spectra vs kevlar?

Discussion in 'Materials' started by sigurd, Feb 18, 2009.

  1. sigurd
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    sigurd Pompuous Pangolin

    hello,

    Kevlar is sometimes used in the outer laminate of a cored hull, right next to the core, for impact protection.
    At least for carbon outer layer, I guess that the kevlar will do nothing until the carbon is already broken. Right?

    I have seen a few impact tests for spectra and kevlar (unfortunately only ballistic speed - not sure if low speed impact calls for different laminate attributes) - spectra does a lot better, and is lighter too. It is stiffer than kevlar, which leads me to think that it could take a much larger amount of load before the outer carbon or glass layer is broken. Am I thinking right?

    Are there other caveats? For this application, which is preferrable, spectra or kevlar?
     
  2. robherc
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    robherc Designer/Hobbyist

    I'm not much of an expert on spectra, so I really can't compare the two that well, but isn't spectra FAR more expensive than kevlar? If cost is no object, and the spectra has higher ballistics ratings, and is stiffer, then my (semi-educated) guess would be that yes, the spectra would benefit your CF layup more...but I really don't know any more about spectra than that it's better than kevlar in ballistic vests.
     
  3. sigurd
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    sigurd Pompuous Pangolin

    not much more expensive. about 10-12 dollar per sqm. discountcomposite. Unfortunately I am unable to get in touch with them by phone or email.
     
  4. apex1

    apex1 Guest

    Holiday for Kevlar?

    Cannot reply to your last question, I have no experience with both. But the assumption above is wrong, sorry. All the matrix, means every member of the matrix, has to carry the given load, simultaneously and transfer the load to the core and opposite matrix.
    So, your Kevlar layer does´nt enjoy a boat trip, til the neighbour failed.;)

    Regards
    Richard
     
  5. sigurd
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    sigurd Pompuous Pangolin

    Don't understand what you mean?
    Transferring pressure from a sharp object directly into the core will ofcourse happen, but... I think what happens is that the fibers' tensile stiffness and strength is responsible for spreading the point load to a larger piece of core..?
    If I used rubber instead of carbon, the rubber would be just pushed straight through the core without puncture. So with this assumption I conclude that "stretchy" kevlar will not delay the puncturing of the carbon, but it will delay the puncturing of the core.
    Spectra on the other hand is almost as stiff as carbon so when the carbon is almost rupturing, the spectra is also almost rupturing, so both are protecting each other. make sense?
     
  6. apex1

    apex1 Guest

    Yes, if you like to describe it so.
    What usually is´nt understood, is the fact, that you have no Kevlar or carbon, glass or whatsoever. You have a completely new material. The matrix, does neither behave like carbon nor like kevlar, it´s a complex mix of addition and subtraction of the different materials characteristics.
    Naturally if you shoot at it (as an example of puncture), the outer carbon layer gets destroyed before the inner kevlar bears a load. If you now substitute the (softer) Kevlar with the stiffer (I dont know) Spectra, I would guess you pay for bacon and get belly. Now you have two materials in the matrix that are not as puncture resistant as the former combination, your matrix will fail sooner, not later! Although it was´nt wrong that both materials still are protecting each other, that was better achieved by the K/C combi.

    I hope I could express that clear enough?

    Regards
    Richard
     
  7. Landlubber
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    Landlubber Senior Member

    Basically what Apex is trying to say is that the sum of the two products is not equal to the sum of the individual products added together.

    They form an entirely new substrate, the characteristics may not necessarily represent the addition of the individuals characteristics.
     
  8. sigurd
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    sigurd Pompuous Pangolin

    Thanks for the effort, both, but I don't understand.
     
  9. Landlubber
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    Landlubber Senior Member

    OK, I will try to do a better job.

    If material "a" has certain charasteristics on its own, and material "b" has certain characteristics, on its own, then the combination of materials "a" and "b", need not necessarily be the same as the total of "a" and "b" individually, sometimes the characteristics are greatly improved above the sum of the individual parts.
     
  10. Landlubber
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    Landlubber Senior Member

    Take for instance, a composite lamination of 300gsm CSM and 25mmfoam.

    The two layers of 300, individually, are not very stiff (as an example).

    The 25mm of foam is not very stiff either, but laminate one layer on each side of the foam, and the total stiffness is greater than the total stiffness of the sum of the individual parts.
     
  11. apex1

    apex1 Guest

    may not necessarily
    No, they never do !
    To try again, sigurd wrote: So with this assumption I conclude that "stretchy" kevlar will not delay the puncturing of the carbon, but it will delay the puncturing of the core
    That exactly is wrong! You look at two materials, you do´nt have two! In your matrix the carbon fibre will hand over a lot of load to the kevlar fibre wich now acts as a bumper, spring, or so, giving the carbon fiber attributes she originally does´nt have! To a certain point! (very dilettantic, but I cannot express it better in english)
    You look at say a Steel and a Alu plate connected. If you could amalgamate them, adding some copper and lead, you came out with a totally different material having nothing in common with the former.
    Was that better?
    Regards
    Richard
     
  12. sigurd
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    sigurd Pompuous Pangolin

    I think I get your point.
    But, I wish I knew more about *how* the materials interact? If I knew, maybe I could deduce which combination would be preferrable? It seems like apex1 thinks carbon/kevlar is better, but I still find it a bit hard to conclude.
     
  13. apex1

    apex1 Guest

    Yes, you´re right. Unfortunately my ability is´nt sufficient to calculate a matrix, I otherwise would have done already, to assist you, and to back my statements with some figures.

    Regards
    Richard
     
  14. sigurd
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    sigurd Pompuous Pangolin

    Here is one drop test. I'm not sure how much real world impacts with a core relate to this.
    edit: don't you think it's curious that the kev hybrid fails before the glass hybrid?
     

    Attached Files:


  15. sigurd
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    sigurd Pompuous Pangolin

    Here is a chart for materials properties by themselves.
     

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