# Question for BoKu

Discussion in 'Composites' started by RSD, Oct 14, 2019.

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1. Oct 14, 2019

### RSD

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Hi BoKu,

Did any of these fabrics look like the one you mentioned that you are using in your glider cockpits?

A

B

C

2. Oct 14, 2019

### BoKu

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I don't remember if I've used any of those particular weaves, but they all look similar to what I've use. In general, the weave of a cloth has a relatively small effect on its strength or stiffness. The big driver there is mass (weight per unit area). I think what we've generally used is about 6oz/yd^2, maybe an ounce or so more or less. We use a ply on the inside surface of the fuselage shell in the cockpit, and also on the expose surfaces of the cockpit longerons and canopy rail closeouts.

We've lately started using a clear gelcoat product from Fiberglass Supply on some of the "aesthetic carbon" parts we make for the racing jets. Going forward, we will probably start using it on the molded cockpit parts so that the hybrid fibers aren't right there on the surface where any bit of sanding turns them into hateful fur patches.

Thanks, Bob K.

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3. Oct 15, 2019

### RSD

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Cheers Bob - very informative.

4. Oct 15, 2019

### BoKu

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Correcting myself here: That's generally true for weaves with the same mass of fiber on both the warp and weft. But where there is a substantial difference between the two (for example in a unidirectional like 7715 fiberglass), the strength and stiffness will be much greater in one direction than the other. I've been bitten hard by this with fabrics that look uniform to the casual glance, but actually have like 80/20 fiber distribution when examined closely. Since glass and epoxy have slightly different thermal coefficients, that's a good way of unintentionally making large thermometers.

--Bob K.

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5. Oct 16, 2019

### RSD

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Also very informative - many thanks Bob.

FYI I have just stumbled across an interesting 3:1 CF/K - the one in the bottom right of the picture -

6. Oct 16, 2019

### BoKu

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Be careful with that one and the one right above it; request a sample before you commit to using a lot of it. It looks like the kevlar fibers are only in one direction. If so, whatever good it does after the carbon starts breaking, it'll only do in one direction.

--Bob K.

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7. Oct 16, 2019

### RSD

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I will ask them to send me a better photo - I tried zooming in but it was too pixellated to see anything. Certainly the one above as you pointed out only has the kevlar going in one direction.

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8. Oct 16, 2019

### RSD

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Better photo they sent me - I think the kevlar goes in both directions

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9. Oct 16, 2019

### wktaylor

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RSD... what are the weave design #s for these various fabrics?

Caution... Kevlar has a known tendency for moisture absorption along the fibers... which can cause the composite to swell and affect the epoxy [matrix] strength... not to mention gain weight.

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10. Oct 16, 2019

### Lendo

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RSD,
Is that RED in the Red and Black photo also Kevlar?
George

11. Oct 17, 2019

### RSD

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I didn't ask for the weave designs for the other fabrics unfortunately but I can find out for you.

Many thanks for the heads up on that - wasn't aware of that! Will try to put some measures in place to stop that being a problem hopefully!

12. Oct 17, 2019

### RSD

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Hi George,

I believe so - as far as I am aware all of the photos are of various carbon fibre/kevlar fabrics that they have, and they do seem very reputable to deal with.

For those that are interested in prices - US$11 per metre for the carbon fibre/kevlar hybrid, and US$11.30 for 3k twill carbon fibre, postage works out to about $2 per metre - delivery takes about 2 weeks - unless you live in Canada apparently. 13. Oct 17, 2019 ### Vigilant1 ### Vigilant1 #### Well-Known MemberLifetime Supporter Joined: Jan 24, 2011 Messages: 4,069 Likes Received: 1,805 Location: US It might be worth looking for a weave with some of the UHMWPP and UHMWPE fibers (Innegra, Dyneema, Spectra, etc). They have properties similar to (or better than) Kevlar/aramids, but (as I understand it) without the same water/moisture issues. Some of these are now used in ballistic body armor in preference to Kevlar. It doesn't bond as well to epoxies as CF and fiberglass do, but if they are incorporated into a weave with the other fabrics, maybe things will hold together pretty well. BoKu likes this. 14. Oct 17, 2019 ### Hephaestus ### Hephaestus #### Well-Known Member Joined: Jun 25, 2014 Messages: 1,184 Likes Received: 258 Location: YMM The colored weaves every time I've looked have been kevlar. There's a super sharp looking blue one And yes Canadian shipping sucks donkey doo 15. Oct 17, 2019 ### RSD ### RSD #### Well-Known Member Joined: May 20, 2019 Messages: 209 Likes Received: 42 LMAO!! Did you get my message? 16. Oct 17, 2019 ### Hephaestus ### Hephaestus #### Well-Known Member Joined: Jun 25, 2014 Messages: 1,184 Likes Received: 258 Location: YMM Missed it amongst others, no I didn't pay 2$/m shipping. It was flat rate to about 30lbs. Don't know what Canadian/us/Aussie $is these days. I think they hit me about 35$ up to 10kg.

17. Oct 17, 2019

### RSD

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They didn't calculate it per metre, but that's just what it worked out to when I was seeing what the all in costs were per metre. $35 for 10 kilos might explain the delivery time - its on that slow boat from China! Australian$ has fallen a bit lately due to suffering from the fallout of the various trade wars, currently one Canadian buys 1.1 Aussie.

18. Oct 17, 2019

### Vigilant1

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19. Oct 17, 2019

### RSD

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20. Oct 26, 2019

### RSD

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Just a quick update - my carbon fibre cloth and carbon fibre / kevlar hybrid cloth arrived from China yesterday so shipping took exactly 7 days. Haven't opened the package yet to see what it is like but I'm confident that it will be good quality.