foam-fiberglass cloth sandwich for hull

Discussion in 'Fiberglass and Composite Boat Building' started by petethai, Jul 14, 2013.

  1. petethai
    Joined: Mar 2009
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    petethai Junior Member

    your opinion on making foam-fiberglass sandwich hull

    fiberglass cloath
    5mm of foam
    fiberglass cloath
    5mm of foam
    fiberglass cloath
    5mm of foam
    fiberglass cloath

    the foam can be perforated so the resin may go between foam layers
    any opinion?
     
  2. tunnels

    tunnels Previous Member

    ok can you please explain in detail why you would make such a laminant ??? is it the shape you are making that is difficult to bend over ??
    why not simply use-
    2 x cloth ?
    15mm core ?
    2 x cloth ?
    :confused:
    simple is always best !! :)
     
  3. petethai
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    petethai Junior Member

    this why I want use 5mm foam
    1/ shape
    2/ with multi layers of foam I can build panels with different characteristics ( by using different cloth)
    3/ I have access to 5-6 mm foam

    isn't multi layers panel stronger?
     
  4. tunnels

    tunnels Previous Member

    what is you deffinition of strong ???

    The weakest part of any foam sandwich laminate is the bond line between the core and the glass !! the density and type of foam core plays a really big part in all this as well !!
    not all foams are made equal and there's a daunting choice to pick from !!
    so choose wisely read and o your home work !!
    so how many bond lines will you have with your panel ??
    with the one I gave there only two !!
    what are you actually wanting to make ??
    or just asking questions in general ??
    we will help in any way possible !!

    The word strong can have many different meanings and be interpreted in lots different ways also . strong by its self means nothing !! :eek:
     
  5. rwatson
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    rwatson Senior Member

    Tunnels has made some valuable points to think about.

    As he said, if you can get 5mm foam for cheap, you will get just as strong a hull as laminating the foam into a 15mm core, then glassing the outsides.

    "The word strong can have many different meanings and be interpreted in lots different ways also . strong by its self means nothing !!" is your major question. there are lots of ways of measuring 'strength'
    ,
    http://en.wikipedia.org/wiki/Strength_of_materials


    "the foam can be perforated so the resin may go between foam layers"

    This idea doesnt have any merit. If you don't put the majority of the glass weight on the outside, then you will most certainly have multiple weaker layers damaged on an impact, and also the weaker layers in the centre will not contribute to the important structural integrity.
     
  6. PAR
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    PAR Yacht Designer/Builder

    Petethai, it would be best if you studied the concept of cored structures, before attempting to engineer a new type of laminate. The whole idea between cored sandwich structures is the distance between the surrounding laminate skins. Decreasing or adding multiple, internal skins will just setup shearing loads and internal delamination issues, within the sandwich, which is something to be avoided at all costs. If you need to get the core to bend over compound surfaces, there are several techniques that will work, including diagonal planking, scoring and others. In short design the sandwich around solid engineering principles, so there's no surprises come launch day. If you have a bunch of 5 mm foam, glue it up so you bulk the core to it's desired skin separation thickness. This will permit things like double and triple diagonal planking of the core, which will solve any fitment issues. If the surfaces are too compound for even this approach, you can still score the core or simply use narrower strips in the diagonal planking method.
     
  7. petethai
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    petethai Junior Member

    so in your opinion it will be no advantage from multi-layers, because shearing loads problem
    I was thinking that it will be no problem if I perforate the foam, so the resin will make cross matrix between all glass cloth after infusion
     

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  8. PAR
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    PAR Yacht Designer/Builder

    Make up some 12" square test panels and see for yourself.
     
  9. tunnels

    tunnels Previous Member

    no matter what approach you take with multi layers of thin foam there will always will be the core surface peel problem ,always !! same with the type of resins it will still peel !!!
    in the case of impact and bending and movement shear is 5 times more likely to happen at the point of impact ,after the foam has sheared then peel will take over and if there is still further movement from bending almost total destruction of the panel end to end once shear/peel starts there is no way to stop it !! non at all so could lead to a complete loss :confused::eek:

    The point you are missing is the foam is the weakness !! the foam comes apart has little to do with what ever resin is used be it poly vinyl or epoxy and same with the layers of glass its the foam that's the problem when you see the structure of all kinds of foams under a microscope its scary stuff !!!
     
  10. rwatson
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    rwatson Senior Member

    The real point is, if the outer layers cant resist damage, you cant rely on the inner foam to provide very much help.

    The foam only provides compression strength, the outside layers do the lions share of the work.
     
  11. petethai
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    petethai Junior Member

  12. HakimKlunker
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    HakimKlunker Andreas der Juengere

    That makes little sense to me:
    The size of your construction is not known, fibreglass cloth can be 200g/m2 or 2000 g/m2, and the building method is unclear (mould? jig? )
    A multi-layer core - especially in stronger curvatures - gives you a risk of bonding failures and would require a reliable vacuum application.
     
  13. petethai
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    petethai Junior Member

    I just finished small test
    because I don't have now any resin, I am using 2 and 3mm plywood, glue and 300xps ( I only have ~1, 2 and 3 mm plywood

    so I have 3 panels -size 40cm x 40 cm
    1/ ply / styropore (15mm) /plywood
    2/ 2mm plywood / 5mm styropore- perforated/ 1mm plywood / 5mm styropore- perforated/ 2mm plywood
    3/ 3mm ply/styropore 15mm - perforated/ 3mm plywood
    in panel nr 2 and 3 styropore was perforated, so glue can make cross matrix between plywood

    I don't have any lab, so 5 Kg hammer from 3m, and hydro car jack for bending was used

    looks like the matrix between make the panel stronger and stiffer
    panel nr2 was a winner and the nr 3 was better as nr 1
    net of glue helps with shear loads
    no panel survive full loaded concrete truck (we pour the concrete in my house foundation), but I must say most damage was from gravel base
    I think the multi layer panel with perforated core is much stronger and stiffer as simple core
    is it true for epoxy composites - I DON'T KNOW

    A and B - skin
    between A-B-A we have core (perforated)
    C- glue (epoxy) makes panel more shear and bending resistant and stiffer
    please correct me if I am wrong
     

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  14. rwatson
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    rwatson Senior Member

    Great innovation, but there is one obvious flaw in the methodology.

    The plywood is not 'proportionally comparative'.

    What you should have tried is say - 3mm ply on the outside, and 1mm ply in the number 1 tests. Multiple 2mm ply is significantly more material.

    The evaluation should also compare the weights of the sample tests, as this is an important factor in boat design.

    The 'approved' panel test is not that dissimilar to what you did. Support the test panel on all four edges.

    Then get a spherical weight, and drop it from a preset height until the panel shows serious damage. The one that lasts the longest is the winner.
     

  15. HakimKlunker
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    HakimKlunker Andreas der Juengere

    I like what you did. There are so many here, who only TALK, but seldom touch ...
    What I cannot see in your 'report':
    - In what manner did the test panels fail?
    - you say ...'net of glue helps with shear loads'... is that an assumption, or how do you come to the conclusion?
    A reinforcement (ply; glass; ...) in the centre of a sandwich contradicts everything I was ever taught; but I admit that I simply believed and never tested myself.
     
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