FRP vs CF

Discussion in 'Fiberglass and Composite Boat Building' started by Seafra, Aug 24, 2006.

  1. Toot
    Joined: Jul 2006
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    Location: Chicago

    Toot Senior Member

    Another problem with CF is the fact that fabrics don't give you nearly the strength as straight rovings.

    Tensile strength for well-manufactured solid rod will exceed 320ksi. Because of the small size and difficulty with inspection, following appropriate standard practice might be to cut that amount in half for design purposes. So let's assume 160ksi and just be happy that we have a nice safety margin. By comparison, a wet-layup of unidirectional E-Glass will be around 40ksi. Although carbon can be brittle, it is very strong, so it can be used within a laminate structure to give a HUGE boost to strength without exposing it to damage on the exterior.

    Graphlite is a tradename for pultruded rods that hit the above stength specification- most cheapo rods do not. Graphlite costs $2 a foot in low volume for a rectangular pultruded rod with the dimensions being: 0.092x.220, or .02024sq.inches, which means each rod has a tensile strength of 3,240 pounds. The cost, in low volume, is $2 per foot. I think in high volume, you can get it for half that amount. That's makes it a really good option for making I-beams and such where you embed the rods into a fiberglass structure. Here's a link to a guy that's doing it with sailplanes... Jim Marske


    By way of comparison, most homemade CF rods will struggle to get above 60ksi and some may fail at half that! And most woven CF fabrics will struggle to outperform S-glass because the crimps/curves in the weave dramatically decreases the strength of the CF, much more so than with fiberglass. I read somewhere that a 1-2 degree variance in the straightness of CF will cut it's strenght in half. Scary!

    As a result of the above, I'm pretty well sold on the idea that, at least where cost is a concern, the only justifiable use for CF in a small homebuilding type of operation, be it sailplane, boat, car, plane, skis, or helicopter, is in pre-manufactured rods and then only for unidirectional load absorption. Otherwise, stick to FG or maybe Kevlar.
     
  2. John ilett
    Joined: Nov 2003
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    John ilett Senior Member

    This is usually the case to save weight and cost as the carbon is so expensive. For any boat a builder would hope to improve their boats in any and every way possible so I think there is a compromise of using less material and still having a boat that is lighter and stronger than its fglass equivilant.

    The stuff (carbon fibre) is really stong especially when it's pre pregs (2 x better resins) and when it's used properly. With aerospace and race car builds they follow strict guidelines when processing to remove the human error factor so carefull attention is given to fibre orientation with engineered laminates, monitored vacuum and cooking set ups.

    When you start to use less material you then have to be sure that the loads are spread (good fibre orientation). Average fglass boats often simply rely on the laminate thickness as a guide to strength.

    When you hear of broken carbon parts very often the cause is a processing mistake. Not enough fibre in the right place pointing in the direction. Holes drilled in the wrong place etc.
     
  3. Toot
    Joined: Jul 2006
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    Toot Senior Member

    Right now, for a project I'm working on, I'm 80% settled on the idea of creating an I-beam using the following method-

    Take an 8 foot aluminum/steel U-channel, waxing and PVA'ing the inside surface, laying up graphlite rods inside the channel to acquire the necessary strength, laying unidirectional glass rovings on top to make a uniformly-sized piece. Then creating 2 knitted-glass (+45/-45 orientation) flat panels using peel-ply on a flat surface to serve as the spar web. Then bonding the sparcaps in between the sparwebs, with a foam core taking up the rest of the space. I figure I can create a 16 foot long spar, capable of carrying 20G's on a 700 pound aircraft and keep the spar well under 20 pounds.

    And best of all, the carbon won't be exposed to the surface, so it's brittleness won't be a factor. It sounds like just about a perfect structure!

    Try that with E-glass! ;)
     

  4. frosh
    Joined: Jan 2005
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    Location: AUSTRALIA

    frosh Senior Member

    Using CF selectively in boat building

    It seems to me that a good compromise cost wise and also durability and weight wise can be achieved as follows.
    Make the hull skin a foam sandwich using an appropriate thickness PVC structural foam, Airex, Klegecell etc.
    As most of the loadings on the hull skin are pushing inwards or torsional, it makes sense to have the inner reinforcement in the strongest lightest material, such as carbon fibre. This takes advantage of the tensile modulus of carbon which is higher than all other fibres.
    The outer skin is probably less loaded than the inner skin, but is exposed to the outside elements, and environmental nasties.
    For excellent strength, flexibility, and fairly good abrasion and impact resistance, I like using a biaxial knitted fibreglass. If the orientation of the carbon fibre is 0,90 degrees, then the biaxial glass is best in a different orientation, that is: +45, -45 degrees. Resin will be always epoxy, never polyester.
    Any opinions? As this is just my idea of an excellent hull skin composition.
     
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