Vacuum Bagging On Plywood and Balsa Permeation?

Discussion in 'Boatbuilding' started by zstine, Jan 29, 2021.

  1. jehardiman
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    jehardiman Senior Member

    Weave or linear? One layer of weave at 45 is all you need.
     
  2. zstine
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    zstine Junior Member

    I'm not sure what you mean that "one layer of weave at 45deg is all you need". Weave can be as light as 2 oz/yrd, or like 26 oz/yrd for roving. I'm looking at 2 layers of 1708 above the waterline and 3 layers below. The inside is 3 layers of 9 0z carbon weave... maybe 2 layers of 11oz carbon. 1/2inch x 4lb divinycell core. I haven't done any analysis other than looking at weighs and it looks like I can obtain my light ship target of 700lbs with foils using this laminate schedule... 1100lb all up weight, so 400lbs for 2 crew.
     
  3. jehardiman
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    jehardiman Senior Member

    An issue is over using carbon. The inside is generally the tensile side, 3 layers of carbon inside may move the NA too far towards the carbon. Whose analysis program are you using?
     
  4. zstine
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    zstine Junior Member

    I have not done the structural panel analysis yet. The above was an educated guess (roughly 1/8in thick E-glass outside and 1/16in carbon inside). The target weight is based on the weight of similar size and construction boats. Can you advise me of any free composite analysis software?
     
  5. gonzo
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    gonzo Senior Member

    Why the expense and complication of carbon on the inside if you are using the cheapest fiberglass on the outside?
     
  6. zstine
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    zstine Junior Member

    E-glass on the outside and carbon on the inside is a good compromise for cost and strength. This is how Morrelli & Melvin designed the Rapido Trimarans. The E-glass provides good impact resistance on the outer skin, which is generally in compression, while the high tensile strength of carbon on the inside provides rigidity. A thin carbon outer skin would be easily pierced when confronted with a bolt or corner of a dock. What makes zero sense is mixing E-glass and Carbon in the same skin, which I see people do all the time. In that case, the glass fiber takes virtually no load since the modulus of elasticity of carbon is so much higher than glass fiber.
     
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  7. fallguy
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    fallguy Senior Member

    yes, but

    Is it not true that the outside skin n foam can move more than the carbon?

    So, in an impact, does the carbon fail first?
     
  8. gonzo
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    gonzo Senior Member

    That is not correct. Both skins can be in compression or tension. In fact, when there is an impact, the area that is deformed will have parts in compression and parts in tension. What you are suggesting is a laminate that will be grossly unbalanced. The difference in modulus between the fibers will create large shear stresses in the core, which is likely to make it fail.
     
  9. fallguy
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    fallguy Senior Member

    It is indeed an interesting concept to develop a technically unbalanced sandwich. If, for a moment, we consider plywood the inside skin; then an impact to the exterior would crush the core. Carbon would not behave much different except at failure; it would elongate and break and the core would shear on the inside.

    I'm am a little surprised you wouldn't want to or advocate to test it more Gonzo.

    One must admit the weight saving potential is high.

    OP does not suggest where the idea hails from.
     
  10. zstine
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    zstine Junior Member

    Hi, The concept is from Morrelli & Melvin. It is how they built the Rapido Trimarans and is not a 'new' concept.

    When I was speaking of impacts, I specifically said a bolt head or corner of a dock where you expect localized deformation of the core (indentation) and the outer skin to buckle or tear. A thicker E-glass skin is going to be more resistant to hitting a bolt on a dock than a thin Carbon skin. The panel doesn't act like a sandwich beam (2 flanges on a shear web) in this failure case.

    As jehardiman said, the lower modulus of the glass compared to the carbon will move the neutral axis of the sandwich toward the carbon. This is not necessarily a problem. Assuming the thickness increase of the outer skin is proportional to the difference in material strength (approx double the thickness of E-glass to Carbon), then the NA shift is proportional to the ratio of young's modulus, ~ 3:1. So the NA on my 1/2in thick core is 1/8in from the Carbon and 3/8in from the E-glass. This NA is the same distance from the E-glass skin as it is in a 3/4in thick panel made of E-glass on both sides symmetrically. Without a detailed analysis, I don't see why my panel would fail with the neutral axis at 3/8in from the skin when another panel with the same 3/8in NA distance and same skin would not fail?
     
  11. gonzo
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    gonzo Senior Member

    That is not a good comparison to a cored laminate. Plywood has layers of similar physical characteristics. Also, it depends on the impact and what it is impacting. A sharp, hard object will generate a different forces than a blunt soft one.
     
  12. gonzo
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    gonzo Senior Member

    I didn't say it couldn't be done. My question is why use a cheap fiber on one side and a very expensive on the inside. An impact will create tension and compression forces at different locations. Also, you are not considering that the core the strength of the skins is irrelevant if the failure is on the core.
     
  13. zstine
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    zstine Junior Member

    I did answer your question of why I think mixing E-glass and Carbon is a good idea, post #52. It helps balance cost with light weight and it helps 'toughen' the outer skin compared to a thin carbon skin. As it is, my cost estimate is $4200 for Carbon and about $800 for E-glass (about 100yrds each 11oz C & 1708). If I did all carbon, It would increase cost by very roughly $3400... I expect the internal carbon skin to save me about 50lbs. (light ship 700lb target)

    If you wanted a light weight construction but also want to control costs, what laminate schedule would you suggest?

    Core failure is a potential failure mode. I have yet to analyze what the failure mode of this panel is... that's going to take some time.
     
  14. gonzo
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    gonzo Senior Member

    If I were to control costs, I would laminate with either vinylester or polyester resin and S-glass.
     

  15. fallguy
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    fallguy Senior Member

    The comparison is relevant because the behavior of carbon would be similar to plywood. The exterior core would flex more than the inside core. Under extreme loading, the core and carbon would crack before the exterior skin. The idea is at least intriguing. And a plywood skin of say 4mm ply on a core is still a cored laminate if we are going to be nitty here.

    I think the idea is very interesting and the only thing that worries me is how they perform under things like heat and accidents. I had a problem here where I failed to vent an enclosed compartment and my laminate got stretched. It would be quite horrible to post cure a hull with different skins and fail in the oven, for example.
     
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