Author Topic: Mechanical Properties of Wood for Building Bows and Arrows  (Read 1665 times)

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Offline PatM

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Re: Mechanical Properties of Wood for Building Bows and Arrows
« Reply #15 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.

Offline Aussie Yeoman

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Re: Mechanical Properties of Wood for Building Bows and Arrows
« Reply #16 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.
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Offline gfugal

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Re: Mechanical Properties of Wood for Building Bows and Arrows
« Reply #17 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.
Greg,
No risk, no gain. Expand the mold and try new things.

Offline joachimM

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Re: Mechanical Properties of Wood for Building Bows and Arrows
« Reply #18 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 :-)

Tillering is easy. Problems arise when a bowyer thinks he's right and the wood is wrong.

Offline Aussie Yeoman

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Re: Mechanical Properties of Wood for Building Bows and Arrows
« Reply #19 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.
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Offline willie

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Re: Mechanical Properties of Wood for Building Bows and Arrows
« Reply #20 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.—Moisture 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.
« Last Edit: November 11, 2017, 02:36:57 pm by willie »

Offline Aussie Yeoman

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Re: Mechanical Properties of Wood for Building Bows and Arrows
« Reply #21 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.
Articles for the beginning bowyer, with Australian bowyers in mind:

http://www.tharwavalleyforge.com/articles/tutorials

Offline Jim Davis

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Re: Mechanical Properties of Wood for Building Bows and Arrows
« Reply #22 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.

Jim Davis

Kentucky--formerly Maine

Offline PatM

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Re: Mechanical Properties of Wood for Building Bows and Arrows
« Reply #23 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.


Offline willie

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Re: Mechanical Properties of Wood for Building Bows and Arrows
« Reply #24 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.
 

Offline Badger

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Re: Mechanical Properties of Wood for Building Bows and Arrows
« Reply #25 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?

Offline Marc St Louis

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Re: Mechanical Properties of Wood for Building Bows and Arrows
« Reply #26 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
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Offline Jim Davis

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Re: Mechanical Properties of Wood for Building Bows and Arrows
« Reply #27 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.
Jim Davis

Kentucky--formerly Maine

Offline PatM

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Re: Mechanical Properties of Wood for Building Bows and Arrows
« Reply #28 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.

Offline Marc St Louis

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Re: Mechanical Properties of Wood for Building Bows and Arrows
« Reply #29 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  :)
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