Combining E-Glass and Carbon (or any cloth with different stiffnesses)

Discussion in 'Materials' started by bryson, Dec 14, 2020.

  1. fallguy
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    fallguy Senior Member

    Pay close attention to rx advice. His expertise is exceptional.
     
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  2. rxcomposite
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    rxcomposite Senior Member

    The bow takes a lot of wave beating and should be stiffened. The outer shell skin is always thicker as it is mostly in compression. Composites are generally low on compressive strength and made thicker to compensate for strength loss. The keel takes a lot of load and abrasion. It is generally 1.5 X thicker than the bottom laminate and about 1/10 of the beam width.
     
  3. bryson
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    bryson Junior Member

    @rxcomposite @fallguy thank you both very much for your advice! This has been very helpful. Also, thank you for the heads up on the biax -- I have no experience with it and will keep that in mind for future projects.

    It appears that (as usual, unfortunately) the most effective actions are those requiring the most work. Adding a floor and stringers requires a re-work of both bulkheads and the drain system. Doubling the core is definitely more work than adding glass, but I'll need to extend the core into the front compartment and slightly re-work the front bulkhead to prevent low areas. There is also risk associated with getting a good bond between a large sheet of core and the existing floor, but I don't think it will be an issue. Additional glass is the easiest but the least effective. The tough part now is quantifying everything.

    So, which equation is more accurate for stiffness/displacement of composites? The reference I'm using seems to show stiffness primarily as a function of (face thickness*height^2), where height is the total thickness of the composite. Other info shows stiffness as a cube function of (height^3).
     
  4. rxcomposite
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    rxcomposite Senior Member

    Just finished my study based on the other thread which has gone stale.

    Plywood is not stiff. It has very low elastic modulus. Cored composites although thicker will always be stiffer and lighter by about 30% for the same panel size and pressure.
     
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  5. fallguy
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    fallguy Senior Member

    versus a core no glass is my comparison; so plywood 6mm beats core of 6mm standing alone

    of course, noone uses a core standing alone, but in his case; simply adding 6mm ply and a 10 oz woven would be stiffer than adding a 6mm core and same glass

    all I was trying to say...
     
  6. rxcomposite
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    rxcomposite Senior Member

    Stiffness= Height/Thickness (high moment of inertia, watch out for core shear), Modulus of the face (use WR, the twill or knitted variety, not the basket weave) or stitched 0/90 uni plus biax near core, low plate width or span (reducing bending moment).

    Biax is just WR rotated 45 degree. As you rotate the direction of the load, the fiber loses strength though gains shear modulus. It can be likened to diagonal members in a bridge. Stitched biax is the worst as it is two uni's stiched at an angle. Uni's are notorious for losing its strength at off axis load. Wovens on the other hand has low modulus/strength to start with but does not lose much strength at off axis load. The woven fibers tends to tug at each other on a macro level.
     
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  7. fallguy
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    fallguy Senior Member

    I am mostly out of the running here, but based on rx statements; I would add core up to the bow and if needed aft for drainage. My back decided I needed to lay down, but fingers working.

    How much? Probably at least 12mm, but I am just guessing based on my knowledge of cores.

    I don't know if going to H120 would help and minimize the thickness needed, but an H120 panel is never as flexible as H80. H120 is pretty stiff stuff. Again, rx is the expert.

    As for bonding; I'd be tempted to try and use vac or even a shop vac.

    Bonding needs to consider the core. And you certainly need to trowel thickened resins on both the sibstrate and the core. I like a 1/16" vee trowel. It will take a fair amount of resin. If you even have a shop vac; you ought to consider vacuum or at least weighting well.
     
  8. rxcomposite
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    rxcomposite Senior Member

    Yes of course you are right.
     
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  9. rxcomposite
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    rxcomposite Senior Member

    Vacuum bagging or infusion always increases modulus. Makes panels stiffer.

    From previous projects, H80 seems to be adequate.
     
  10. bryson
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    bryson Junior Member

    I figure short of adding stringers, the span/width is constant. The Modulus of both the core and the skin is not going to change either (assuming the options are increasing skin thickness or doubling core). I ran some quick calcs with the two equations I mentioned earlier: 1: (stiffness = face thickness*height^2) vs. 2: (stiffness = height^3) and some rough estimates from local composite vendors.

    And if the two options are:
    A) Doubling skin thickness with no change in core thickness
    B) Doubling core thickness with no change in skin thickness

    Using equation 1, option A increases stiffness by 2.6x, and option B increases it by 3.5x
    Using equation 2, option A increases stiffness by 1.4x, and option B increases it by 6.6x

    Option A is significantly less work and has the added benefit of abrasion/impact resistance. If equation 1 is correct, then the extra work of option B isn't worth it. If equation 2 is correct, then it just might be.
     
  11. rxcomposite
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    rxcomposite Senior Member

    A 16 mm H80 with a panel size of 700 x 1400 mm will take a beating of up to 22 kN/m2 of pressure on the bow with 2 mm inner skin, 2,5 outer. It will deflect by 7 mm which is just about the span to deflection ratio limit. If the boat is driven harder say up to 28 kN/m2 pressure, 16 knots, then go to 20 mm core.
     
    Last edited: Dec 16, 2020
  12. rxcomposite
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    rxcomposite Senior Member

    What is the span, length of the panel? What speed are you looking at and what is the LWL of your boat? What is your present core size?
     
  13. bryson
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    bryson Junior Member

    The unsupported panel is approx 48" x 90" between the chines and bulkheads. Maximum speed is around 32mph, LWL is I believe around 180", but I would need to check. Core was 3/4" thick, but was sanded a bit to prepare for glass. There are overlaps in the layup but I haven't been accounting for them.
     
  14. rxcomposite
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    Whoa! High speed boat. 5g's of vertical acceleration and 50 kN/m2 of pressure. You have very large span. Is that in midship or bow area?

    I will answer tomorrow as I look in my database.
     
    Last edited: Dec 16, 2020
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  15. fallguy
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    fallguy Senior Member

    yeah, even a guthunch figured it was light R
     
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