Thoughts on Carbon Fiber

[Xander]

So I have lots of carbon fiber sheet and have been reading up on using it in a Hill style flat bow.  Needless to say, not much out there.  I have seen some tensle strength numbers posted on various sights but everyone says, just experiment til you like it.  No set formulae.  Well I found this handy chart online with the mechanical proprties of various composites...

  http://www.performance-composites.com/carbonfibre/mechanicalproperties_2.asp

So now back to the hill bow.  The high tensil strength of CF tells me I need to adjust a few things in the stack recipe to get close to my desired outcome.  First off, I am not looking to do CF under glass, I want CF back and belly only.  Now, I would need to adjust either one of and likely a combination of, skin thickness (lam thickness) total stack thickness.

The CF I have is roughly .020" thick.  I would need a high compression capable and thin core lams to keep the forces relatively balanced between back and belly.

Now this is where I need the help, I have been recommended one parallel and two .002" tapers for a build like this, but that was by only one person.  Any thoughts on a stack recipe to get me in the 40# range. I'm willing to experiment a bit, but I would like to get close.  I don't care about fancy figure woods, the cheaper the better.

Thoughts?

[bjansen]

What will your riser length and total bow length (ntn) be?   I could probably get you close

[Xander]

Well to be a Hill bow I understand riser should be 16" and my draw is 29", so that should be 66-68 ntn, correct?

[MoeM]

Guess the high surface travel and the deep wood-rich core would make a prepreglam glass belly much more suiting the c-fibres.
I know a guy who build mild R/Ds and longcurves this way, using c rovings in lamination raisin covered by veneers for protection- with good results so far.
On a Hill you could use very thin split boo over the c for c`s max power and boo`s vintage design.
Would be funny like the most recent cobra or old musclecars with e.g. SRT10 engine and electrics   :D
Damn I`d love to shoot her...

[LittleBen]

First regarding the stack, whether you use carbon or glass or wood, the taper should always be the same as long as the stack thickness is the same. So a .500 carbon limb, a .500 glass limb and a .500 wood limb will have the identical taper for a proper tiller. Obviously the weight will be drastically different, but they will all bend identically (essentially). So long story short, use whatever taper is recommended for a normal glass bow of the same stack thickness.

Well I looked at the mechanical properties chart ... first some questions.

Is the CF you have pre-impregnated with resin? or do you have just the raw fabric?
Trying to use CF fabric, in my opinion is not a high success percentage plan. The ratio of resin to CF will substantially alter the strength or the composite. It's unlikely you will be able to get the maximum strength.

Unidirectional CF is roughly 3.3x Stiffer than unidirectional E-glass. The sheet does not state whether this is compressive or tensile stiffness. However, assuming that compressive stiffness and tensile stiffness between the two show a similar ratio I think you can expect the following:

At the same stack thickness, carbon should give you about 3.3x more draw weight. However since the carbon is tyoically .020 and the glass .040 or .050, there may be a slight deviation. So maybe 2.50-3x the draw weight with carbon vs glass at same stack thickness.

my 2 cents. I think you may still have to do some experimenting ... If you want a hill bow I'd make it 70", and shoot for 35-40#, if it comes in too low start piking until you're at 40#.

[Xander]

Good thoughts.  The CF I have is prepreg lam and cured twill.  It was vacuum and heat cured at 120*C. It is aerospace skin material.

Yes I understand that the bend characteristics do not change, but I was hoping I could get the stack height down a bit and rely more on the higher tensile of the CF.  Decreasing the stack will balance the tensil and compression strain on the back and belly skins.  I was thinking if I could cut the stack height down 1/3 roughly I should be close, somewhere in the .250-.300 range is my guess, but that's a half educated guess.

[LittleBen]

According to my calculations on this, assuming the carbon limb is 2.5-3x the draw weight at the same stack thickness as the glass as I suggested above,

To convert a glass bow design to carbon and maintain the same draw weight, the stack thisckness of the carbon bow should be ~ 68%-75% of the thickness of the glass bow. (68% represents the carbon being 3x as stiff and 75% represents the carbon being 2.5x as stiff)

so for a .500 stack glass bow, you'd want between .345 and .375.

A .250 - .300 stack carbon limb should be equivelant to a .362 - .435 stack for glass (assuming 3x stiffer for carbon).

Hope that makes sense

[Xander]

Ok, so we are on the same page then.  The numbers I used are based on 1/3 the stack of the binghams numbers, but I understand they use a 18" riser, so I may have to lean towards the upper end for a proper 16" riser Hill bow.

Now th next question is, where do I remove the thickness from?  Do I take away from all lams evenly, or do I take the tapers down, how about the parallels?

The idea of a bow with only .250" stack height, .020 CF back and belly really sounds like a feather weight in hand!  I suspect arrow speeds will be similar to that of traditional stack recurves or possibly higher.  This has really intrigued me.

If we can come to a consensus of where to start I will begin to gather materials and hopefully get a working prototype soon.  I will also check with my aerospace engineer buds to get their input on the matter, since I am only a metrologist and don't have the practical experiance with the material specs that they do.

Ok, so now I am cautiously optimistic about this idea, let's get a collective brain trust together and figure it out.


Thanx!

[LittleBen]

You're going to need to adjust the taper. I'd say if the glass bow needs a total of .004/" you should be thinking about .003/" for yours. If you use two .020 carbon lams, your wood will need to be .210 for a total of .250"

I'd probably just go with 3 .070" .001" tapers.

or I guess you could do like 1 .100" .002" taper, 1 .070" .001" taper, and 1 .040" parallel.

Frankly it makes absolutely no difference as long as the total stack is correct and the total taper is correct.

[Xander]

Yes, I was originally recommended two .002" tapers.  I could conceiveably use 4 .001" tapers, properly adjusted for total stack.  I bet more tapers with the modern adhesives would yield a stronger, faster bow allowing less sheer forces to enter into the tapers/parallels.  Considering that modern epoxies have higher sheer strength than common bow woods and with greater consistancy I would be more inclined to go that route.

[LittleBen]

If you taper .004" total over the length of the limb, your tips will be extremely thin (.114 at the tip for a 68" bow), and I think you will end with a more whip tillered bow. Not bad for a hill style. Again, personally I'd probably go .003" but i don't think you can go wrong either way.
all tapers will definately work too ... it's really all gravy in terms of what exact lams you use. 4 equal thickness tapers will allow you to make it look cool ... line alternating walnut and maple, or alternating carbonized and natural actionboo etc.

[bjansen]

When taking into consideration the stiffness differential for CF vs. Glass, I don't think a simply 75% x the old glass stack will work.  Remember every x change in lam stack thickness with have an 8x change on the weight. Thus the calculation may need to be a bit more precise taking that into consideration.


Here are my thoughts on making a Carbon fiber hill style, 16" riser length, 66" NTN length bow of 40# @ 29":

-Total butt thickness of .395 (including all lams and Carbon Fiber)
-Total taper rate of .005 per inch
-Straight line taper from fades (1.125" wide) to tips (.400" wide)
-Trap the bow from back to belly (i.e. thinner back to equalize the tension/compression.  I put a pretty heavy trap on mine.


This is based on some data that I have from Hill's I built and a little guesswork and I can see that it is much higher than the stacks mentioned above.  Of course, the way you finish a limb will have a big effect on weight. The amount of trap that you put on the limb can adjust the poundage significantly.

[LittleBen]

Brad,

limb stiffness is proportional to limb thickness to the third power. Stiffness ~ H^3 where H = stack thickness.

Assuming a carbon bow is 3x stiffer than a glass bow having the same limb thickness, if we want a carbon limb of EQUAL draw weight to the glass bow, we have to reduce the stack.

So to reduce the stiffness of a given limb to 1/3 the draw weight

1/3 * StiffnessOriginal = StiffnessFinal

1/3 * H_original^3 = H_final^3, when H_original=1,

1/3 = H_final^3

(1/3)^(1/3) = H_final

~.69 = H_Final, when H_original is 1.

Therefore if a carbon limb of equal thickness to a glass limb has 3x the draw weight, the carbon limb would need to be 69% as thick as the glass limb to be of equal draw weight.

I think my math is correct, the only question is how stiff a carbon limb is compared to glass at the same stack thickness. If you know that, you can calculate exactly what stack you need based on the glass bow charts.

I proposed that it would be 3x as stiff ... this may not be accurate, it's just a starting point.

Likewise, if someone has a carbon bow and a glass bow of the same design (off the same form) even if they're not the same weight or stack thickness, you can also calculate exactly how much stiffer the carbon makes the limb.

[Xander]

Ben, your math looks correct to me and follows what I "figgered" was "about right" based on tensile strength and beam theory in the simplest fashion.  My limited experiance is in actually building a bow, but we all know that "in theory, practice and theory are the same. In practice, they are not."

[Dan Bonner]

I have built a bunch of carbon rigs. It's pretty complicated and they don't tend to last all that long. Failure rates are very high due to shear forces between the back and belly. I can point you to a fair amount of info if you PM me.

Bonner

[Xander]

Bonner, PM inbound.

The shear forces between back and belly are exactly why I am looking to reduce the stack height.  Also, starting with a longer bow I can limit the amount of shear even more by reducing the amount of bend, but at the same time eliminatin any gain in limb speed.

[Xander]

Originally posted by bjansen:
When taking into consideration the stiffness differential for CF vs. Glass, I don't think a simply 75% x the old glass stack will work.  Remember every x change in lam stack thickness with have an 8x change on the weight. Thus the calculation may need to be a bit more precise taking that into consideration.


Here are my thoughts on making a Carbon fiber hill style, 16" riser length, 66" NTN length bow of 40# @ 29":

-Total butt thickness of .395 (including all lams and Carbon Fiber)
-Total taper rate of .005 per inch
-Straight line taper from fades (1.125" wide) to tips (.400" wide)
-Trap the bow from back to belly (i.e. thinner back to equalize the tension/compression.  I put a pretty heavy trap on mine.


This is based on some data that I have from Hill's I built and a little guesswork and I can see that it is much higher than the stacks mentioned above.  Of course, the way you finish a limb will have a big effect on weight. The amount of trap that you put on the limb can adjust the poundage significantly.

So as I said before, I haven't built a bow yet, but some observations on your theory.

Trapping limbs concentrates the stresses into a smaller area and since the limiting factor of using CF belly skins is the shear forces between skin and core this appears inadvisable.  Most natural materials have compressive strength less than 10K psi while using CF belly skin would generate 25-35K psi (50# 20" limb .030" thick skin).  That leaves a tremendous load to be carried by the wood = failure.

Trapping also lends itself to unstable limbs.


I think using equal skin thickness is the way to go, and lowering the stack height will better manage the shear forces on the core lams, but not store as much energy as a narrow deep limb would.

Just thinking outloud.

[LittleBen]

Interesting thoughts. I'll have to try and use my brain on this later ...

[Xander]

Now i just got off the phone with Bonner, he did bring up a good point, a Hill style bow is going to be as mild a build as it gets, having such long woring limb length will reduce much of the extreme forces generated by CF overpowering the core mterials.  His suggestion was to use core lams equivalent to a glass bow, but because the CF is thinner the total stack will be .040" less than a glass stacked bow.

No real performance gain is likely to be noticed, but for me it will be a cost saving matter as I already have the CF.

[MoeM]

Looking forward at your experiences!