Aramid location in sandwich composite advice needed

Discussion in 'Fiberglass and Composite Boat Building' started by isvflorin, Feb 7, 2015.

  1. isvflorin
    Joined: Nov 2009
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    isvflorin Junior Member

    Hi everyone,
    need some advice on the location of an aramid ply in a foam sandwich hull. To be very clear - the aramid fabric is used to thicken up the outside skin (building laminate thickness with low weight).

    Question is - should it be next to the core or the last layer outside ?
    If it is the last layer outside - I'm worried about fiber damage, as aramid soaks up water if the skin is dinged.

    If it is right next to the core (on the outside skin) - I'm worried that it will delaminate faster from the core than glass would, when impacted.

    What is the consensus / advice ? I guess that , ideally, aramid should be sandwiched between 2 layers of glass (or carbon). But a 3rd ply would make the hull a bit on the heavy side.
  2. TANSL
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    TANSL Senior Member

    One question that occurs to me is not where to place the aramid, but whether I should use it. I am assuming that if you use aramid is because you have a problem and aramid is the best way to solve it.
    That said, stress distribution in the laminate thickness is linear, so that the value is zero at the neutral axis and increases towards the outside / inside. According to this principle, the most resistant layers should be placed towards the outside / inside the laminate. I would place the aramid as outward as possible, but after it I would put, at least, two layers of mat and then gelcoat.
  3. isvflorin
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    isvflorin Junior Member

    Thanks TANSL,
    good point about whether to use it or not. I just need a lightweight fabric that builds up thickness. It could be Diolen or even wood veneer. If I place either Diolen or Aramid on the outside then I will have to cover it. I don't like mat, but probably 100gr of plain glass should be ok.

    I've learned that most boats built as a foam sandwich on a slat mould are only finished with a paint after final fairing. So probably no gel coat for me.
  4. FishStretcher
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    FishStretcher Junior Member

    If by aramid you mean kevlar, and the goal is adding bulk without mass, then the my opinion would be this: don't use kevlar. It is much more difficult to work with than glass or carbon. I would be very reluctant to have it near a lay you might ever need to sand into for a repair later on. It also has a reputation of being more difficult to bond to than glass or carbon.

    If you want it for particular properties, like resistance to penetration in an accident, that's different.

    And if you have a foam sandwich cored hull, then any layer you apply will be fairly far from the neutral axis compared to the layer thickness, so if you did have a difficult to rework material like kevlar, then I personally would be inclined to bury it in a few layers below glass or carbon. I think this might make fabrication and rework a lot easier.
  5. Stumble
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    Stumble Senior Member

    If it's just to thicken the composite why not use thicker foam? It would be a lot cheaper and much easier to work with.
  6. PAR
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    PAR Yacht Designer/Builder

    The use of these types of fabrics typically is for penetration resistance, so if your laminate needs it, use it and it should be on the very inside, as far from the impact point as practical. Kevlar also has good abrasion resistance, so some may employ it on the outside, such seen in a canoe or kayak, but mostly, you need to justify these types of fabrics, when developing the scantlings, not as an ad hoc amendment to a laminate schedule.

    So, why do you want to bulk up the outer skin of the sandwich? The answer to will usually direct you to an appropriate laminate schedule.
  7. Eric Sponberg
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    Eric Sponberg Senior Member

    Isvflorin, if your only reason to include aramid (Kevlar) in a laminate is to bulk up the outside skin, you are approaching the laminate engineering all wrong. Kevlar has certain mechanical properties which contribute to the strength and stiffness of the laminate, but unfortunately, these are not balanced properties. Kevlar is very good in tension, but lousy in compression. To make matters worse, sandwich laminates experience both tension and compression on both the inside and outside skins when under normal hull loading, so at some point where the laminate experiences compression where the Kevlar is, you could experience some premature failure (if the laminate is not engineered properly) which can lead to later progressive failures.

    Personally, I never advocate Kevlar in a laminate unless a client wants to add some bullet-proofing, in which case I would accept a single thin ply of Kevlar on the outside skin, but against the core with the other layers on top of it. Kevlar is a real bugger to repair, and if it is on the outside of the outside skin, minor abrasions will break through the surface, and you'll end up with a fuzzy wad of Kevlar in the damaged area that you cannot cut out easily and you cannot repair easily.

    If you need to bulk up a laminate, use chopped strand mat or layers of fabric that have mat stitched to the back.

    And do some proper laminate engineering to select the right fabrics, and be sure to balance the fabrics either side of the core--the lay-ups should be mirror images of each other through the central plane of the laminate. This ensures the least amount of panel distortion and twist under load.

    I hope that helps.


  8. TANSL
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    TANSL Senior Member

    This is something that should be emphasized for the "designers" of structures understand that calculating scantlings of hull plates is not only put a layer on top of the other until a certain thickness.
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