Elongation Characteristics on Hull vs Bulkheads?

Discussion in 'Materials' started by Iridian, Nov 7, 2021.

  1. Iridian
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    Iridian Junior Member

    If you had a hull made of fiberglass foam sandwich, but bulkheads and fore aft foam sandwich supports running made out of carbon fiber to decrease weight and increase stiffness, would the bulkheads / fore & aft laminates break before the hull takes much of the load, making the idea not useful?

    https://i.imgur.com/MhNWOo8.jpeg
     
  2. bajansailor
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    bajansailor Marine Surveyor

    Is this in relation to the hull in the photo in the link above?

    If the bulkheads and fore aft foam sandwich supports (longitudinal girders?) are made out of carbon fibre foam sandwich, and they are designed to have the same strength as heavier bulkheads and girders made out of fibreglass foam sandwich in relation to the foam sandwich fibreglass hull, then I don't see why they should break before the hull takes much load?
     
  3. Ad Hoc
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    Ad Hoc Naval Architect

    That is for YOU the designer to establish and thus, which member you wish to fail first.

    Structure fails owing to insufficient: stiffness, shear area, material strength, fatigue.

    Therefore one designs each member accordingly. Starting with the global structure, then secondary structure and finally the tertiary structure.
     
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  4. Iridian
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    Iridian Junior Member

    Mostly just my own curiosity. That image is an example of where I thought that potentially the bulkheads and longitudinal supports could be made of carbon fiber to reduce weight.

    Since the carbon fiber backbone and bulkheads would have much lower elongation to failure (1-1.5% vs 4-5%), my conjecture would be that the carbon fiber would support the vast majority of the forces before failing, and then the fiberglass hull would take the rest before failing. Essentially, they wouldn't be working together additively, but rather independently to a large extent.

    It's less about wanting one to fail first and more about if the hull will add much strength to the backbone/bulkheads because of the greater backbone/bulkhead stiffness and lower elongation characteristics. If the carbon fiber takes all of the forces with the hull taking almost none, and then the backbone/bulkheads fail before the hull takes it all, it might be better just to have the whole thing be fiberglass?

    I feel like I'm probably missing something though.
     
  5. Ad Hoc
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    Ad Hoc Naval Architect

    You are discussing primary or global load analysis.

    As such, if you wish to make the hull (the shell/plating) stiff, the question you must ask is, will those long.t be full depth?
    Forget elongation - you're missing the point.

    If you apply a load to a simply supported beam, and the beam is an I-beam, with the flanges being material A and the web being material B, you can design the I-beam to do what you want.

    Since given that the dimensions of the I-Beam remain pretty much constant (like your hull) then the depth is 'about' the same, and the flange is 'about' the same, you can decide how and where the first failure occurs.

    If material A is rubber and material B is steel and you apply a load, what will happen?
    The flanges, being rubber, will flex not absorb any load and thus will be unable to transfer any load to the web, which will be in steel.
    If the load is applied directly over the web, then the flanges, being rubber, still take no part, but now the steel web, will take the load. So the stiffness (2nd moment of inertia) of the web is important.
    And since we are taking about the structural stiffness the "EI", you have calculated the I of the web, and then you just add the E, of the web, in this case steel, and you get the direct bending stress, shear stress and deflection.

    But in this scenario, if the steel web is able to take the load with minimal stress and minimal deflection (inside material yeild properties), the question then becomes, what about the flanges... well, it is rare to have the load applied directly above the web, since the load transfer is via the flange, which means these rubber flanges, will be no good, too flexible and unable to carry the load.

    If the reverse were now applied....the now steel flange can take the applied load, but the web, now rubber, will flex.
    The question then becomes how much flex is acceptable, and can the rubber web carry the load and transfer it to the opposite flange?
    Most likely not... so even in this case, this is still not acceptable.

    The point being, you need to establish the load paths and as such, establish the direction you with the load to go.. the load path.

    The objective being, the flange and web work as one member, and that the transfer of load is possible from upper to lower flange, so the beam bends an an I-Beam, and not a flange on its own.

    Then it becomes a simple matter of how much material of web/flange combo you want and feel will do the job, so that under any applied load the I-beam will always work as one member and failure at the same value of the other material.

    That is it in a nut shell.

    So back to your hull... you make the hull fail and the same rate as the long.t backbone/BHD....whatever the material.

    So the applied load is then a secondary load, i.e a local load and no longer a primary/global load problem.
     
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  6. Iridian
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    Iridian Junior Member

    Thanks @Ad Hoc . I think I am following now. I hadn't been thinking about the load transfer in context of the beam.

    Will leave the designing to the pros, but I want to know enough to second guess when something doesn't look/sound right, and know when to ask the right questions about what I can do given a budget.
     
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