Is carbon fiber / kevlar fabric of any benefit?

Discussion in 'Composites' started by RSD, Sep 20, 2019.

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  1. Sep 20, 2019 #1

    RSD

    RSD

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    Was just wondering if anyone can see any benefits to using the carbon fiber / kevlar hybrid fabric in the construction of a composite plane? I know that adding kevlar adds impact resistance but will it add sufficient impact resistance to make a difference to your survival if you crash? Kind of an open-ended question I know but was just wondering if anyone had seen any benefits or was using it?
     
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  2. Sep 20, 2019 #2

    John wadman

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    its beneficial if your plane is being shot at.
     
  3. Sep 20, 2019 #3

    spaschke

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    It would be benefitial as the outer layer on an amphibian hull or floats
     
  4. Sep 20, 2019 #4

    RSD

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    LMAO! Hopefully not! However bird strikes seems to be a popular topic here at the moment...
     
  5. Sep 20, 2019 #5

    RSD

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    Certainly a useful idea for those.
     
  6. Sep 20, 2019 #6

    FritzW

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    It might be worth putting a few layers on the boom under the prop arc of airplanes like the J1-B.

    a3fd0a8e.png

    Pure Kevlar (not CF/Kevlar) might be good for making things like custom fairleads.
     
  7. Sep 21, 2019 #7

    Rik-

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    In boats, we use a combination of the Kevlar and the CF. Pure CF will tend to crack under impact and pure Kevlar tends to be not rigid enough. So the Kevlar tends to compensate for the impact shocks when combined with the CF.
     
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  8. Sep 21, 2019 #8

    Voidhawk9

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    Yes, absolutely, if applied appropriately. 1:1 substitution of other materials is not appropriate.
     
  9. Sep 21, 2019 #9

    RSD

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    Totally agree with that!
     
  10. Sep 21, 2019 #10

    RSD

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    The hybrid cloth seems to be the way to go
     
  11. Sep 21, 2019 #11

    RSD

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    An interesting thought - would a layer of the carbon fiber kevlar hybrid fabric on the inside and outside of say a fuselage solve stop/contain some of the carbon fiber splintering action that occurs in an accident?
     
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  12. Sep 21, 2019 #12

    Voidhawk9

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    It would probably help to some degree, but if it was just a thin layer outside the carbon primary structure it would probably not be worth the weight and effort. If it was designed into the structure, it may work, if that was a necessary driving factor in the design.
     
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  13. Sep 25, 2019 #13

    TMann

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    Each type of fabric has it's strengths and weaknesses.
    Carbon Fiber is very stiff, lightweight and wonderful to work with but it's expensive, transmits vibration, blocks RF and breaks into shards when it reaches it's failure point. It also eats aluminum (a sort of galvanic reaction.)
    If you integrate some Innegra material with the Carbon Fiber, you will eliminate the tendency of the CF breaking into shards which eliminates a ton of concerns that I have when building something like a Long-EZ out of this material. There are some weaves already available that incorporate Innegra with the CF.

    Kevlar is very good in areas that may be subject to abrasion (wingtips), is sound deadening and pretty affordable. The downside is it's difficult to work with and will require a layer of glass if there is any chance you may hit it with sandpaper when finishing. Yes, floats, wheel pants, cowls etc. would be a good application.

    Perhaps a choice that is overlooked a lot is Basalt (sometimes called the poor man's carbon fiber.) It's comparable in weight and density to S-Glass (so stiffer and stronger than E-Glass.) It's RF transparency is another positive so you can still use your in-skin antenna. It is also heat resistant so while the epoxy may degrade, the basalt will still be there.
     
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  14. Sep 25, 2019 #14

    bmcj

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    For crash protection, MAYBE if it preserves the integrity of the cockpit enclosure against crushing and intrusion (by tree branches or broken metal), but I think a large percentage of injuries from a crash come not from cabin failure, but from sudden deceleration and (to a lesser degree) impact with the panel.
     
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  15. Sep 25, 2019 #15

    Vigilant1

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    I do think if a composite cockpit could be made to have high ductility (so that it deforms in a plastic fashion after yielding so the structure continues to absorb energy without total failure) that it might improve crashworthiness. Innegra, Dyneema, Spectra are some of the trade names of UHMWPE and UHMWPP that have very good tensile strength and, especially, elongation (and energy absorption) prior to failure. They have been incorporated into weaves with CF to offer improved "toughness" for parts that need to remain intact even after the CF has failed (e.g. that might be a handy property in a racing bicycle frame, etc). Because of their properties, they won't add significant strength to the part under normal loading (the CF will be doing all the work there due to its greater stiffness and resistance to initial elongation).

    Here are some spots where this has been discussed, for reference.
    Thread: Impact tolerant/high ductility composite hybrids (Dyneema, Spectra, etc)
    Posts:
    Wittman style landing gear options (26Jan2019)

    Billski, thread above: Adhesion to some of these fibers can be a problem
     
  16. Sep 25, 2019 #16

    BJC

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    The thought of a CF bicycle splintering in a crash sent shivers up my spine. Please be gentle with us old ... more mature citizens.


    BJC
     
  17. Sep 25, 2019 #17

    Pops

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    Have built 3 Fiberglass Kayaks and used a layer of Kevlar to keep the Kayak from breaking into two pieces in hitting a large rock.
     
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  18. Sep 25, 2019 #18

    bmcj

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    They already do in the current metal construction. They found that out while researching fatal accidents with hard flat impacts that looked like they should have been survivable. High speed video, slowed down, showed that the cabin deformed to such a great extent that cabin roof collapsed down to the seats such that the passengers would have received fatal crushing blows to the head and upper torso. It wasn’t apparent from the original accident investigations because after deformation, the cabin sprung back to its original shape.

    I don’t recall the models they tested, but I would assume that the high wing cantilever models might be more prone to this. If anything, you would want build a stiffer cabin, not a more flexible one.
     
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  19. Sep 25, 2019 #19

    TMann

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    I was thinking of something more likely, such as an off-field landing.
    A friend of mine performed a off-field landing in a Cassutt (welded frame) and spent about an hour, soaked in fuel, with his foot pinned in the twisted steel frame before he was extracted. I would say that each solution has it's good points and failure points but none is perfect.
     
  20. Sep 25, 2019 #20

    Vigilant1

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    With composites, the designer would still have the option to make the cabin as stiff as desired using CF. I think the question is: At some point, is it worth the cost/weight/effort to add some other material (UHMWPP, UHMWPE, etc) to keep the structure together after the CF has failed, and to reduce the chance that CF shards will injure to occupants? Or, is the weight/cost/effort better spent in increasing the cabin stiffness still more with even more CF and no other materials?
     
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