft total length.

For a bow that's quite a lot of poundage to pull! But still short of the heavier Roman ballista ;-) (replica shown here https://www.youtube.com/watch?v=jn2wbZow8cw) that pulled an astounding 870 000 pounds (give or take a few thousand)
Title: Re: Mechanical Properties of Wood for Building Bows and Arrows
Post by: Badger on November 20, 2017, 03:20:03 am
   I don't have my paperwork handy, but I think it was about 15 to 20,000# at 32 feet and about 120,000# stored energy. I may redo the plans for 60 ft total length.

For a bow that's quite a lot of poundage to pull! But still short of the heavier Roman ballista ;-) (replica shown here https://www.youtube.com/watch?v=jn2wbZow8cw) that pulled an astounding 870 000 pounds (give or take a few thousand)

      I think they measured the 870,000# at some point that was not reflective of stored energy. I know the ballista's did store a lot of energy but I doubt they stored quite that much. A 1,000# crossbow may store as little as 100# energy depending on the draw length. Where you read the stress it not so important as much as how much is being applied to the projectile. I am still thinking a much lighter wood might be in order for a giant bow as the cell structure of wood may need to be considered when scaling something like this up.

     I don't remember the exact numbers as it has been a while since I played with it but I think on my mass calculator it calls for limbs that weight about 500# where in reality they would weight closer to 2,000# likely even more than that.