CF tubeing ...info overload

Discussion in 'Composites' started by FritzW, May 19, 2019.

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  1. May 19, 2019 #1

    FritzW

    FritzW

    FritzW

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    I need to source CF tube spars for an ultralight(ish) Volksplane style stab and rudder and I'm running into information overload on the different types of tubing (roll wrapped twill, roll wrapped uni, braided, etc.).

    Reading the descriptions on the interweb they all say "ideal where bending and torsional strength is required". Obviously there's a best choice, especially considering all the considerations I haven't considered yet (glueability, machineability, shock resistance, etc.)

    The tubes will have fairly concentrated bending loads and distributed torsion loads. If anyone has actual knowledge of this stuff I'd welcome your 2 cents.
     
  2. May 19, 2019 #2

    Victor Bravo

    Victor Bravo

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    Composite Structures Technology in Tehachapi, CA is a very knowledgeable source. They've been around for a lot longer than most of the internet sellers.
     
  3. May 19, 2019 #3

    wsimpso1

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    Way outside the scope of a forum to teach this... You are asking how to select beams in composites. Read my sticky on how beams work, then my sticky on composites. Now both of those stickies were intended to help us understand what is going on without math. To actually pick a tube that you know is strong enough without being excess takes math. Composites mechanics is a senior/ graduate course in ME/AE.

    In beams we have the fiber in the caps aligned the long way, while the fibers in the web is at +/- 45. In tubes, the wall is the same all of the way around, and the fiber orientation is also the same all of the way around. The primary thing to be concerned about is bending strength of each of the tubes being offered.

    So you have two tasks before you can select suitable tubes.

    First is to figure out how much bending moment you have at the worst place on each spar. THe old version of CFR 23 had simplified methods for figuring out the max inflight loads on the tail surfaces. Then to the Mechanics of Materials or AE structures book to figure out the max bending loads within your flight envelope.

    Second task is to obtain the bending strength of the candidate tubes. You can learn all of the composite mechanics or maybe you can get the supplier to tell you the bending strength of each.

    In aero structures, we use FOS of 2, which means the strength needs to be twice the max bending moment. Take the lightest one that is at minimum 2 time the moment.

    A few things about composite tubes.

    Composite fibers are strongest along their length. Start winding the fibers at an angle, and strength goes down.

    Tubes must have the fibers at wrapped at alternating positive and negative angles, or they can be fragile. Woven cloth wrapped on the bias is at 45 degrees and a lot lower strength than stuff at zero degrees. If the tube is braided, but the fibers are still at 45 degrees to the long axis, the material is about the same. 45 is perfect if the tubes carry only torsion. If you have mostly bending and shear, with little torsion, and you can get braided tube at smaller angles to the long axis, you might be able to save some weight. A tube of the same diameter and wall thickness with smaller angles will be stronger than one wound at 45.

    All of this is nice if you are designing tubes for a task, but I got the impression you were selecting tubes out a catalog. Then you pick the lightest one that does the job.

    Billski
     
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  4. May 19, 2019 #4

    FritzW

    FritzW

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    I'm not asking how to design a spar or figure out what size tubing will be strong enough. I'm asking about "glueability, machineability, shock resistance, etc." for the different styles of CF tubes that are available. Like I said: "all the considerations I haven't considered".

    There's a lot more to selecting the tube than just strength. I've got the strength part handled.
     
  5. May 19, 2019 #5

    wsimpso1

    wsimpso1

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    FritzW,

    It is in your best interests to not get all snippy with someone who read your words and tried to answer about the topics specifically asked about...

    Now that we know what you are interested in, someone knowledgeable who has not recently been told "You did not answer my question!" might respond.

    Billski
     
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  6. May 20, 2019 #6

    FritzW

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    Your right, my bad. I only read the first paragraph. I know the loads I need to design for and how to pick a tube "by the numbers" that will handle the loads. That's the easy part.

    What I'm trying to do is find all the little "gotch-ya's" that noobs don't know about and aren't in any stickies. ie. don't load bolts parallel to the grain in unidirectional tubes (what if it's in a bias sandwich?), don't glue a bias sleeve to a uni tube because of different CTE's (is that really an issue?), don't glue aluminum to CF (is that a real problem or a theoretical "best practice"?, what if it's anodized?) etc...

    It's not the obvious stuff that I'm worried about, it's the little things I didn't think about (and/or didn't know were an issue).
     
  7. May 20, 2019 #7

    pictsidhe

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    There are thousands of gotchas with composites. You may do better by telling us how you plan to use the carbon tubes. Then let the hexperts (I have it on good authority that witchcraft is needed for composite design) pour scorn on it, before suggesting something that will work.
    Yes, aluminium in contact with carbon is a big no-no. Seperate them with glass and you are good. Once you have a reliable method of bonding aluminium, anyway...
     
  8. May 20, 2019 #8

    proppastie

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    Well if you know the size you need tell us how much weight you will save vs what you do know how to build, (wood, aluminum ?).....Do not forget the FOS of 2 for plastic, vs 1.5 for say Aluminum or steel....You may decide it is not worth the trouble.....I guess otherwise buy the plans or construction manual for a plastic airplane and copy their techniques. .....Go to BoKu instruction get-togethers.....NM must get boring.
     
    Last edited: May 20, 2019
  9. May 20, 2019 #9

    Victor Bravo

    Victor Bravo

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    I second the motion for asking these questions directly to Bob K. While he may not have a master's degree in engineering, or any of the resume credentials that Billski has, Bob has a LOT of experience bonding, laminating, sanding, prepping, selecting, and "dealing with" this type of material in a homebuilding environment. Not that Billski doesn't, but I know Bob does.
     
  10. May 20, 2019 #10

    wsimpso1

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    We are good here.

    BoKu has way more practical experience in graphite.

    UNI tubes as beams tend to be fragile. Usually they are pultruded and tend to be heavy.

    Once you start putting fibers at an angle (wether wing alternately positive then negative angle, braided tubes, or woven cloth rolled on mandrels), both strength and stiffness are lower, but they are sturdier. Exactly how you get the fibers on is less important than what angles you have...

    Graphite fiber has this odd characteristic of negative thermal expansion coefficient in the direction of the fibers, but is conventional across the fibers. The result is that once you build something of graphite with angles varying, temperature difference between when it was assembled and current temperature stresses the parts before you even apply your flight loads. It is real, but usually not huge. If you have huge temp differences, it can add up. Bond an aluminum plug inside a circumferentially wound graphite fiber tube, and it might matter...

    More later,

    Billski
     
  11. May 20, 2019 #11

    proppastie

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    once asked Marski about glass transition temperature of his rods he sells......specifically sitting out on the ramp in AZ in summer would the wing droop, ....he said yes.....
     
  12. May 20, 2019 #12

    BoKu

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    Unfortunately, re-fabricated carbon tubes are outside my realm of practical knowledge. The best approach is probably to just get some samples and try them out to see what works. As you do so, be aware that the weight you save will have a pretty high cost both in the materials and in the time you spend developing and validating their application.

    As for glueability, my guess is that pretty much all commercially-available CF tubes will develop about the same bondline strength. For bonding, you'll probably end up doing the usual carbon fiber prep: Sand down to (but not into) the knuckle tops with coarse sandpaper, degrease with acetone, wet both mating surfaces with your bonding agent, and clamp together for good squeeze out. You can make your own bonding paste using laminating resins plus the usual fillers, but for critical applications you're probably better off using an engineered mix like Hysol EA9430 (pricey) or EA9360 (pricier). I try to design bond areas so that the epoxy doesn't see more than about 2k psi in shear at ultimate load. Most of these high-strength mixes go to 4k psi, but that's best case on lab-prepared specimens and is not always achievable under workshop conditions.

    --Bob K.
     
  13. May 20, 2019 #13

    flyboy2160

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    To try to answer your original question:
    'Twill' is a woven material, like the cloth in your shirt. The fibers in any layer cross over and under each other in both directions. 'Roll wrapped' twill is a tube that is made by wrapping layers of twill around a mandrel like you roll a home-made ciggy from a fixed width strip. It has starting and ending edges. The layers may or may not be aligned with the tube axis. It's the cheapest. You can use prepreg. You can cure it by simply wrapping it in a bag, but you have to know the 'art' of this to keep from wrinkling it.

    'Uni' is a material that doesn't have the fibers crossing over and under each other. A stack of unis is like making layers of chopsticks. All the chopsticks in any layer are parallel to each other. 'Roll wrapping' is the same as roll wrapping twill. Uni layers typically are NOT parallel, i.e. you have some closer and some farther from being aligned with the long axis of the tube. Changing the alignment gives different bending and torsion characteristics. It's more expensive than twill. The same curing 'art' applies, to an even greater degree.

    'Filament winding' is wrapping uni material around and around the mandrel like you load thread on to a sewing bobbin instead of starting with a fixed-width strip. Some vendors offer this at a reasonable price.

    'Braided' is a 3D weave in which all the fibers cross over and under each other, just like a Chinese finger trap. It too can have different alignment of the weave to give different bending and torsion characteristics. It doesn't have seams. It's the most expensive.

    Uni is the most difficult to prep. If you abrade the surface just a little too far, you have fibers popping out of the matrix. Woven and braided are more forgiving.

    You're much better off using media blasting instead of 'hand' sanding for bonding prep.
     
    Last edited: May 20, 2019
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  14. May 20, 2019 #14

    flyboy2160

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    You seem to have already decided on carbon tubes, but allow me to relate a story from work about carbon tubes vs. steel tubes.

    I had to design a structure that just cried out to be a truss. It had a tight weight goal, numerous hard point requirements, a very severe cost requirement, very severe frequency requirements, and a small design budget.

    I looked at CF tubes, including going through what you're doing with types and vendors. I rejected CF tubes and went with welded 4130. The CF tube costs were ok, but the labor involved in joining the tubes and in fabbing the hard points was outrageous, as was the cost of certifying the design up front and in production. The hard points and joints were also a nightmare to analyze.

    A welded, very thin wall 4130 truss was the clear winner. It was similar in size to a typical aerobatic welded fuselage (the fuselage vendors gave the best prices!) It met the weight and cost goals. The analysis was also much, much easier. It was easy and fast to tweak the tube ods and walls to get the desired frequency response.

    So don't give up on steel…….or aluminum (from a former composites weenie!)
     
    Last edited: May 21, 2019
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  15. May 20, 2019 #15

    pictsidhe

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    Flyboy. I briefly considered a composite centre section for my project, but rejected it for the same reasons. it would be a PITA to design and build in composite with the numerous joints and hardpoints. I'm going with aluminium and gussets instead, though steel would have worked too.
     
  16. May 22, 2019 #16

    wsimpso1

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    All graphite laminates using epoxy or vinylester resins will sand, saw, or otherwise machine easily. Taping the area to be sawn will make for nicer edges. Pultruded tubes tend to be thick (and heavy), but also fragile, as there are no crossing plies. Once you get into wound tubes, whether it is wound of cloth or braided tubes or braided on a mandrel, the crossing plies make it sturdier. Here is why. When ALL of the fibers are parallel, you only the have the strength of the resin involved, and the resin is on the order of 7-11 kpsi strong. Tensile strength accross the fibers (at 90 degrees to the fibers) is thus just the resin strength. When you get cross plies, the fibers start reinforcing each other...

    Concentrated bending loads were mentioned in the OP. I am curious as to how you get those, but how you deal with point loads is the whole deal of making hardpoints in composites. Hard points get kind of big in composites and should be minimized. They take extra plies in the vicinity, same as with sheet metal of wood. If it is a bolted joint, the usual path is a bushing that will stand the bolt loads on the ID and other faces that pickup bolt loads. Steel and garolite seem to be preferred. Aluminum is not so good as most of our bonding methods will eventually come loose. Use a snug fit and then bond the bushing into the composite. Some placesa a pair of sandwich plates are also used. The ID of the bushing and thickness of the interface between bushing and composite should handle the suite of loads with FOS of at least 4. Then you go out in radius from the concentrated load - as stress gets low enough, you can drop a ply, keep going until you either run down to base part thickness or run into adjacent hard points.

    Graphite tubes are usually easily bonded using good technique - use adhesives intended for the resin used in the tubes. Composite material bonding requires fine sanding - 200 grit is usually considered optimal. Sanding must immediately preceed bonding - 90 minutes is considered max delay, re-sand with 200 grit if bonding is delayed. Surfaces should be cleaned of sanding dust and all oils, preferably with a wet solvent wipe followed by an immediate dry wipe to pick up the solvent and contaminants. Some experts recommend reagent grade acetone for this as hardware store acetone usually has a touch of oil in it. If in doubt as to how clean your acetone or other solvent is, wipe it on a mirror and as the acetone evaporates, look for colors from a very thin layer of residual oil on the surface.

    Shock resistance? Anything can be broken, just hit it hard enough... Pultruded tubes will split rather easily with greenstick style fractures. Everything else is tougher to unintended loads, but overload anything and it will fail. Most composite structures with cross-plies or a layer of woven cloth on the outside will tend to require big energy for failure. Design it strong enough and it can be awesome indeed. F1 racers are carbon tubs, noses, seat structures, belt attachments, etc. They allow for some spectacular crashes with the drivers getting out under their own power after everything comes to a stop - do not believe that carbon means shattered stuff everywhere and no protection...

    Billski
     
    Last edited: May 22, 2019
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  17. May 22, 2019 #17

    proppastie

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    Does it corrode...... why?
     
  18. May 22, 2019 #18

    wsimpso1

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    Corrosion? Epoxy is really resistant to anything resembling corrosion. It is one of those preferred materials for use in medical implants - the body is a really hostile environment for just about everything...

    Got this at an Oshkosh forum - sanding breaks some of the cross-linking of resin molecules right at the surface. If you cover it with fresh resin immediately, you get some level of chemical bonding and a lot more strength than you get by delaying, using coarse sandpapers, etc. Sand and wait until tomorrow, the broken molecules find new connections, and you are left with the mechanical bond. Testing has demonstrated that 200 grit is as good as it gets, and strength drops rapidly if assembled with delays.

    Billski
     
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  19. May 22, 2019 #19

    BJC

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    Bill:

    Does vinyl ester exhibit the same behavior as epoxy?


    BJC
     
  20. May 22, 2019 #20

    wsimpso1

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    Byron,

    I do not know. I suppose we could look up the preferred repair methods for parts made with vinylester resin...

    Bill
     

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