Question for any Engineers or Materials Scientists out there

[Flem]

The composite I have in mind contains no longitudinal reinforcement. My idea is to reduce the mass, since all that unidirectional fiber is overkill on the belly of a bow. A well made prestressed sheet of  elastic epoxy would likely be enough by itself, but it could be made even lighter with some aggregate that would increase a couple of desirable properties. Glass fiber is nearly twice the mass of epoxy, but HGMicrospheres would cut in half the mass of epoxy alone. So I'm thinking if it has a more optimum mass and retains adequate physical properties for its task, it going to be more efficient.

Ray, I get most of my fabric from Fiberlay, but thats if Im near a store. There are a lot of retailers with great selections on line. If I need to mail order, I use Composite Envisions. They have the most extensive selection of fabrics I have seen to date.

[Shredd]

What Alan said pretty much explains it...   Microspheres with epoxy is pretty much only good as a filler or core material...  It is pretty fragile when made into a lam...  Do some research on some guys that have worked with it...  Maybe someone will chime in...  It is also called syntac foam or something along those lines...

[mmattockx]

The composite I have in mind contains no longitudinal reinforcement. My idea is to reduce the mass, since all that unidirectional fiber is overkill on the belly of a bow.

The fibres are what carry all the load, on both the back and belly. They aren't overkill, they are the only way lams can work.


Mark

[Flem]

You guys sound like the voice of experience. Did somebody try this?  What ratio of spheres to resin was used? What was the composite intended for? I got questions?
I did mention I was considering a thin biaxial layer for structure. Think elastic concrete.
So if someone has tried this and it failed, details please. Otherwise I might just try it. Unlike the other composite I need to make, this would be a piece of cake and inexpensive.

[williwaw]

Heavy weight crossbow limbs and compound bow limbs are other good examples where glass can be used to its true potential. In contrast, it is much harder to take full advantage of the properties of glass in bows that require relatively low draw weight and long draw lengths (typical of most target and hunting bows).
Alan

Microspheres with epoxy is pretty much only good as a filler or core material... 

but wouldn't innovation in core materiels be just as important as innovation with front or back lams? especially if glass or carbon is not being used to its full potential?

[williwaw]

Think elastic concrete.

microballons in laminating resin gets quite runny, and some cabosil  (fumed silica) in the mix would be useful to stiffen the uncured mixture.  Maybe even stiffen the cured mixture. Think of the sand needed in addition to the larger aggregates to make good concrete

[onetone]

Mark, thanks for the information re fiberglass and carbon. Joel

[Shredd]

Yes...  I am familiar with micro balloons as in what williwaw called it and which I always called it...  I did some research on it when I was in search of a lighter core material...  My experience with micro balloons is as a filler because it is super light and easy to sand...  I used it when making surfboards, sailboards and other projects...  I found out that they make different grades of it and was gonna order some to make cores for limbs but I found out how fragile it was from other bowyers and decided not to mess with it...  Plus my own experience with it as a filler told me that it would not have the strength that I desired for a strong limb...  If it was the best stuff everybody would be using it...  It's tough to beat good ol' hard maple...  When you think of micro balloons and epoxy, think of styrofoam but maybe more rigid, denser and more firm...

[avcase]

You guys sound like the voice of experience. Did somebody try this?  What ratio of spheres to resin was used? What was the composite intended for? I got questions?
I did mention I was considering a thin biaxial layer for structure. Think elastic concrete.
So if someone has tried this and it failed, details please. Otherwise I might just try it. Unlike the other composite I need to make, this would be a piece of cake and inexpensive.

Yes, there is test results available that document the mechanical properties of epoxy filled with a range of 3M glass microballoon sizes, wall thicknesses, and volume ratios of micro balloons to epoxy.  The tensile strength, compression strength, and elastic modulus all suffer in relation to increasing percentage of microballons.  Density gets a little better with higher volume of micro balloons, but it is not enough to offset many of the downsides.  As mentioned above, this is the recipe for the syntactic foams used in some bow limb cores.

I agree with Shredd’s comments above. My bet is you are better off using a piece of straight grained wood for a back, belly, or core laminate.  It makes sense to me to me using syntactic foam cores in things like surfboards because it isn’t going to rot out if some moisture happens to seep in.

Alan

[Flem]

Well I learned something new, syntactic foam. Sounds perfect to me. Not sure why you guys are so adamant it won't work. To know that you would have to be able to compare the actual stresses generated  by a bow limb and compare it to the properties of the laminate and or its components. Has anybody done that? Can anyone say that the amount of compressive force incurred drawing a bow, exceeds the limits of epoxy or Hollow glass microballons? I can appreciate what you guys are saying, but without that knowledge its just speculation. If you have some comparative numbers or first hand experience using a HGM filled epoxy composite on the belly of a bow, please share that info.
 
Density should decrease as the volume of microballons increased. HGM .06 g/cc , epoxy 1.1g/cc
and the addition of HGM to epoxy with not render it porous. Its used in flotation applications! Neither glass nor epoxy are subject to rot from water contact.

[mmattockx]

Has anybody done that? Can anyone say that the amount of compressive force incurred drawing a bow, exceeds the limits of epoxy or Hollow glass microballons?

Yes, both Alan and I can confirm that the compressive stresses on the belly will very significantly exceed the strength of your epoxy (I believe you stated it is ~26,000psi?). Bending stresses can run upwards of 50,000-60,000psi depending on the design.


Mark

[Shredd]

  Flem...  Let me get this straight... You are attempting to build carbon backed limbs with a syntactic core  and belly??  If I got this wrong please explain your exact limb lay up schedule...

[Flem]

Has anybody done that? Can anyone say that the amount of compressive force incurred drawing a bow, exceeds the limits of epoxy or Hollow glass microballons?

Yes, both Alan and I can confirm that the compressive stresses on the belly will very significantly exceed the strength of your epoxy (I believe you stated it is ~26,000psi?). Bending stresses can run upwards of 50,000-60,000psi depending on the design.


Mark

Thats too vague a statement. Do you have some figures to validate that statement. "Can run upwards" leaves a lot to the imagination. Whats typical? Specifically which stresses are you referring too?
And what design are you correlating those "figures" too?

Rich, I am not building that bow. Not sure how you came to that conclusion. I am getting ready to experiment with composites. One is the carbon/epoxy "lam" the other is the minimally reinforced epoxy composite "lam"with HGM used as a yet undetermined amount of filler. I would not call what I am planning syntactic foam. I'm not trying to make a floating bow. I'm not attempting to make any core material. All I can tell you is that ultimately if I use any of these test composites, they will go into making a straight limb, slight string follow ASL.

[Roy from Pa]

Ya need to get some of sticky-pops wood stiffener, if it works for him, it otta work for you...

Just sayen:)

[mmattockx]

Thats too vague a statement. Do you have some figures to validate that statement. "Can run upwards" leaves a lot to the imagination. Whats typical? Specifically which stresses are you referring too?
And what design are you correlating those "figures" too?

It depends entirely on the design, they vary from soup to nuts depending on the details. The stresses I am referring to are the bending stresses seen on the belly and back surfaces of the limb. Typical is pretty hard to define because there are so many different designs possible.

The red oak lam bow I am working on now has maximum stresses in the low 16,000psi range because red oak can't take much more. My maple pyramid bow I posted a week or so back has maximum stresses of ~18,000psi because it is shorter than the oak bow. I have a 61" lever bow designed for FG lams that has a maximum stress of ~54,000psi. From the modeling I have done of 'typical' FG recurve designs the stresses are never much less than 50,000psi.

If you want another reference point, ask Alan what the stresses are in the foot bow he built and shot this year to shoot an arrow 1600+yds.

If you want to know what your Hill style bows run PM me the specs and I will analyze it and let you know.


Mark

[Flem]

I have to disagree with you again. I don't think that most non-exotic bows and certainly less so for the long working limb style bow, are approaching the limitations of the composites. If you have ever physically bent a piece of bo-tuff or other laminate, you will find that the epoxy will start to fracture long before the composite its self will fail. So even though the composite as an entity un to itself has not failed, you have still exceeded the elastic limits of the epoxy component. Yet, I cant remember seeing a bow thats experienced that kind of permanent deformation. Of course environmental degradation could not be ruled out as a potential factor, but barring that or a manufacturing/assembly defect, I don't see it happening. Which I why I question the efficacy of modeling. Too many variables left unaccounted for. Bow making for me is an organic exercise. The bow takes on its form and function thru visual and tactile input. Minimal engineering involved. I would be interested to know if anyone has designed and calculated the properties of a hypothetical bow, built the bow and then checked the accuracy against test data of same bow in a laboratory?

[mmattockx]

Which I why I question the efficacy of modeling.

You're welcome to question it all you like, but I use it every day to design and analyze much more complicated structures than a bow.

I would be interested to know if anyone has designed and calculated the properties of a hypothetical bow, built the bow and then checked the accuracy against test data of same bow in a laboratory?

My maple pyramid was designed using software. I simply cut the board to the dimensions predicted by the software. It was accurate to under 1% in terms of predicting weight, which means that the stresses are predicted to the same accuracy. If you don't believe me, ask Alan Case about his bows.

Just because you don't like the answers it gives doesn't mean it is faulty or invalid.


Mark

[avcase]

I would be interested to know if anyone has designed and calculated the properties of a hypothetical bow, built the bow and then checked the accuracy against test data of same bow in a laboratory?

Yes, every bow I build.




You might be able to get a workable bow with your micro-balloon belly strip. After all, you can make a bow out of concrete with the right dimensions. It may not be much fun to shoot or carry around, but it can be done.  The elastic modulus of the no-glass-fiber material you described will be very low, kind of like putting a piece of taffy on the bow belly.  Probably a best-case result will give you something like a piece of PVC for the belly lam of your bow (best case).  It will require much more than typical stack thickness to achieve draw weight.

Good luck.  Make your material, use it on a bow, and let us know how it goes.

Alan

[Flem]

Mark, your cherry picking.
Would you not like to address my assumption that if we were exceeding the elastic limit of the epoxy in the composite, that we would be seeing signs of plastic deformation?

This is the bow you are referring to?

“Thanks. It had quite a saga and it is likely a miracle it became a bow at all. There was a pin knot on the back that concerned me, so I initially backed it with cotton fabric and TBIII. The rest of the board was nice, straight grain and I wouldn't have backed it except for that knot.

Got it worked in some on the tree and then decided to heat treat the belly and put a touch of reflex into it. During heat treating I scorched the edges of the backing a bit in a couple spots. I didn't trust it after that, so I sanded it off (sanding all that off by hand was certainly penance for being a muppet with the heat gun...) and put a small patch of vectran fibres over the pin knot and left the rest of it unbacked. It was OK after that and I finished tillering it out to 28". First crisis averted...

Similar to Roy's recent BBO build along, a knot had appeared on the belly side of the lower limb when I was removing wood at the beginning. It was small and didn't transfer through to the back, so I soaked it with CA and carried on. After a few hundred shots that spot started hinging. So I rasped off some of the belly wood at the hinge and glued a patch over the knot area. I didn't pull it back perfectly flat when I glued the patch on and you can still see where the hinge was on the limb, but the patch has held for 1000+ shots now and nothing has moved, so it is hopefully going to stay that way.”


How is it possible to account for all that modification?

Do you have an example other than your Maple bow, one with fiberglass that you have done the same design and testing procedures?

I would be interested to know if anyone has designed and calculated the properties of a hypothetical bow, built the bow and then checked the accuracy against test data of same bow in a laboratory?

Yes, every bow I build.




You might be able to get a workable bow with your micro-balloon belly strip. After all, you can make a bow out of concrete with the right dimensions. It may not be much fun to shoot or carry around, but it can be done.  The elastic modulus of the no-glass-fiber material you described will be very low, kind of like putting a piece of taffy on the bow belly.  Probably a best-case result will give you something like a piece of PVC for the belly lam of your bow (best case).  It will require much more than typical stack thickness to achieve draw weight.

Good luck.  Make your material, use it on a bow, and let us know how it goes.

Alan

Sound like you have access to a lab?

And thanks, I think I will just to spite you guy's

[mmattockx]

Would you not like to address my assumption that if we were exceeding the elastic limit of the epoxy in the composite, that we would be seeing signs of plastic deformation?

I missed this in the exchanges. Are you referring to the epoxy that is holding the unidirectional lams together? If the modulus of elasticity of the epoxy is much less than the fibres then the epoxy will just follow the deflections and the load will be carried by the fibres. A typical recurve bow will not induce more than about 1% of strain in the limbs. I expect that is significantly less than the strain limit of the epoxy. If you have the material properties info for your epoxy we could see what it will be.

I think I understand now where you are coming from on this. When I say a bow has maximum stresses of 50,000psi on the limb surfaces that does not mean all of the material in the lams will see 50,000psi. They all see the same amount of deflection, but that does not equal to the same amount of stress. Stress is calculated by multiplying the deflection (the strain) by the modulus of elasticity of the material. A stiffer material (higher MOE) will see higher stresses than a less stiff material for the same amount of deflection. Because the fibres are hugely stiffer than the epoxy (probably a couple orders of magnitude) they end up carrying all the load and having the high stresses while the epoxy is doing almost nothing aside from holding the fibres in place in the lam.

This is the bow you are referring to?

Yep, that's the one. The 1% accuracy and weight I referred to were measured before I screwed it up with my heat treating attempt and the hinge started to appear. Stresses won't be a lot different now but the light heating and bit of reflex added (and then lost) likely changed the starting point a bit.

Do you have an example other than your Maple bow, one with fiberglass that you have done the same design and testing procedures?

Nope, I haven't done a FG bow yet. If Kenny is on it then I should have a bow kit and extra materials in the mail today sometime.

And thanks, I think I will just to spite you guy's

I look forward to it, experiments are always cool to see. My red oak lam bow is nothing but an experiment in Perry reflex along with a different style of form and slightly unusual construction method. As you have read, there have been some speed bumps on the trip so far but I am learning a bunch and that is mostly the point.

Neither Alan nor myself are trying to be difficult about this, it is the reality of the situation that we understand as engineers. You can be skeptical about our methods, but if our understanding of the physical world was significantly incorrect buildings would be falling over on a daily basis and machines would almost never work properly.


Mark