Question for any Engineers or Materials Scientists out there

[Flem]

Mark you did not read very carefully what my intent is. Unidirectional carbon lam. The point is to avoid having to put carbon under a glass lam, not interested in cosmetics. My motivation is curiosity and to see how it would compare to a fiberglass lam. I like to push boundaries, never been one to accept the status quo

[Roy from Pa]

My motivation is curiosity and to see how it would compare to a fiberglass lam. I like to push boundaries, never been one to accept the status quo

Noooo, say it isn't so:)

[Flem]

Its twue!, its twue!

[williwaw]

How much pretensioning do you want the carbon to have?  if you lay up the carbon on the inside of a curved lam, and then straighten it, the carbon will be in tension

[Flem]

Usually pretensioning is just getting the slack out, but I have with fiberglass roving, intentionally over tensioned during lay-up to get a arched lamination off the form. Did not really seem to be any advantage that I could tell in the finished bow. Don't know if it would be any different with carbon tow?

[williwaw]

just getting the slack out,

seems like a good idea for consistency.

silk was once used as a backing by hickman, before glass.  it was prestretched before gluup, but of course it's quite stretchable compared to carbon

[flyonline]

Flem

have you looked at the uni-directional fabrics? They come in a fair range of weights and tow count and might make it easier to get a nice straight layup. The stuff I had was backed with scrim to hold it together, but I was doing foot wide wet layups. There is uni tapes but the one's I've seen all have scrim which I'm guessing won't be what you want.

How are you pre-tensioning your glass now? I know someone who has done this (CF not glass) and from memory he was using a bar type clamp to hold each end then tension by pulling the clamps apart over some kind of form. Don't think he was getting any arching either.

Steve

[zhangkeyang]

looking forward to your success

[Stagmitis]

Hey Flem couldnt you just use an arrow spine tester that measures deflection? Just bring the posts closer together.

[Flem]

Steve, uni fabrics would no doubt  work, just not for what I am doing. Mine is not an open lay-up, so clamps are not an option. I have been trying to as closely as possible mimic the process used by manufactures, pultrusion, to make my composites. The main issues with an open lay-up are fabric saturation, surface finish and excess resin content, which is why I have started doing a resin infusion process. Do a search on that subject and you will see what I mean.
Charles, I have used my tensiometer to measure deflection with homemade glass composites and checked them against Gordon's. Gordons is stiffer, or should I say it has a higher MOE. But it would be nice to have more info and some real numbers for comparison. If you look at property charts for manufactures laminates their is a lot of info that I have no ability to acquire at home. Tensile modulus and strength, flexural modulus and strength, compressive modulus and strength and shear modulus and strength. I'm just a goofball grunting around in my cave with sticks and stones for equipment.

[avcase]

Flem,
There are a couple of different ways to obtain the Elastic Modulus. The easiest method is to use a 3-point bending method with a dial indicator to measure deflection under a known weight applied at the center of the sample.  It is hard to tell for sure, but it looks like your device pictured on the previous page might work in a similar way.  The rest of the info required is the span between supports, sample thickness, and width.

Elastic Modulus = (deflection/load)*(span^3)/(48*I)

I = sample_width * (sample_height^3)

I have a spreadsheet that I can share which does this and other calculations. I also measure the mass of the sample in order to see the stiffness or elastic modulus to mass ratio.

Another small detail. I removed the return spring from my dial indicator.  The return spring in skews the results because it is counteracting some of the applied load by a varying amount depending on the amount of deflection of the sample. My simple test results agree very well with the Bingham specs for their glass and carbon. The Bearpaw samples were much less consistent.

I also have also found it useful to test composite sample layups and compare the results to those I calculated.

Measuring the strength properties is the other important property, but this is not as simple a process.

I find that the slightest misalignment of carbon fibers makes a big difference in the stiffness of the carbon layup. This is why a layup using woven carbon fiber cloth has just a fraction of the bending Modulus of unidirectional.

I hope this helps.

Alan

[Longcruise]

It's official!  We now have two mad scientists on the Bench.      

[williwaw]

If you look at property charts for manufactures laminates their is a lot of info that I have no ability to acquire at home.

True, but do you need all the data?
the flexural modulus (which is what Alans formula will give), seems to be an average of the tensile and compressive, and would be adequate if you were to use your lams on both back and front of the bow.

In a case where your limb design utilizes your lam on only one face, you could make an adjustment up or down from the results obtained from the beam (flexural modulus) test described by Alan.

[Flem]

Thanks for weighing in Alan. Their are some things I can’t wrap my head around and some things I just can’t do. For instance the tensiometer I made is for building and truing a bicycle wheel. I put it on a 2mm X 290mm stainless rod and measure the deflection. The only point of relevance is that all 36 spokes measure the amount of deflection. On my MTB thats .025” But the problem is, I don’t know how much force it took to get that deflection. It’s inconsequential for the purpose the tool was made for. The other thing is I have no base line by which to calibrate it and I don’t know how to go about getting the correct material with known properties to do any calibrating, if possible. It’s likely a mathematical thing, which will be a problem for me. I  do elementary math, but my brain won’t make the  connection from the intangible  to an equation. Even still with that tool or a spine tester, as a couple of guys have suggested, it would only be very limited amount of information, restricted to the elastic zone of the composite. I did, one time measure a piece of white .050” Gordon’s and a piece of .050” homemade. I cut them into 3/8” X 11” pieces to approximate the size of material the tool was designed for, measured a bunch of pieces and took the average. Gordon’s measured .031” on my meter and homemade measured .042”. But once again it useless information, I only have the amount of deflection, don’t know how much force it took to achieve that measurement. So I'm asking myself, if I end up making this test composite, is it just for posterity or will I be able to get some meaningful data?

[williwaw]

do you have a dial caliper or micrometer and accesses to gram scale? It would be all you need really.  The tensionmeter has a spring and would need to be dissembled and re purposed. from what I can see, it may have a short range of like 1/4 inch.  a new dial indicator with a 1" range would be useful for making deflection measurements, and can be had from amazon for 20 bucks, but If you already have a dial or vernier caliper you could get by

the math is fairly straightforeward, and I expect someone on the forum could help, given the needed dimensions.

Are you hoping to crunch numbers so that you can eventually calculate a stack height with your custom lam?

[avcase]

Flem,
Do you have some specs on the carbon fiber that you are using?

If the fibers in your layup are well distributed and aligned, and you know the percentage of fiber by weight, then we can calculate a pretty decent estimate of the mechanical properties.

Will it be useful?  It depends on how you use the info. I find it useful.

Alan

[Flem]

I think I am slowly wrapping my head around this. So if you had estimations of all the variables, would you be able to extrapolate a dynamic loading sequence chart or graph, similar to what the material  would be encountering in actual usage or could be replicated in a materials lab? Thats the vanishing point for me.
I can calculate the mass and volume of the materials and I can create a static loading and measuring device, but building something that could take the material thru its elastic range into deformation, to failure, is way beyond my current ability. To me that the most useful information, we as bowyers could possibly put to use.
 

[williwaw]

but building something that could take the material thru its elastic range into deformation, to failure, is way beyond my current ability.

If wood is part of your design, them I would think set taking would be where your your concerns for deformation would be prudent.  A bow design that pushes carbon to stresses outside it's elastic range, or even plain glass, for that matter, would be a radical departure from most bow designs.

[Flem]

It's not about design, I just do one. It's curiosity about how this hypothetical composite will perform and compare to a published, but unavailable product. I won't be pushing past the elastic range, but I would still like to have a visual idea what it looks like.

[williwaw]

I guess I do not understand what kind of improvement you hope to bring to your design.  You only work with one?  which is it, btw?