FYI: Carbon Fiber tubes - how they are constructed - Enjoy ;)

[RotoTwit]

I simply wanted to know and then thought I would share.

Basic Fact #1: Think of CF as a rope, it is ONLY strong in 1 direction, tension, exactly like a rope! Push (compression) a rope - weak, pull (tension) a rope - strong! CF in shear (across) not so strong, CF is brittle & does not like to bend, it shatters much like glass/ceramics.

Note: Metals are the same strength in all directions of tension (not necessarily compression but better than CF), unlike CF which is ONLY strong in only 1 uniform direction.

Basic Fact #2: The 3k, 6k, 12k, etc refers to the # of 'thousands' of filaments (around 7 microns) woven forming a strand of CF in the cloth/fabric, loose CF it seems a bit harder to quantify this aspect, but would imply the 'groups/chunks/threads' of fibers together of similar count - I believe.
(cool pics of CF fabric - http://www.fibreglast.com/category/carbon_fiber_all)

Basic Fact #3: Air/bubbles/globs of epoxy/etc, ie VOIDS weaken CF and must be avoided/eliminated. Many CF objects (tubes-maybe, but certainly complex shaped CF things) use vacuum sealing inside of a 'bag' where a strong vacuum attempts to eliminate 'voids' by pressing out these voids.

Basic Fact #4: CF must be heated to remove ALL but the carbon in the fibers/epoxy/material to get greatest strength (epoxy is weak).

Basic Fact #5: CF MUST be constructed to behave (show strength) as desired, meaning, construction creates the strong direction(s) unlike metals which are uniform in strength.

Basic Fact #6: There are 3 major categories of CF (how the actual CF filaments are made), 1) High Modulus Carbon Fiber (Aerospace Grade), 2) Intermediate Modulus Carbon Fiber, 3) Standard Modulus Carbon Fiber (Commercial Grade). It is the later that copters use and most anything we see as 'normal' ppl, and the price is in the same order obviously.


Carbon Fiber Prepreg - High Performance and Quality (but cost effective)

The majority of the carbon fiber tubes produced are made using multiple layers of unidirectional (UD) carbon fibers (loose not woven fabric) in an epoxy resin. This form of carbon fiber is called prepreg, where the carbon fibers (loose or fabric) are pre-impregnated (prepreg) with an epoxy resin.


UD carbon fiber prepreg is used because it ensures the best properties possible of carbon fiber tubing in terms of strength and stiffness, and it is also a cost effective form of carbon fiber. This provides the best value in carbon fiber tubing - bar none. Pre-preg helps with eliminating voids as well.


Carbon fiber fabric/cloth prepreg is also used extensively, but mainly on the exterior of the carbon fiber tubing for aesthetic and cosmetic reasons.
(My impression of the difference is that the non fabric is a slurry of aligned CF strands in epoxy, whereas the CF fabric is woven just like clothing cloth)



The above is likely how all the Chinese CF tubing is made, the base being NON fabric CF with a surface of CF fabric.

The above is likely NOT the strongest tubing but a cost compromise & generally good enough for most uses.

The strongest CF tubing may or may not have an 'inner' tube of some kind (strong plastic, alum, cro-moly, etc) ie - w/ CF wrapping, but would be much more completely made of wrappings of Carbon fiber 'fabric' not just Carbon fibers inpregnated material, with wrapped layers w/ different strand orientations at some angle but with CF strand emphasis along the axis of desired maximum stiffness/strength.

For complex shapes, proprietary software designs the layers, shapes & sizes of CF fabric (or CF non fabric base) to give the desired stiffness, elasticity & directions of strength. Very complex stuff - ask a Formula One chassis designer or the quite elastic front wing designers, seriously !

Tubes are simple ! Or are they?

Sometimes absolute stiff is NOT what you want, some give is needed in the real world for many uses, but stiff is what we want for motor beams/structs/cantilevers !! Flexibility is what we want during a crash!! See the compromise issue!

This is OBVIOUSLY a cursory glance, NOT by a CF expert but it answered many of my questions, now how to compare to CF tubes to metal/alum motor beams/structs for strength/stiffness/weight/cost ??

Correct me where I am wrong !

Thanks!

 

[bensid54]

Thanks for the info it sounds to me like you know what you are talking about. It also seems to me that aluminum is a better choice if weight is not a factor and with that my dreams of carbon fibre construction are going to be modified. I also find that aluminum is easier to come by without the problem of being toxic.

 

[RotoTwit]

Thanks for the info it sounds to me like you know what you are talking about. It also seems to me that aluminum is a better choice if weight is not a factor and with that my dreams of carbon fibre construction are going to be modified. I also find that aluminum is easier to come by without the problem of being toxic.

There are square CF 'tubes' as well, they are pultruded 'tubes', meaning they are pulled thru expoxy/heat while being formed into a square - a bit pricey but cool and better than CF tubes probably !!

Any tube, especially metal (heavy) is strongest if its diameter (interior size - round or square) is large as practical, which allows thinner walls (lighter weight) for the same strength. IOW, the large interior tube/square can have thinner walls and thus be lighter. While a rectangular alum 'tube' can have more strength in the up/down (good for copters) while square is same strength all directions.

I suspect with a nice size alum sq/rect tube, thin wall, of high strength/weight formula (7071, 6060, etc) you could have about the same weight or lighter as CF w/o the toxic or inconvenience of purchase! Maybe cheaper too. BUT, I don't know this for a fact but suspect.

Good Luck!

 

[Electro 2]

I am unaware of carbon fiber products being "toxic". Getting a strand(s) stuck in your skin is certainly an issue, but I don't think this stuff is toxic. Also, weight is *always* an issue in any aircraft, full scale, or miniature. I've used Al tubes and they simply don't compare with CF. In the typical sizes and shapes used in multis, even 6061T6 simply isn't a rigid as CF. I suspect this is the reason all the leading frames on the market are CF. They've tried other materials and CF is the best choice for what we are doing here.

 

[Benjamin Kenobi]

What a great topic, thanks!

 

[15crewdawg]

CF is only "toxic" when cutting or sanding. The flakes or dust can cause health issues if inhaled/ingested.

 

[jes1111]

fwiw - the Health & Safety Police in UK list MDF dust as hazardous, but not CFRP dust or debris.

On the other hand - aluminium dust is highly toxic and highly explosive!

 

[gtranquilla]

RotoTwit and all,
If you want to know all there is to know about carbon fiber plate and tube etc. join the following Yahoogroups forum
a most valuable resource: http://groups.yahoo.com/neo/groups/CompositeRockets/files

I participated and contributed there as well and have made my own cf plate and larger 4 inch diameter tubes for the HPR activities.
The following can also be made available to anyone requesting it (see photo). If there is a way for me to attach this pdf here I would.
UD is the type of tube construction used for archery arrows and shatters into lengthwise strands and not good for MRs. The CS6 and CS8 use a plain or twill weave fabric in their composite construction which is much stronger.
The optimal plate construction configuration is transversely isotropic (commonly referred to as quasi-isotropic) meaning that it is equally stiff in all x/y directions. CF is weakest in the perpendicular direction which is why kevlar is sometimes mixed in to improve perpendicular, i.e., z direction strength.

Optimum ratio of cf to epoxy is 60/40 to achieve max strength to weight ratio. To squeeze out the excess epoxy one needs a 2 ton press or can get by with a vacuum bagging system which can achieve 14.7 lb/sq inch but only at sea level. Most will be luck to achieve a CF to epoxy ratio of 50/50 such that the extra epoxy is marginal parasitic weight. Like asbestos, CF is not toxic but guess what? There are concerns that it can become carcinogenic if the extremely fine and sharp fibers get trapped in the lungs.
Polyester resin must not be used with CF(strength is compromised). Polyester resin is most commonly used with fiberglass plate and G10 etc.View attachment 15435

 

[sledge57]

Another interesting idea for strength to weight would be light weight aluminum wrapped with CF or maybe even kevlar.

The company I works for uses an aluminum tube wrapped in CF in one of our machines.

One issue they found if you try this yourself, is if the tube had the CF wrapped around in one direction it would tend to unwrap or wrap it self tighter with changes in temp. They solved that by adding a second wrap in the opposite direction.

Might make zero difference to us as the amount of movement is very small but we make CMMs and the machine is accurate to about 1 micron (39 millionths to those unfamiliar with metric) and they could see it move.

However our MR's subject to much wider temp changes so it could very well pose an issue.

 

[jes1111]

One thing I've discovered about CF - if you want to see just how amazing this material can be then do one simple thing: pay more for it! The cheap, nasty crap that's sold through so many online hobby stores, particularly tubing, is just a waste of time. Unscrupulous vendors will sell you GF sheet/tube with a CF skin to make it look "cool". They'll sell you "CFRP" with 20% CF and 80% resin. They'll sell you production seconds (and even thirds) with air pockets, pin holes, delaminations and so on. Not all CF is born equal! Like everything else in life, the good stuff costs good money!

 

[PeterLester]

That is good information sledge57,

BTW - you can now purchase a kevlar/carbon fiber fabric weaved as plain or twill..... twill works best for complex curves and one or more MR manufacturers offer an Octocopter made from this material.
The kevlar adds weight but resolves the z- axis weakness inherent in cf composites should that even matter.

T6061-T3,4,5,6 aluminums are aircraft grade aluminum alloys intended to provide a better strength to weight ratio but will still flex easily. Layers of cf help to stiffen these tubes.
Unfortunately aluminum, although lighter than most metals still has a very high specific gravity relative to the cf composites.
The aluminum must be pre-treated in an acid bath to improve adhesion to epoxy/cf layers, i.e., reduce risk of delamination.
In the right application such as very large scale systems such as the Dreamliner, the combination of aluminum layer with carbon fiber layers proves to be superior in terms of construction costs and ability to withstand penetration since carbon fiber plate is most prone to damage in the z axis re: the NASA documentation. At smaller scales the benefits of aluminum taper off quickly since the specific gravity of aluminum is so much higher than cf. The aluminum also doubles as a form or mold to help reduce construction costs.
Another benefit of the cf/aluminum fuselage is slightly better tolerance to hail damage but only up to the point where the cf begins to fail.

Some RC heli booms also use the cf/aluminum method but that is almost entirely a cosmetic solution (chips off easily)! Currently the lightest hybrid plate material is beryllium/cf/epoxy which has been used in the aerospace industry but beryllium is a very dangerous material to work with. Within a couple of years we will be seeing CF plate and tubes impregnated with nanotubes to further increase the strength to weight ratio.

I always come to MRF looking for answers but here I go again getting carried away spouting off all this detail.

 

[RotoTwit]

I am unaware of carbon fiber products being "toxic". Getting a strand(s) stuck in your skin is certainly an issue, but I don't think this stuff is toxic. Also, weight is *always* an issue in any aircraft, full scale, or miniature. I've used Al tubes and they simply don't compare with CF. In the typical sizes and shapes used in multis, even 6061T6 simply isn't a rigid as CF. I suspect this is the reason all the leading frames on the market are CF. They've tried other materials and CF is the best choice for what we are doing here.

Thx,

Toxic is also defined as anything that can cause harm - pathologist would classify CF as toxic - especially in internal organs, eyes, etc. And remember it is NOT just CF but CF & epoxy - doubt epoxy good stuff either - could even be carcinogenic. Personally I don't like breathing anything but air!

I also was talking about sq or rect alum, not round, shape matters VERY much - shape allows maximum use of the properties of materials for a purpose!!

I have also asked for analysis results of CF tubes vs thin wall Alum tubes/sq/rect - and no responses. (my searches for real world results have had lousy results - biking has a long history of CF vs alum vs steel issues)

Comparison of properties:
http://www.carbonfibertubeshop.com/tube properties.html

So please, publish or point to some testing results of 'typical' Chinese CF copter tubes vs alum tubes/sq/rect of 'typical' formulas. - Thanks!!

Note: Looking at properties of materials helps, but actual real shaped products results is what I would like to see & is what is important, it could be surprising!

 

[RotoTwit]

Some interesting statements:
From: http://www.clearwatercomposites.com/faqs-carbon-fiber-tubing/

Is carbon fiber tubing stiffer than aluminum?
When designed for bending, carbon fiber tubing is typically around 1.5X stiffer than aluminum, assuming the same dimensions (diameter and wall thickness). This will though depend on the carbon fiber grade and laminate design. For example, high modulus carbon fiber can be designed to be over 3X stiffer than aluminum in bending.

Me- I always see it stated - same size. But Alum props. state that a LARGE interior/thin wall is the strongest/lightest config for strength/weight.
And also I would assert the real test as - same/similar cost, optimum shape for both materials - what are the results then !!! ????


Is carbon fiber tubing stronger than aluminum?
Here again, the answer is it depends. Generally speaking though, if the tube is to be used in bending, carbon fiber tubing will be stronger than aluminum, assuming the same dimensions (diameter and wall thickness).

Me- same size again. Same question - see above.

** Of course this is just a technical interest topic, but I do believe sq or rect shaped 'tubes' of ANY material are best for copters, less bracketing needed (weight), can still use interior for wiring, vs round tubes, I believe aero advantages are irrelevant at copter speeds as wind/breeze removes any aero advantage of round vs sq/rect - I suspect at least.

Another interesting bit:
http://www.clearwatercomposites.com/properties-of-carbon-fiber-/

 

[jes1111]

Don't know if you're gong to find any meaningful comparisons/tests. In general, a round tube will have greater torsional stiffness than a square/rectangular - the square/rectangular will have greater rigidity (irrespective of material). But there are so many variables, you can't characterise "CFRP tubing" by testing one type, or even a few.

IMO there's no real need to dig into the science on this, the choices are fairly straightforward: aluminium for good performance at low cost (but it bends), CFRP for lightness and rigidity at higher cost (but a tube crushes relatively easily). Even wood is a great material - light, resilient, absorbs vibration, cheap, etc.

If low weight and high rigidity are your priorities, then CF is going to be hard to beat for this application (outside of Boeing). Avoid simple pultruded tubing (square or round) - it has damn-all resistance to crushing. Many tubes sold are pultruded inner with sleeve or wrapped outer - a bit better. Prepreg form-wrapped tubing (like yacht masts) is expensive but very strong. Top of the tree would be filament wound - very expensive.

Just remember - if you crash hard, none of these materials are going to survive - so your main consideration then will be replacement cost

 

[Scott Drysdale]

The ability of a tube to resist crushing or rupture is referred to as the hoop strength. The need for a quasi-isotropic cf fabric layup is applicable to cf plate but becomes much less relevant when the structure is tubular. The epoxy is simply a two component plastic such that CFRP stands for carbon fiber reinforced plastic but you already know that I am sure....
Where flat plate material is not used for structural integrity such as for a base for only mounting the electronics the puncture or hole tear strength is of no concern such that sandwiched plate materials provide all the strength required. In my case I creates a cf plate with an aircraft grade inner plywood layer to help to minimize both cost and weight since CF is so expensive. Plate strength and stiffness is primarily determined by the bottom and top layers, the inner layers provide almost no benefit. Here is a photo of the sandwiched plate created using vacuum bagging View attachment 15437

 

[sledge57]

I don't want to take this to far off track but I have a question about CF sheets. I have a nice sheet of CF (2mm) I was going to use on my RC rock crawler but I think I'm going to use it to make a plate to mount electronics on.

My question is what's the best way to cut this stuff and still have a nice clean edge? I'm thinking scissors maybe?

 

[gtranquilla]

Carbon fiber will plate dull cutters, milling and router bits very quickly because the friction produces excess heat which destroys the tool temper.
One method to extend cutting tool life is to cool down the bits with "Super Cold" available from your local electronics store.
Or you can use a can of computer dust remover and hold it upside down when spraying the bit to get the release gas to cool off the bit faster.
Doing the cut under cold water works well and eliminate the dust issue but can create a mess and/or short circuit your electric tool since cf is conductive etc.
If the material is thin enough, try a utility razor knife or old pair of scissors that you are prepared to discard after use.

You will get a cleaner edge by simply sanding the edges with sandpaper glued to a straight piece of wood. It typically sands quite easily.
For your personal safety wear a dust mask and safety goggles and work in a well ventilated area.

 

[bensid54]

I don't want to take this to far off track but I have a question about CF sheets. I have a nice sheet of CF (2mm) I was going to use on my RC rock crawler but I think I'm going to use it to make a plate to mount electronics on.

My question is what's the best way to cut this stuff and still have a nice clean edge? I'm thinking scissors maybe?

Do you have a Dremil? A Dremil with thin cutting discs should do the trick as long as you have a vacumn removing the dust you produce while cutting.