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

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

  1. bryson
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    bryson Junior Member

    I built a fairly small skiff (17.5' LOA, 30hp) and have been testing over the last few months. There is no "floor" and no stringer system -- the entire skiff is built from 3/4" PVC core, with 30 oz e-glass on each side. The cockpit floor is just the interior of the hull.

    Unsurprisingly, the floor flexes a little bit in choppy conditions. The cockpit is about 4 ft wide and 7.5 ft long, so it's a fairly large unsupported span. I don't think it flexes enough to cause any issues, but it's a little irritating and makes the skiff feel less "refined".

    It's been suggested that I consider adding strips of carbon (or any stiffer material) to act sort of as a stringer -- there would be no additional core, just epoxy/carbon around 1-2 ft wide down the length of the skiff in the cockpit, faired into the surrounding floor.

    I am a mechanical engineer by trade, but this skiff is about my only experience with composites design. I understand not using materials of different stiffnesses in the same layup, but am trying to wrap my head around what will happen if I lay a thin carbon "stringer". I know that the stiffer material will take a significant part of the load under low deflection and will need to be strong enough to withstand it, but I'm not sure how I would want to deal with the transition from carbon to glass.

    Thank you for your help, and let me know if you need any more info on the skiff or its current construction.
     
  2. fallguy
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    fallguy Senior Member

    Well, it will probably eventually delaminate as is.

    what is your 30 oz laminate and why so light inside no stringers?
     
  3. fallguy
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    fallguy Senior Member

    How did you arrive at that schedule? I am mostly surprised the inside laminate is so light.

    3x10oz woven? Only laid 0-90? Or something else?
     
  4. bryson
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    bryson Junior Member

    Plans from the designer called out 3/4 H80 with 3 layers of 10 oz inside and out. Currently laid 0-90, aside from the tape at the bulkheads. The deck and bulkheads add a considerable amount of stiffness. Total hull weight is probably around 300-325 lbs - I believe it was around 215 before the deck.

    Photos for design reference. This is a Chris Morejohn skiff, a revision of the original Hell's Bay Whipray.

    20200616_233255.jpg

    20200709_181948.jpg
     
    Last edited: Dec 14, 2020
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  5. bryson
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    bryson Junior Member

    To add - the skiff was "strip built" over stations using 2" wide strips of basically H60. Then glassed, pulled off the stations, flipped, and the interior glassed. Bulkheads were glassed on a table, then bonded and taped in. The deck was made in a mold (basically same lamination schedule), then puttied/taped on.

    Edit - forgot to mention: the lack of stringers was primarily to keep the build simple, but also to aid stability and to give the feeling of more freeboard when moving around inside the cockpit. Also helps keep the openings in the bulkheads slightly larger. @fallguy
     
    Last edited: Dec 14, 2020
  6. fallguy
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    fallguy Senior Member

    I have a builder knowledge about 10 ounce woven and basic laminating. Other posters here can answer better, but I have done testing on plascore full inch for a 4' wide cat cockpit.

    A couple layers of 12 oz biax, for example on a 12mm core is pretty stiff im two foot spans, You can feel a 220 pound man push the deck down a tad. More for sure in 4 foot spans. It would honestly be scary.

    Also, the laminate is only affected where there is no bulkhead and widespans ftmp I'd say in your boat. My original vision was a 17' skiff wide open all the way and this was the basis for my comment on a light layup; sort of..

    I am a little surprised the designer did not want glass in any other direction or stiffer tows. 10 oz woven glass has 5 ounce yarns each way and those are pretty light.

    I am a also a little surprised the exterior and interior are equal. I would have expected a difference.

    I can tell you that my gut says you are light in the cockpit. But you might not like my answer. Personally, I would want one of two things. For a one inch thick core spanning 4 feet; I would want forty ounces glass multiple directions. Tows at 0/90/+45/-45. This is based on personal experience and testing. My cockpit, in fact has exactly that layup at 42-44 ounces each side, but not subject to water.

    Since the bottom of the boat is already done, you cannot achieve more glass easily on the exterior. And so the best way to add stiffness now is to make the hull thicker. I would add more core to the cockpit; perhaps 12mm or 18mm if you have it. Then, I would glass that with a wear layer. The core would be bedded in thickened resin.

    The other way which is not as good would be more glass alone inside. A 1700 db would offer tows of 8.5oz and another direction.

    I am not a fan of your carbon stiffening ideas and expect them to disappoint.

    Another thing you could do that might turn your stomach entirely is a 6mm okume plywood bedded in resins and glassed above for wear. But kind of a sin eh?

    Also, sorry this was rambling so bad. I am rather tired.

    Do not worry about delamination. I am not worried after seeing the bulkheads. Like I said, I had visions of a wide open skiff.

    If you skimped on the designers wishes or plan; adding core probably best.
     
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  7. rxcomposite
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    rxcomposite Senior Member

    You basically commited two major mistake. Using a lower shear modulus H60 against H80 and omitting the stiffeners.

    H60 has only 20 N/mm2 of shear modulus while H80 has 27. "Stiffeners" are just as the name implies, to stiffen it. Adding carbon strips on top of of the laminate would do you very little good. It is too close to the surface and needs to be raised a distance to be effective, thus going back to a need of stiffeners.

    While carbon is good because of its high strength, it is also very poor in compression much like a steel cable. In bending analysis, you have tensile and compression on the outermost surfaces.

    Carbon with its high strength comes with low strain. It cannot be "stretched" as much as glass. Glass has about 2.5% elongation while carbon has about 1.1%. This is called strain. When mixing different materials, the lower strain of the two is used. With carbon/glass, glass will have lower tensile strength usable using the strain criteria or the carbon breaks first before the glass.
     
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  8. fallguy
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    fallguy Senior Member

    I am glad you posted as the local composites expert.

    What about my idea to simply bed 12 mm or 18mm core over the cockpit?

    He would retain the flat bottom and closely maintain his freeboard and especially the 18mm would stiffen it.

    Or do you think he needs stringers and sole?
     
  9. fallguy
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    fallguy Senior Member

    Rx-is shear and flexural ridigity related always or in composites?

    I think the flexural rigidity of the h60 would be lower, but is that simply derived from shear?

    Good catch on the h60; I completely missed that he lowered that....definitely a factor.

    If he adds to the cockpit; can he add H80 or a higher shear material?
     
  10. bryson
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    bryson Junior Member

    Thank you both for the insight. I talked with the designer before I made the deviations -- the H80 was selected as the core mostly due to availability and his experience with it with the design/production of the Hell's Bay skiffs. I think that the H60 is plenty strong from a load carrying standpoint, but figured the thickness of the sandwich would be more important than the stiffness of the core for most of the loading I'd see on the skiff. The removal of the stringers and sole was approved, but he didn't mention a modification to the layup.

    Thankfully the boat is only in primer -- I wanted to run it for a bit and see how it performed before really finishing out the fairing and paint. I also considered the ply since that's one of the designers options for "topping" the sole, and that's a good backup plan. I like the idea of getting some 45-45 cloth in there too. Just from some more basic calculation, it seems like the biggest improvement will come from increasing skin thickness (and thus the overall sandwich thickness). There are other things that would help, but they are out of my control (or at least, too much work to change).

    Here are the options I've come up with so far, in order of my preference -- I would definitely appreciate any advice or suggested modifications.

    1) To both the exterior of the hull (transom to front bulkhead area, below the waterline) and to the interior between the visible fwd and aft bulkheads, add 1 layer of 17 oz 45-45 cloth, then another layer of 10 oz 0-90 to help with fairing and to thicken the skins (additional 27 oz each, inside and outside).
    2) To add another layer of core to the sole, either 1/2" or 3/4" thick, then 3 layers of 10 oz cloth on top.
    3) To add 6mm ply, then 2 layers of 10 oz cloth on top.

    I am not too terribly concerned with weight because the skiff came out lighter than I expected, but it's a factor. I am leaning towards the first option because it requires the least work, and changes the layout of the skiff the least.

    edit -- I could also use 1 or 2 layers of my 10 oz cloth just laid down on a 45 degree in lieu of the 17 oz biax. I don't know if woven cloth has an advantage over the biaxial or not, but it seems like it might.

    edit again for reference -- I know this is overly simplified, but the basic calcs I'm referencing are just bending calcs for simply supported beams: Bending stiffness should increase with the square of the overall thickness, as well as linearly with the increase in skin thickness. I can't change the modulus of the skin material.
     
    Last edited: Dec 15, 2020
  11. fallguy
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    fallguy Senior Member

    Actually, changing the skin thickness is the worst option.

    Stiffness is a function of the cube rule.

    The stiffness of the core can be compared as follows.

    1" core ^ 3 = 1
    2" core ^ 3 = 8

    So, by doubling the cockpit hull thickness; it will be 8 times stiffer. You will never achieve that with a layer of glass or two..

    plywood is inherently stiffer, but 6mm alone not so much

    rxcomposite is 12 hours different than us giv r take; give him time to respond; I can't commit him to work, but he might give you an idea

    forget messing with the bottom of the boat

    for me, I'd double the core; perhaps even change the top to h80 AND I'd glass that with 1200 db or 10 oz woven laid on 45s and os or some such. Mostly for a wear layer. Make sure to sand before bonding the core. Primer is not very strong vs epoxy. Run the added core to the edges and infill with thickened resins... I think you'll like the result.. I am not sure you need to get too crazy tabbing the lifted core to the sides; I am on the fence as to whether it matters much. As a matter of good build practice; sure, but it isn't going to do much for stiffness.

    Because your bottom is a bit light; adding some db glass to the existing skin prior to adding core might actually provide some tension OR, you could put some db on the bottom of the piece you intend to add unless there is curvature and piecing...
     
  12. bryson
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    bryson Junior Member

    @fallguy thank you, I was actually just looking over those equations! I should have said biggest "bang for my buck" rather than biggest "improvement". This is the document I was referencing: https://www.hexcel.com/user_area/content_media/raw/Honeycomb_Sandwich_Design_Technology.pdf

    I agree doubling the core is definitely the most effective solution, but I need to decide if it's worth the extra work. It will involve modifying the front bulkhead, and if I don't add core to the front compartment as well, it will create a low spot for water to collect. I'm also planning to strip the boat back down in a couple weeks, and will be flipping anyway to finish fairing, sharpening chines, etc. Adding glass on the bottom isn't too much of a concern, and I wouldn't mind the extra impact/abrasion resistance. This boat is made to be poled in very shallow waters, and we drag the hull across oysters frequently.

    The floor doesn't deflect excessively -- I actually didn't even notice it my first few trips out, but my buddy pointed it out to me recently. Slight use of the trim tab to drop the bow makes it go away, once the sharper entry is taking the chop.
     
  13. rxcomposite
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    rxcomposite Senior Member

    Too much exchanges have gone by since I posted so I will answer topics of interest.

    Shear and ridgidty is always related, The more flexible the material is, the more the neutral axis goes past one another.
     
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  14. rxcomposite
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    rxcomposite Senior Member

    In glass sandwich laminated with polyester, the core shear is less important as it is the skin that gets stressed first. Following the calculations, based on the modulus of the skin used, the skins are always increased by a factor.

    In calculations you have to satisfy core shear first then skin stress. The last but not the least is deflection which is often overlooked. To reduce deflection, there are several ways of doing it.
    1. Add stiffeners. This will reduce the plate area and reduce bending.
    2. Add skin thickness by adding more layers. How many layers to add? Only the calculation will show. Biax is a bad idea. It has low tensile strength though high in plane shear modulus. Only used when torsion is a problem and it is laid near the core.
    3. Increase the modulus of the skin. Use STITCHED 0/90 uni cloth. Or use CF from the very start. Uni or CF has a very high modulus. Increasing skin modulus will increase stiffness.
    4. Add a floor. This will increase the moment of inertia of the whole central portion.
     
    Last edited: Dec 15, 2020
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  15. rxcomposite
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    rxcomposite Senior Member

    Composites follows the equations of the first principle. It is basically the same except for some fudge factors.

    Increasing core thickness will increase moment of inertia and reduce the stress on the outermost layer (skin thickness can be reduced)

    Plate calculations always follow the fixed end beams and not simply supported. Hatch covers follow the simply supported bending theory.
     
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