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Old Dec 02, 2010, 09:51 PM
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Resin/Fiber Ratio Math

Can someone double check my math?

Since Resin/Fiber ratios are done in terms of volume, not weight, a little math is in order. I've always calculated it with the math below, but I just want to make sure that I'm doing it correctly.

Example

I have a completed composite structure that contains only fiber and resin. The total weight of the part is 20 grams. The weight of the E-glass in the structure is 14 grams. This means that the remaining weight is resin=6grams.

The specific gravity of E-glass is 2.55 grams/cc
The specific gravity of the Resin is 1.10 grams/cc

To calculate the volume of each i divide the weight (grams) by the specific gravity.

E-glass is 14 grams/2.55 = 5.49 cc
Resin is 6 grams/1.10 = 5.45 cc

Total volume is 10.94 cc.

Resin volume 5.45cc/by the total volume 10.94cc = 49.81% resin

Is this correct?

Adam
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Old Dec 02, 2010, 10:00 PM
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yes, the math looks perfect to me.
Are you sure e-glass is that much heavier than resin? A cc of glass weighs more than twice a cc of resin?
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Old Dec 02, 2010, 10:11 PM
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Originally Posted by dmcquinn View Post
Are you sure e-glass is that much heavier than resin? A cc of glass weighs more than twice a cc of resin?
Well according to the internet it is.

Specific Gravity - E-Glass
Specific Gravity - E-Glass
Specific Gravity - E-Glass

Most resins seem to range from 1.06 to 1.20 g/cc
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Old Dec 02, 2010, 11:25 PM
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Quote:
Originally Posted by wyowindworks View Post
Can someone double check my math?...
The specific gravity of the Resin is 1.10 grams/cc
Adam
Looks right to me. A minor point, but assume you've already considered that netting out 'resin' technically means 'resin + hardener' mix at their respective densities. Typically they dont vary much, but for ex:
resin = 1.18 g/cc
hardener = 1.04 g/cc
ratio = 40 PBW
resin 'mix' density = [(1*1.18) + (0.4*1.04)] / 1.4 = 1.14 g/cc
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Old Dec 02, 2010, 11:47 PM
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Originally Posted by ptxman View Post
Looks right to me. A minor point, but assume you've already considered that netting out 'resin' technically means 'resin + hardener' mix at their respective densities. Typically they dont vary much, but for ex:
resin = 1.18 g/cc
hardener = 1.04 g/cc
ratio = 40 PBW
resin 'mix' density = [(1*1.18) + (0.4*1.04)] / 1.4 = 1.14 g/cc
Yep. Wouldn't you divide by 1.volume mix ratio rather than divide by 1.mix ratio by weight? See bold italic in your qoute.

Adam
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Old Dec 03, 2010, 12:10 AM
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Adam,

I have always used (and seen used) the weight ratio for fiber to resin. When specifying prepreg it is as well done in weight ratio.

The accepted "standards" are:

50/50 for glass and/or Kevlar to resin in wet layup: this starts to be on the resin rich side and more adapted/acceptable for mold making.

60/40 for glass and/or kevlar to resin is what one should aim at in a wet layup.

For carbon to resin the starting point is at 60/40.

Prepreg can go down to 70/30 (glass/Kevlar and carbon) and I have been involved with one manufacturing where a carbon prepreg had a resin content significantly below 30% (in weight). Now the tricky part: the ratios might be different if using a woven fabric or a UD tape (less resin per weight for UD tape).

Usually, when establishing a production, you start with the above values and refine the amount of resin to be mixed after the first couple of pull as the resin content is as well dependent of the type material used and the complexity of the molding.

You can do an experiment: put a piece of fabric between two perfectly flat and smooth surfaces (glass) and make sure that the fabric is touching both sides without be squashed. Measure how much resin is needed to fill up all the voids (no air bubbles). You will have the optimal fabric to resin ratio for this particular material.

In your case you are using 14 grams of glass fabric (I assume it is fabric): wet layup good starting weight ratio 60/40 gives 5.6 grams of resin. In this case, you might be on the "rich" side. But the finish and complexity of the part might require this. When it becomes more critical is when you work on large(r) layups. If you have 1400 grams of material you will require 560 grams of resin. With your method you would be 40 grams heavier. If you use it as a starting point and refine the process after it is all right. But if you stick to this "rule" you might end up with overweight structures.
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Old Dec 03, 2010, 12:35 AM
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Thanks, fnev. I feel much better about my parts now. I was really getting worked up when I was calculating my stuff by volume. The kevlar calculations looked alright but my glass ones looked ugly.

My figures in post #1 were only an example. If I look at it by weight, my last kevlar/glass part was 73/27 and my last glass part was 69/31. Neither of the parts contained any sploog, but they were painted in the mold. If I take paint (which increased the final weight of the part) into consideration the resin content is even lower. Is the resin content of my parts too low? Maybe the pressure in my bladders is too high?

I've been using a really tight and thin H8 weave and it has reduced my resin content but the hoop deflection modulus of the fuse seems lower.

Adam
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Old Dec 03, 2010, 12:59 AM
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You are welcome and I am glad I can help somehow.

Ideally the least resin used the better as long as ALL the voids are filled otherwise the fibre won't work properly in the matrix. If the end product is not satisfactory for the loads no increase in resin content will improve its strength/stiffens. It might "feel" stiffer but it is NOT stronger. The layup schedule has to be modified or (at worst) the structure redesigned. This is where the different types of treads, weaving become handy as they have an influence on the resin content and the way the fibers work under load.
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Old Dec 03, 2010, 01:08 AM
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Originally Posted by fnev View Post
IIt might "feel" stiffer but it is NOT stronger.
That has been my experience. I've found that the low resin content parts feel a little softer when they are squeezed (hoop modulus), but in failure tests they perform very well and seem a bit more resilient with fewer visible stress zones when being heavily deflected. Do you think that the resin content plays a part in the flexural modulus of a tail boom? How about buckling resistance when under compression?


Adam
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Old Dec 03, 2010, 01:13 AM
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Originally Posted by fnev View Post
IIt might "feel" stiffer but it is NOT stronger.
That has been my experience. I've found that the low resin content parts feel a little softer when they are squeezed (hoop modulus). In failure tests they perform very well and seem a bit more resilient with fewer visible stress zones when deflected. Do you think that the resin content plays a part in the flexural modulus of a tail boom? Buckling resistance when under compression?

I feel the need to do some more testing......after I tidy up some projects of course.

Adam
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Old Dec 03, 2010, 02:10 AM
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Compression is not the strongest characteristic of any composite laminates. The best fibre for compression is "S" glass with only some high modulus carbon being substantially stronger. Resin will help in this particular stress situation as it is the stabilizer (holder) of the fibers to prevent buckling. Unfortunately as soon as a slight deflection is combined with the compression everything collapses.

The solution for a tail boom is to increase the diameter while reducing the thickness of the layup. This would be an interesting test to make: different boom diameters combined with different layup. Just from gut feel I would bet money that you can get lighter booms than the existing ones used for DLGs if the diameter was increased (compare to what is used today). The aerodynamic penalty due to the increased wetted area should be negligible. Of course, the availability of adapted material (fabrics, tape, socks. etc) is a limiting factor.

For a wing layup "unbalanced" layup between top skin and bottom skins could be carefully considered. But the best way to take care of the compression loads in any composite structure is by thorough structural design in combination with the proper use of the adapted materials. Not always a simple task and often leads to complex structures...
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Old Dec 03, 2010, 03:04 AM
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Adam,

not completely on topic, but have you seen the R&G Handbook of Composite Materials?

It is 238 pages full of data, in both German and English, and available as a free download from http://www.r-g.de/en

Regards,

Martin.

P.S. How are the videos coming on?
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Old Dec 03, 2010, 05:02 AM
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heli_bee thanks for the link: this is a very good, practical and pragmatic document. It should answer 99% of all the questions related to composite work within the model community.
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Old Dec 03, 2010, 08:20 AM
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In that case, perhaps it might be useful to make the link into a sticky!!
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Old Dec 03, 2010, 10:49 AM
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Compression is the weakest property of a structural laminate. Carbon, for example, performs brilliantly in tension but is prone to material failure when subjected to compression.

Hoop strength will depend on the number of crosswise filaments that are dissipating the "squeeze" force. In this manner, the ideal orientation for hoop strength is to have bias fibers and hoop fibers.

Adam, you say you are able to see fewer stress zones when doing hoop modulus testing on lower-resin content parts. This is because loads are being transferred more efficiently without forming any "hotspots" or concentration zones. Load concentration often happens at areas with high resin content and low fiber density.

Adam, what 8-Harness cloth are you using? I'd like to try out a stiff, tightly woven fabric for wet seaming, as the looser 4-oz and 2.3-oz cloth I have on hand makes seaming quite difficult.

-Alex
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