How Stiff is Carbon Fibre?

[rsterne]

I've been using Carbon Fibre tubing for many years for R/C Model Yacht masts, and parts for R/C Sailplanes, and also "pull-truded" (extruded) shapes of various cross sections for things like spar caps in the wings of model Sailplanes, plus I have made fin keels for the R/C yachts from laminate CF.... so I have a pretty good working knowledge of the material.... Using it for sleeves to stiffen barrels, or as a material for shrouds, makes sense because of it's high stiffness-to-weight ratio.... What is stiffness?.... Well basically it is the cross sectional area times the Tensile (Young's) Modulas.... so for a given cross section, the higher the Young's Modulas the stiffer the part.... Here are some examples of Young's Modulas....

Oak (along the grain): 1.6 x 10^6 .... (10^6 is a million, so that is 1,600,000)
Fibreglass Rod: 2.5 x 10^6
Concrete: 4.3 x 10^6
Woven CF (50% fabric) 4.3 to 7.3 x 10^6
Aluminum: 10 x 10^6
Kevlar strands: 10 to 16 x 10^6
Brass: 14 to 18 x 10^6
Titanium: 16 x 10^6
Unidirectional CF (70% fabric) 26 x 10^6
Steel: 29 x 10^6
Berylium: 41 x 10^6
Tungsten: 59 x 10^6
Tungsten Carbide: 65 to 94 x 10^6
Diamond: 152 to 175 x 10^6

Since Carbon Fibre in its raw state is a fine filament, and many of them are used to make up a strand (3,000 is typical, making a 3K "tow"), it is either woven into a bid-directional fabric, or oriented with all the strands in the same direction (called unidirectional, or "uni"), and the strands are glued together in a "composite" matrix, typically by an epoxy resin.... In the list above, you can see CF appearing twice, once as a woven fabric, where 50% of the composite is CF and 50% is epoxy, and the other as a "uni" product, with all the CF strands in the direction of the load, and 70% of the composite being CF (it packs tighter because it isn't woven) and only 30% being resin.... The former is less stiff than aluminum, and the latter is nearly as stiff as steel....

The tubes we often get to use for our airguns are of two types, "uni", which should have a Young's Modulas of about 26 x 10^6, or a tube with a "twill" weave on the outside for appearance and to improve the crush (hoop) strength, but underneath most of those the fibres are "uni", because they are cheaper and give greater resistance to bending, which is the great thing about CF tubing.... Many of the CF tubes available commercially are advertised as "1.4 times as stiff as aluminum", which would indicate an overall Young's Modulas for the tube of 14 x 10^6.... Therefore, it is about half as stiff as steel of the same cross-section....

Have you seen references to "High-Modulas" Carbon Fibre tubes?.... It is exactly what it sounds like, a stiffer tube for the same cross section.... Typical CF strands, in their bare state, actually have a Tensile (Young's) Modulas of 33 x 10^6, and it is known as 33M (for million).... Also available are an "Intermediate Modulas" CF which is 42M (called "IM"), and a "High-Modulas" CF which is typically 55M.... That means it is 60% stiffer than regular CF, so a well constructed tube, made mostly of 55M CF "uni" with just one "twill" layer on the outside should be stiffer than the same cross section of steel tubing.... and a "uni" High-Modulas tube should be quite a bit stiffer than steel....

I hope that gives you a rough idea of what Carbon Fibre tubing can and can't do, and the wide variety of types available.... In summary.... "uni" is stiffer than "woven" or "twill".... and "high-modulas" can be either 27% (for IM) or 60% stiffer than "regular" carbon fibre....

Bob

[PakProtector]

Bob, who sells this 42M and 55M tubing? The data from Clearwater shows their high modulus at 30M.
cheers,
Douglas

[sixshootertexan]

I didn't look through the size chart but they 57 and 66.

http://www.rockwestcomposites.com/browse/round-carbon-fiber-tubing/hi-mod-fabric-carbon-tubing

[sixshootertexan]

This is neat website.
http://www.cstexpress.com/carbon-fibre-tubes-c-23.html

[rsterne]

I notice that once you exceed 42M tensile, you trade off strength for stiffness, which is not an issue for us.... The 66M is only used for filament wound tubing, such as for driveshafts, designed to be stiff in torque, which is not what we need.... However, look at the prices for a 1/2" ID x 5/8" OD tube that is 72" long.... the 57M tubes are $117 each.... The same size tube in 35M fibre is $62 for woven and $56 for uni....

Bob

[Bill G]

I have to ask.  What kind of fit is necessary?  Just plain slip amd twist it on with epoxy filling the voids as you advance the liner into final position?

Bill

[rsterne]

Yep, just a slip fit.... I actually plan on using Loctite 638 to glue on my liner, it is thinner than most expoxies, and designed for slip fits (retaining bearings in worn housings).... I heard from Sean Pero that on composites a post cure in a 175* oven for 3 hours is a good idea.... I plan on doing some test pieces and testing to destruction prior to doing a barrel.... starting with a room temperature cure for 24 hours as you would for steel to steel.... If it hasn't cured properly, then some heat as recommended by Sean....

Bob

[vinenator]

a friend sent me this link to a company selling 3d-printers that can apply carbon filaments in conjunction with typical other materials.

looks promising.

https://markforged.com/