RATIO OF STRENGTH TO MASS IN WOOD SPECIES

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Dennis La Varenne
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RATIO OF STRENGTH TO MASS IN WOOD SPECIES

#1 Post by Dennis La Varenne » Sat Jun 30, 2012 10:39 pm

Some of you will have noticed that I have corrected the positions of the numbers in my example for relationships between the density and strength of specific species of wood in the thread on making a bow from Massaranduba wood - http://www.ozbow.net/phpBB3/viewtopic.php?f=34&t=12641. The numbers used are derived from published data which I have cited (and will always continue to do so of course).

This mathematical error was pointed out by one of our hawk-eyed members - with thanks (Rod). He also pointed out that when I first proposed the method some years ago which he remembered quite well, that I used it in reverse, ie strength units per unit of mass which gave a number of less than one which he thought would be more intuitive for the average person. Here is the example above rejigged that way -

Massaranduba: 188/950 = 0.198
Spotted Gum: 150/950 = 0.158
Red Ironbark: 135/1220 = 0.110
Grey Ironbark: 181/1210 = 0.149

The ratio can be read either way and the result is still the same for practical usage. Of the fours species cites as examples, Massaranduba is the strongest per unit of mass, followed by Spotted Gum, Grey Ironbark and finally Red ironbark.

Again, for practical purposes, the relative strengths could be cited as whole numbers for convenience as a sort of index. For instance, Massaranduba could be cites as having a S/M ratio of 198 compared to Red Ironbark's 110 so long as we all understand the derivation.

When I first posted the ration in Alex's thread, I was concerned that numbers less than one may prose problems of visualisation for most people and hence chose to reverse the units (poorly as it turns out) to yield numbers greater than one.

However, henceforth, when I use this ration again, I will use it as a S/M ratio.
Dennis La Varénne

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Re: RATIO OF STRENGTH TO MASS IN WOOD SPECIES

#2 Post by rodlonq » Sun Jul 01, 2012 10:55 am

When you put it like that it is clear that Massaranduba is a clear favourite.

Perhaps adding some units would help eg;

Massaranduba S/M ratio = 188/950 = 0.198 MPa/(kg/m^3) = 198 kPa/(kg/m^3)

The units are not so intuitive until you considers you would be using the S/M ratio to compare the relative strength of two bows of the same design but different woods, therefore the volume would be equal and you can mentally cancel out the m^3 terms in the denominators of the units (i.e. kPa/kg).

Thanks again for sharing the benefits of your research with us Dennis.

Cheers....... Rod

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Re: RATIO OF STRENGTH TO MASS IN WOOD SPECIES

#3 Post by bigbob » Sun Jul 01, 2012 3:06 pm

My head hurts :wink: :smile: :mrgreen:
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Re: RATIO OF STRENGTH TO MASS IN WOOD SPECIES

#4 Post by Dennis La Varenne » Sun Jul 01, 2012 7:35 pm

bigbob,

This ratio is just a way of comparing the suitability of one wood species to another for bowmaking.

Relative to the mass of the wood (its air dried density expressed in kilograms per cubic metre) which is an industry standard, I have just divided the mass into the number for its modulus of rupture which is a number comparing the strength of a sample of wood against breaking, usually (but not always) by bending it in beam form in the same way that a bow does.

Thus, in the case of Massaranduba, for every unit of mass (kg per cubic metre), there are 0.198 MoR units (Megapascals). So, relative to its mass, it is significantly stronger than the other 4 examples cited.

It won't take you long to get your head around it if you give it a little bit of thought. I devised it so intending bowyers could look up the mechanical properties of various Australian woods for suitability for bowmaking and which is better than another by using a calculator.

The Americans still use specific gravity (SG) instead of air dried density (ADD) such as we and the rest of the world uses, but I convert SG numbers using the factor which Keith Bootle uses in his book 'Wood in Australia' 2nd edition 2010 on page 24, Table 3-1 where he gives a list of relationships between SG and density. There is a constant conversion factor of x 1120 to convert SG to kg per cubic metre which is what I use when confronted with SG numbers.

I am preparing a table with a list of common northern hemiphere bow woods and compared their published properties to those of a list of Australian woods having mechanical properties in the same or higher range as them. My hope is that this list will help intending Australian bowyers not spend money unnecessarily on overseas bow woods when they can obtain perfectly suitable and equally good performing Australian bow woods.

From assessing the relative mechanical properties of both local and northern hempisphere woods, ours have every chance of being able to perform equally as well and often be a bit more durable because of their usually higher resistance to knocks and bumps.

We all know that the published mechanical properties of any wood to be used for bowmaking cannot be the whole story. They can only be a guide to selection. Actual usage and information sharing will sort out the bugbears within any specie which is used. There is also the problem that the same tree grown on one side of a hill will have different properties at the individual level to the same specie grown on the other side of the hill because of different growing conditions. Two different bowyers can easily come to loggerheads of such issues, which is why I prefer the published data because of the sampling methods used.

One supposes that they cannot be too far wrong if the whole timber industry relies upon them at least for structural and load-bearing work.

My idea is, that manipulating into some kind of ratio, the published numbers for the mechanical properties of wood species which are most relevant to bowmaking, will enable us to make more prudent choices in a land where there is no bowmaking culture.
Dennis La Varénne

Have the courage to argue your beliefs with conviction, but the humility to accept that you may be wrong.

QVIS CVSTODIET IPSOS CVSTODES (Who polices the police?) - DECIMVS IVNIVS IVVENALIS (Juvenal) - Satire VI, lines 347–8

What is the difference between free enterprise capitalism and organised crime?

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Re: RATIO OF STRENGTH TO MASS IN WOOD SPECIES

#5 Post by bigbob » Sun Jul 01, 2012 7:48 pm

Dennis thank you yet again for a veritable trove of information. Your imput is always valued. Re my flippant remark I actually do have a condition that makes it hard for me to digest information speedily these days [ less kinder people may call it senior onset dementia]. A comparison chart or graph between native and 'offshore' timbers would be brilliant. I did once have a copy of Keith Bootles book on Australian timber species, but alas like my mental prowess its gone missing!
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Re: RATIO OF STRENGTH TO MASS IN WOOD SPECIES

#6 Post by Dennis La Varenne » Sun Jul 01, 2012 11:24 pm

bigbob,

You are far from being on your own having to wade through material to digest it properly. I am doing it constantly. I try to think of it as brain pushups.

When I publish my list here, much of the raw data will be from Bootle's book. The interpolations will be mine though because they need to be manipulated to be suitable for the needs of bowyers. I could also be way off the mark in what I am trying to do, but there will be somebody who will point that out I am sure.
Dennis La Varénne

Have the courage to argue your beliefs with conviction, but the humility to accept that you may be wrong.

QVIS CVSTODIET IPSOS CVSTODES (Who polices the police?) - DECIMVS IVNIVS IVVENALIS (Juvenal) - Satire VI, lines 347–8

What is the difference between free enterprise capitalism and organised crime?

HOMO LVPVS HOMINIS - Man is his own predator.

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Re: RATIO OF STRENGTH TO MASS IN WOOD SPECIES

#7 Post by bigbob » Mon Jul 02, 2012 9:04 am

Dennis, an edited version of his data would be a godsend to anyone making bows and who wishes to use Aussie timber as a matter of preference. I know I cant walk around an archery course these days without getting neck strain from eyeing all the trees. Being able to source some of these woods might be the harder part as quite a few are predominately only cottage industry use or collected by individuals as windfalls or the like. Bob
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Re: RATIO OF STRENGTH TO MASS IN WOOD SPECIES

#8 Post by longbowinfected » Mon Jul 02, 2012 9:34 am

Dennis, will you include figures for bamboo as well as Hoop Pine?

I am not a bowyer but such a list would be great reference when talking to people at Ironfest as I get asked about bow and arrow making timbers by a lot of handymen when at Ironfest in Lithgow each year.


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Re: RATIO OF STRENGTH TO MASS IN WOOD SPECIES

#9 Post by Dennis La Varenne » Mon Jul 02, 2012 9:57 am

bigbob,

The data will be in the form of a list of specie names along with their botanical names so wild-grown wood can be sourced as necessary where it is legal to do so. From Bootle, I will also include the state of origin.

Kevin,
It is pretty hard to get material on Bamboo believe it or not. It is not presented in the same manner using the same testing procedures as does wood used in the timber industry. Also, there are zillions of different bamboo species of which only a few are bow friendly and of those, I cannot source any data thus far.

Data on Hoop Pine is available, but is very poor in every respect compared to the mechanical properties of classic bowmaking species. It rates about the same as Radiata and Slash pine. Compressive strength is abysmal and elasticity is nominal at best.

But as an arrow material, it is quite good and compares well with Sitka Spruce and Douglas Fir (aka Oregon) and much better than Port Orford Cedar by a significant margin. A recently well proven Australian light hardwood is Victorian Alpine Ash (E. regnans) which Forro used to sell on Ozbow, and I have had good results with Bunnings hardwood dowel (often Blue Gum). Bunnings also has plenty of Hoop Pine dowel. But you need a spine jig to match it.

But for my table, I want to concentrate on bow woods for the present. However, like bow woods, arrows can be made from almost anything so long as it is straight and the grain follows almost the full length of the dowel. Heavier species just need to be make smaller diameter and vice versa. There are no BEST arrow woods, just those which have been used and are known and those which have not.

Outside of northern Europe and North America which are our heritage areas, the rest of the archery world was using whatever was to hand quite successfully. Australia is an archery frontier. We have lots of work to do.
Dennis La Varénne

Have the courage to argue your beliefs with conviction, but the humility to accept that you may be wrong.

QVIS CVSTODIET IPSOS CVSTODES (Who polices the police?) - DECIMVS IVNIVS IVVENALIS (Juvenal) - Satire VI, lines 347–8

What is the difference between free enterprise capitalism and organised crime?

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Re: RATIO OF STRENGTH TO MASS IN WOOD SPECIES

#10 Post by rodlonq » Mon Jul 02, 2012 11:00 am

Hello Dennis,

Are published values for MoR standardised as a failure in the tension side or the compression side or whichever fails first?

One often hears of a particular wood being more suited for the belly, rather than the back of a bow indicating it has better compression resistance than tension resistance, how would one know from the available literature?

There is no doubt that the information you are compiling will become a valuable resource for bowyers of all levels of experience all over the world. Well done mate.

Cheers........ Rod

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Re: RATIO OF STRENGTH TO MASS IN WOOD SPECIES

#11 Post by Dennis La Varenne » Mon Jul 02, 2012 11:47 am

Rod,

MoR applies equally to both tension AND compression because of Young's Law . . in theory. But, in practice, you have to try it out for yourself. The numbers are not definitive or necessarily predictive. They are a guide to selection only based on known properties tested in a sample range.

I work on the presumption of MoR working as a guide to compression failure before tension failure, and so far, I have had only one bow of mine break from tension failure that I can recall, and that was a Yew bow in which the sapwood tore apart first.

All other failures of my bows (only a few fortunately) that I can recall have been compression failures where the bellies began to chrysal and progressed to frets which ate through the thickness until the back failed the resulting tension stresses. When examined, it was quite clear from the manner of the breakage how it occurred, but in these few instances, it was expected.

I have read about certain northern hemisphere wood species being weaker in tension, but I have not used any of them. Dave Clarke (Yeoman) has reported elsewhere on Ozbow that he found Eastern Victorian Red Ironbark (Eucalyptus tricarpa) to be weaker in tension than compression in his bend testing, but I have found the opposite in my bows - that chrysals appeared on the bellies of my bows and never any splinters raising on the backs unless there was obvious damage to the surface. So that goes to show that we both must have gotten samples from either side of the same hill perhaps but collected by the same person.

Good bowyery can allow for these shortcomings in the limb layout. For instance, if a wood does prove to be weaker in tension, you simply make the back as flat as possible and wider than the belly - an upside down trapezoid if you like, and vice versa for a bow which is inordinately strong in tension and weaker in compression. Then, there is always backing for tension weak woods.

I am not interested in providing a list of 'sure thing' bow woods. I am only interested in providing a list of suitable candidates based on their mechanical properties compared to the same properties in classic bow woods.
Dennis La Varénne

Have the courage to argue your beliefs with conviction, but the humility to accept that you may be wrong.

QVIS CVSTODIET IPSOS CVSTODES (Who polices the police?) - DECIMVS IVNIVS IVVENALIS (Juvenal) - Satire VI, lines 347–8

What is the difference between free enterprise capitalism and organised crime?

HOMO LVPVS HOMINIS - Man is his own predator.

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Re: RATIO OF STRENGTH TO MASS IN WOOD SPECIES

#12 Post by longbowinfected » Mon Jul 02, 2012 12:34 pm

Dennis, I was thinking along the lines of data wrt say one or two species of boo plus say samples of known boo board. Having the good ones named and compared might help later generations retain the knowledge because it has been recorded. Possibly with the size of this forum someone has access to material testing facilities? If I had a relationship with the boo board guys I would ask them about technical data. Perhaps they would already have some of that data because engineers architects might ask for it. They might not advise of the data unless they know someone who uses theuir product over time.

Hey it is great what you are doing.

Worth considering the arrow material too when you have extra time as if there are not adequate supplies of quality stuff to make arrows out of, that could limit trad archery and the uptake of hand made bows in the future.

I think the quality of available POC is rapidly declining. Hoop Pine could become one of the saviours of trad archery world wide. Hoop Pine also glues very well and might be a great proposition in the laminated arrow sector. Northern Pine which John McDonald sells is a great product as well.

Kevin
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Re: RATIO OF STRENGTH TO MASS IN WOOD SPECIES

#13 Post by rodlonq » Mon Jul 02, 2012 3:26 pm

Thanks Dennis, MoR of wood is something I have yet to find a clear definition for. I am familiar with Young's Modulus, otherwise known as the elastic modulus, which relates stress to strain over the elastic range for a given material. The modulus itself is a line that lies parallel to the linear region of a stress/strain plot and sometimes with some offset (eg. 2% offset, which only changes the location of the elastic limit).

From what I can follow, MoR is not actually a modulus, but more like the yield strength in bending, similar to the way we refer to the tensile strength being the average yield strength of a given material under tensile stress. I cant find any information relating the bending stress to strain, with respect to MoR. Is there a defined relationship you know of, or is MoR actually just the yield stress in bending?

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Re: RATIO OF STRENGTH TO MASS IN WOOD SPECIES

#14 Post by Dennis La Varenne » Mon Jul 02, 2012 3:42 pm

Kevin,

For the time being, I will concentrate on published data for commercially available timber in regard to bows. There is nothing stopping people also sourcing this stuff in the wild either where it is legal to do so which means BOTH sources can be available potentially.

With regard to bamboo, I have hunted far and wide for mechanical data and cannot find any appurtaining to the only specie I know of which is used for bowmaking - Tonkin bamboo. There is a lot which is used commercially in tube form for structural purposes and there is data for that - but only as it is used - in tubular form. I cannot find anything where it is used in laminations for structural purposes.

The commercial producers you write of may have data, but I don't know of bamboo being used commercially in laminated board form for structural purposes - only as flooring. I would be surprised if there is any mechanical data available for it.

Would you like some homework?

Rodlong,

I had a thought about your issue of discerning the tension or compressionability of certain woods. Anybody can easily test a specific piece of wood for these properties quite easily by simply clamping and bending a short lamination of the material over a table edge, making sure that the growth rings in the lamination correspond reasonably to the orientation they will assume in the bow.

By bending the lam very slowly and watching which side started to rupture first will tell whether the sample is tension or compression strong.

The underside equates to the belly and the upper side equates to the back of a bow. If cracks in the form of chrysals and the larger frets show on the bottom first, then the wood is compression weak and if splinters start to lift on the upper side first, then the wood is tension weak.

Cracks are characteristic of compression fracturing and splinters lifting are characteristic of tension fracturing.

MoR, so far as I understand it is as you say, a yield stress at the point of failure which is a long way past the loads we put on bows. We are really concerned with the elastic limit, but figures for that aren't published by anybody unhappily and there is no really direct relationship between the two across wood species. Dave Clarke (Yeoman) noticed that with the wood species he did bending testing on that the elastic limit seemed to be reached at around 60% of the MoR figure he got when he tested to failure.

Have a look at these little snippets I found -
Modulus of Rupture.pdf
(116.58 KiB) Downloaded 209 times
Basic Strength and Elastic Properties of Wood.pdf
(264.05 KiB) Downloaded 225 times
Dennis La Varénne

Have the courage to argue your beliefs with conviction, but the humility to accept that you may be wrong.

QVIS CVSTODIET IPSOS CVSTODES (Who polices the police?) - DECIMVS IVNIVS IVVENALIS (Juvenal) - Satire VI, lines 347–8

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Re: RATIO OF STRENGTH TO MASS IN WOOD SPECIES

#15 Post by longbowinfected » Tue Jul 03, 2012 9:37 am

If anyone can give us the names and contact details for their favourite dealer in boo boards I will drop them a line. They might help given it is extra potential income even if low volume as every extra dollar counts.

Kevin
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Re: RATIO OF STRENGTH TO MASS IN WOOD SPECIES

#16 Post by rodlonq » Tue Jul 03, 2012 10:09 am

Hello Dennis,

Thanks for the resources, I am sure a lot of people as well as myself will find them very useful.

It seems as I suspected that the MoR is an unfortunate naming. The yield/bending/flexural strength is technically a point on a curve, while a modulus is a line (as indicated in the plots in the second attachment). Anyway, what it is called is almost irrelevant, it is understanding of the physical characteristic it represents thats is important.
The underside equates to the belly and the upper side equates to the back of a bow. If cracks in the form of chrysals and the larger frets show on the bottom first, then the wood is compression weak and if splinters start to lift on the upper side first, then the wood is tension weak.
This answers my question from two posts back about whether the Modulus of rupture applies to the tension or compression state. I have read some of Yoeman's work on testing and also Tim Baker has an interesting and quite simple test in TBB1.

Thanks for the discussion Dennis, it has been enlightening. BTW, the last letter on my alias is a lowercase Q, not a g :mrgreen: My alias is a short form of Rodney Lowe, North Queensland.

Cheers, Rod

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Re: RATIO OF STRENGTH TO MASS IN WOOD SPECIES

#17 Post by Dennis La Varenne » Tue Jul 03, 2012 11:03 am

Rod,

This is all a bit of a struggle for me I can tell you. I have no formal training in mathematics past primary school and absolutely nothing from any kind of an engineering background upon which to base anything. It is all very much trial and error and lots of brain sweat working all this stuff out.

In regard to MoR, yes you are quite correct. It is not a modulus at all. The nearest I can find of a practical nature to that concept is something I read in the US Forests Service 'Wood Handbook', Chapter 4 which they term 'Work to maximum load in bending' (WMLB) which seems to translate as a kind of working elastic limit. Here is the quote -
Work to maximum load in bending - Ability to absorb shock with some permanent deformation and more or less injury to a specimen. Work to maximum load is a measure of the combined strength and toughness of wood under bending stresses.
But, the Handbook does not demonstrate how it is calculated, or whether the stress is applied suddenly or slowly. It is listed in their tables of mechanical properties with a values expressed as kJ/m^3. Suggesting a measure of both combined strength and toughness implies to me that the stress is applied fairly rapidly.

That is the problem with all the definitions I come across. Just about everywhere has its own standard with different methodologies which I can't seem to be able to correlate.

For instance, the Handbook lists a MoR for Degame (Lemonwood) as 153,800 kPa, but a WMLB of 186 kJ/m^3 yet I cannot find any kind of reference in Bootle to a similar standard used here in Australia. However, the standard seems to have existed in the US since before WWII at least. I have found references to it in a few old US sources on the web.

We may just be stuck with devising our own. A simple bending test similar to Yeoman's could produce such a number. His method is adaptable to any rectangular beam sample as stress per unit of volume similarly to the WMBL number.

I would presume that the Handbook WMBL figure is based upon a defined resultant degree of deformation, whereas, I would prefer to use a figure based upon that point at which measurable strain occurred. The intention would be to use that figure as the basis upon which to use the Supertiller calculator to derive limb thicknesses which would hopefully produce a bow with the minimum or even zero string follow throughout its life because the bow wood would never be stressed past its inherent elastic limit during normal usage. Such a bow would be almost the exemplar of Comstock's overbuilt bow produced in a much more controlled and predictable way.

Hypothetically, that is quite possible. Anyway, back to other things and sincerest apologies for the 'q'. It is quite obvious now that you point it out. For years I read it as a 'g' for some reason.
Dennis La Varénne

Have the courage to argue your beliefs with conviction, but the humility to accept that you may be wrong.

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Re: RATIO OF STRENGTH TO MASS IN WOOD SPECIES

#18 Post by yeoman » Tue Jul 03, 2012 6:48 pm

Hi all,

it is with regret that I come late to this conversation, but with enthusiasm that I dive in.

Dennis, I think it's a great idea to publish, or at least establish, a manner by which the strength per mass can be compared among woods regardless of their different densities. Even better, that they are all a decimal of 1. Why? because that means the very best strength/density ratio is 1 (an unachievable asymptote to be sure among woods), which is an easy number to remember and a good reference.

It's extremely frustrating that bamboo mechanical properties are not better published. When I get some time, I plan to do some bend testing of it. Because of the cross section though, the maths will be a little tricky. But I think the exercise will be worthwhile.

Reading through the thread, I think there may be room for another number to do the rounds. That being the stiffness per unit of density.

If one board bends twice as far before mechanically failing (this can be through snapping or taking a huge set or cracking in tension etc etc) as another board, it can be said to have twice the strength, no matter whether it broke in tension or compression. However, if one board requires twice as much force as another to bend the same distance, it is twice as stiff.

Yew is not very stiff, but can bend quite far before mechanically failing. This makes it a supreme wood for bows. Sitka Spruce is extraordinarily stiff, but cannot bend far without mechanically failing. In other words, it is brittle.

Woods like ironbark and spotted gum are very stiff indeed. However, their resistance to mechanical failure is much higher than spruce (I have a theory on that penned below) and so they make quite good bows. Depending on your design and tillering.

I haven't done any testing/analysis, but i would like to see the stiffness/density and strength/density of some woods like Jarrah, 'TasOak', others I can't remember off the top of my head...compared with the likes of spotted gum and ironbark. I would not be terribly surprised if, comparitively, as starkly contrasted with absoloutely; the champion woods were not much different to the mid-weight woods.

Why then are they so good? Because they are so very dense. With a denser wood, there are more wood fibres per mm of bow width to resist tension/compression. Because they're dense, less volume is needed to secure the needed number of wood fibres. Because they're dense, and need less volume, bows made from super dense woods can be thinner per-bow than lighter wood bows to the same specs. When a bow is thinner (not narrower) the belly and back fibres do not need to compress/stretch as much to conform to a given bend (tiller shape).

I must dash to dinner. I could rant about this all night.

Dave
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Re: RATIO OF STRENGTH TO MASS IN WOOD SPECIES

#19 Post by yeoman » Tue Jul 03, 2012 9:23 pm

Why then are they so good? Because they are so very dense. With a denser wood, there are more wood fibres per mm of bow width to resist tension/compression. Because they're dense, less volume is needed to secure the needed number of wood fibres. Because they're dense, and need less volume, bows made from super dense woods can be thinner per-bow than lighter wood bows to the same specs. When a bow is thinner (not narrower) the belly and back fibres do not need to compress/stretch as much to conform to a given bend (tiller shape).

I must dash to dinner. I could rant about this all night.
Okay, so to folow on just briefly.

If the fibres do not have to stretch/compress as much, then that means that relatively more brittle woods can be used. Which is one of the reasons SG and IB can make great bows, but much more sparse but still brittle woods cannot. A less dense wood will have to be a thicker limbed bow, which means the outer fibres are much further from the neutral plane of tension/compression, which means stress is loaded onto the outer fibres at a far larger radius of bend.

As an aside, this is why 'mortal' density woods that are somewhat brittle need to be made really long (to increase the radius of curvature of the tiller) or made really wide (to allow them to be really thin and thus get the outer fibres as close as possible to the neutral plane).

That might be enough for the evening. I feel I'm getting away from the original topic.

Dave
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Roadie
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Re: RATIO OF STRENGTH TO MASS IN WOOD SPECIES

#20 Post by Roadie » Wed Jul 04, 2012 8:34 am

Now I'm really confused, is some one trying to make a really super Wood Bow that can shoot for a very long way, say at least 500 yards. Cheers All.

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Re: RATIO OF STRENGTH TO MASS IN WOOD SPECIES

#21 Post by GrahameA » Wed Jul 04, 2012 9:00 am

Hi Grant
Roadie wrote:Now I'm really confused, is some one trying to make a really super Wood Bow that can shoot for a very long way, say at least 500 yards. Cheers All.
What do you want?

A "Super Wood Bow" that can shoot 500yards - and you are walking on deadly ground here - ie a flight bow. Or "Super Wood Bow" that can be pulled to say 150lb is thin and light and doesn't break.

Revelations - Crikey, Yew Bows are light in the hand. Hmm...... could that have anything to do why they are nice to shoot?

There are several component: The material you make the bow from which is what this post is about another is the design of the bow and to that you can add the design of the bow materials.

Those who wants something to while away those cold winter evening could think about something as simple as the appropriate limb shape to achieve whatever.

A lot of the problem is defining the question.
Grahame.
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Re: RATIO OF STRENGTH TO MASS IN WOOD SPECIES

#22 Post by Dennis La Varenne » Wed Jul 04, 2012 9:01 am

Dave,
Reading through the thread, I think there may be room for another number to do the rounds. That being the stiffness per unit of density.
Already thought of that one and it will be included. I have also started a MoR : MoE ratio as well - again as a decimalled number which has started to yield a very interesting set of numbers and to explain perhaps why Yew is so highly rated when its MoR and density are so very average.

Grant,

Don't try to read something into this thread that it NOT there. I am trying to list of wood properties with a set of comparative ratios which will help people to make selection choices using Australian wood species. Nothing else.

It is a different approach and, I hope, based a bit more upon objectively measurable data and not entirely on hand-me-down belief. Although, it may also help explain why some of the classic woods are as good as they are when so much of the belief in the properties of the classic bow woods could not be explained in any very satisfactory way.

I hope it is a start along the track of explaining why one choice is better than another.

As Grahame says above, the type of bow you make and how well it performs has much to do with design. Good design will get a better bow out of lesser materials, but good design coupled with materials having good properties suitable to the project will yield a better performing bow.

There is nothing to be confused about. I hope people are able to go about looking for Australian bow woods in a more systematic rather than haphazard way. I hope it will shorten the learning curve for Australian bowyers.
Dennis La Varénne

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Re: RATIO OF STRENGTH TO MASS IN WOOD SPECIES

#23 Post by yeoman » Wed Jul 04, 2012 3:48 pm

There's another thing I thought of.

The MoR is the stress which causes the wood sample to break entirely. However, the amount of stress in a bow is far less than that, anywhere from 40-65% depending on the design, part of the limb, and wood.

When doing bend tests, I measure the set at every increment of loading, which allows me to graph the increase in tension and compare it with the set. What I have found across a wide range of samples is that the amount of stress that imparts an acceptable amount of set is a bending stress of about 60% of the published or measured MoR. This is only ABOUT, and can drift up and down.

However, publishing a number which is a ratio of density to 60% of a published figure will get a bit annoying I'm sure. I suppose in the long run the numbers are for comparative and not absolute values anyway.

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Re: RATIO OF STRENGTH TO MASS IN WOOD SPECIES

#24 Post by rodlonq » Wed Jul 04, 2012 6:34 pm

Dennis,

I have been chewing on your proposed MoR/MoE ratio. Perhaps you can relate the numbers corresponding to timbers you know to be of good value. I don't have that experience on my side.

The catch I see with the ratio is that Timber A with a low MoR and low MoE will have the same MoR/MoE ratio as Timber B that has twice the MoR and twice the MoE, however, obviously Timber B would build a far better bow.

:idea: Perhaps another way of collecting the two terms is to present the product, i.e. MoR x MoE. (you could call it MoRE...... more MoRE is better :lol: ) In this case Timber B will have 4 times the MoRE product than Timber A, clearly distinguishing it as superior for bow making.

Two timbers with the same product (Timber C and Timber D) may be roughly comparable for bow making. Assume Timber C has slightly higher MoR and slightly lower MoE than Timber D. For the same width, Bow C needs to be a little thicker than Bow D to make the same draw weight, which is compensated for by the slightly higher MoR. If the densities are the same then Bow D will be the lighter (less wood) of the two and possibly a little faster.

This thought led me to MoRE/D being MoR x MoE / Density. It could be a useful collection of properties in that if Timber C and Timber D have different densities, it would advantage the lighter wood.

At this point I realised the number of collected variables one could dream up is quite large and some temperance is required. Any collected term may be useful for generalisation, but the individual properties must also be considered.


I have also been chewing on the definition of WMLB, and I ended up spitting it out :oops: . I don't think I can digest that one :confused: . It obviously has some significance to someone, but I think its use for bowyers is limited.

Cheers.......... Rod

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Re: RATIO OF STRENGTH TO MASS IN WOOD SPECIES

#25 Post by longbowinfected » Wed Jul 04, 2012 10:14 pm

Dennis,

part of my homework about bamboo. Not terribly sure of the species. From
http://www.wood-database.com/lumber-ide ... ts/bamboo/

Average Dried Weight: 31 lbs/ft3 (500 kg/m3) to 53 lbs/ft3 (850 kg/m3)

Basic Specific Gravity: .38 to .64

Hardness: 1,410 lbf (6,270 N) to 1,610 lbf (7,170 N)

Rupture Strength: 11,020 lbf/in2 (76,000 kPa) to 24,450 lbf/in2 (168,600 kPa)

Elastic Strength: 2,610,000 lbf/in2 (18,000 MPa) to 2,900,000 lbf/in2 (20,000 MPa)

Crushing Strength: 8,990 lbf/in2 (62.0 MPa) to 13,490 lbf/in2 (93.0 MPa)

Shrinkage: Diameter: 10-16%, Wall Thickness: 15-17%

Kevin
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Re: RATIO OF STRENGTH TO MASS IN WOOD SPECIES

#26 Post by yeoman » Thu Jul 05, 2012 5:51 am

Rod,

the only flaw with stating the product of MoE and MoR is that a briottle wood and a balanced wood can have roughly the same product if the numbers happen to be the same.

Actually, another flaw is that one wood might have quite high numbers but be brittle, but a wood with lower numbers but balanced will appear to be the worse wood because of the lower overall product.

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Re: RATIO OF STRENGTH TO MASS IN WOOD SPECIES

#27 Post by rodlonq » Thu Jul 05, 2012 8:56 am

Dave,

That is a good point but I believe the same traps apply even if you look at the properties individually. There is no substitute for having the experience or a written guide to tell you which woods are brittle and which are not (I have neither :sad: ). Without this knowledge you may say that looking at a normal table of properties is flawed because the values of MoR can be misleading.

I think of all the possible collection of properties, the one I would like to add to a table of properties most is Dennis's original MoR/Density ratio.

Cheers....... Rod

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Re: RATIO OF STRENGTH TO MASS IN WOOD SPECIES

#28 Post by Dennis La Varenne » Fri Jul 06, 2012 2:33 am

To Everyone,

Here is the table.

There were a lot of surprises for me when the numbers were crunched. I was very surprised at how many Australian wood species scored higher than 0.300 in my table compared to the classic bow woods listed at the start of the table.

There is an explanation of how I allocated a score to each of the woods at the end of the table. One wood specie in particular had an an almost unbelievable score close to 0.400.

Anyway, here it is for you to peruse and have a pick over. Please remember that it is NOT a predictor of how any particular wood specie will perform as a bow. It is only an indicator of which Australian wood species have the potential to be good bow woods compared to known classic bow woods.

But, I am starting to get the impression that some members are beginning to think that I will be providing a list of bowmaking superwoods.

Some woods have inherent problems presenting to bowyers such as gum pockets and gum veins which cause bows to break. Others have a grain structure which is too short for bowmaking unless the bow is backed, and so forth.

In a perfect world where everything else is equal from a bowyer's perspective, this table should help in the selection of suitable wood species from within this country, and potentially, result in bows at least the equal of anything from overseas.

Kevin,
Those numbers are no better than many ordinary wood species. I am surprised at how low they are. I was expecting more for bamboo, especially in the elasticity area. The rupture strength range is equivalent to many wood species. The trick for us is to find those species of bamboo which are at the higher end if we want to make bows using bamboo on the belly.
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Re: RATIO OF STRENGTH TO MASS IN WOOD SPECIES

#29 Post by bigbob » Fri Jul 06, 2012 5:01 pm

Dennis from your comparative table , the numbers suggest black wattle is right up there. i have a section that was felled fairly recently and is about 160mm dia. It has not been sealed before I picked it up and has small amount of checking. What is the likely out come if I was to cut some long thin sections longtudinally from it while still relatively green. My thought would be to make some belly lams for a glass bow.Would this action result in splitting or other undesirable outcome and would these thin sections still need to be seasoned ?
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Re: RATIO OF STRENGTH TO MASS IN WOOD SPECIES

#30 Post by Dennis La Varenne » Fri Jul 06, 2012 5:59 pm

bigbob,

My table doesn't predict anything. I cannot advise on how Black Wattle will work under glass because glass negates most of those properties in wood which make for a good self-bow. The table is a selection guide only. Many of the wood species listed have very good properties, but it remains for bowyers to make self-bows from them to see if they even work as a bow. The properties are only suggestive.

In a glass bow, the glass takes pretty much all the compressive and tensile stresses in a bow's limb. The core adds mass and a separation distance for the glass. Core material needs to be resistant to shearing forces. My table doesn't address that at all.

Whilst the properties of Black Wattle are pretty good, it is a wood which has lots of problems with pockets of sap. This tendency to create sap pockets makes it hard to get good clear specimens sound enough to make a self-bow. The sap pockets are obviously areas of great weakness in a self-bow, but could possibly be gotten away with to some degree in a glass bow so long as shearing stresses weren't too high.

I seriously doubt that the listed properties of Black Wattle will be of any benefit in a glass bow because of their negation by the overwhelmingly higher compression and tension resistant properties inherent in the glass.
Dennis La Varénne

Have the courage to argue your beliefs with conviction, but the humility to accept that you may be wrong.

QVIS CVSTODIET IPSOS CVSTODES (Who polices the police?) - DECIMVS IVNIVS IVVENALIS (Juvenal) - Satire VI, lines 347–8

What is the difference between free enterprise capitalism and organised crime?

HOMO LVPVS HOMINIS - Man is his own predator.

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