Basics of Composite Design

Discussion in 'Materials' started by rwatson, Jan 24, 2018.

  1. rwatson
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    rwatson Senior Member

  2. JosephT
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    JosephT Senior Member

    Nice overview. Calculating material densities is always tricky in the world of developing new composite products. Establishing a weight management formula for bonded assemblies composed of mixed materials (foam, carbon, s-glass, expoxy resin, etc. etc.) all with variable thicknesses is an art. There are many customized analysis methods. With boats the weight issue is a lot more forgiving unless you're designing an Americas's cup boat.

    One construction method I don't see mentioned is fiber placement. This is used by aerospace and allows very precise layup control. It's expensive up front though and would be a waste of money for boats built in smaller quantities. Basically prepreg fiber tows are pulled from a refrigerated supply and placed over the surface. The models are generated in CAD, the tool paths are generated with integrated tools like Fibersim, and all the magic comes together with the fiber placement machine. The last step is usually cooking them at varying temperatures. Fun stuff.

    I personally like putting on a suit and laying the fibers up by hand and using either vacuum bag or resin infusion layup methods. Both are good enough for most boats.

     
  3. Yellowjacket
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    Yellowjacket Senior Member

    Sounds good but the reality is that boats are designed more for resistance to local impacts as opposed to airframes that are designed closer to structural limits. By the time you have sufficient strength to keep the things you hit from coming thru the hull it's plenty strong from most other aspects. Yes you need some ribs and stringers to keep the from bending too much, but if you try to thin the surfaces too much you don't have the impact resistance you need.
     

  4. JosephT
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    JosephT Senior Member

    The thickness of the structure depends on the boat. The Americas's cup, Volvo & Vendee boats, for example, are all quite optimized and very light...much like aircraft. They generally don't handle collisions/bumping as well as boats layed up in fiberglass. The type of fiber used (e.g. satin weave vs. simple fiber) makes a difference too on impact resistance. S-glass satin weave, for example, is excellent for impact strength.

    And fiber placement can be used to lay up very thick structures too...including those with integral reinforced stringers, stiffeners, etc. Add to that a high temp 350F resin system + autoclave curing and you have a very strong hull form.

    On the other hand, recreational & leisure boats typically opt for lower cost fiberglass methods. Some boats today take shortcuts and use cheap chop glass, cheap polyester resin & low temp/room temp curing methods. You get what you pay for.
     
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