Skin thickness

Discussion in 'Fiberglass and Composite Boat Building' started by Tungsten, Dec 25, 2013.

  1. Tungsten
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    Tungsten Senior Member

    After cure of my Infusion samples I;m amazed at how thin they are compared to wet bagging.One sample was 4 layers 400g double biax,infused measured at.055" after cure.I'm no engineer but this seams thin to me.

    I'll have to do some cored samples to see how this affects stiffness/strength.

    I was under the impression that .1" was considered a stout outer skin.If this is true then I'll need approx. 3200g/m2 layup to get that thickness with infusion.

    I hope I'm wrong.
     
  2. PAR
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    PAR Yacht Designer/Builder

    A hand applied 12 ounce (407 GSM) 45/45 biax is about .022" (.55 mm) at 30/70% fiber/resin. If mashed to 70/30 fiber/resin content, this changes to about .013" (.33 mm).
     
  3. Ad Hoc
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    Ad Hoc Naval Architect

    That is 4 x 400gms = 1.4mm in 'normal' units. Which equates to 0.088mm per 100gms

    Last infused project i worked on the yard were able to get 0.072mm @ 100gms, so it is consistent.


    The stiffness is related to the geometric shape, the second moment of area. The structural stiffness is the "EI". Thus change the shape and it changes the "I".
     
  4. Tungsten
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    Tungsten Senior Member

    Oh yes 'normal' measurments,
    So I've since cut the sample in half and measured in the middle.1.45-1.55 mm

    When use too testing hand layups this sure seams thin.

    Thanks,
     
  5. PAR
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    PAR Yacht Designer/Builder

    It is thin and that's the point. It sounds like you're getting good fiber/resin ratios, so . . .
     
  6. groper
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    groper Senior Member

    This is the idea with infusion Tungsten... you get get much higher fibre to resin ratio, which means thinner laminate for the same cloth weight because there is less resin in it.

    If you compare 2 samples of equal thickness, 1 hand laid and 1 infused, the infused sample will be FAR superior in all mechanical properties. It will require more layers of cloth tho for it to come out equal thickness.
     
  7. Ad Hoc
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    Ad Hoc Naval Architect

    Need to be careful what you're saying there Groper.

    It is the structural geometric shape coupled with the thickness (for the area) that provides the stiffness. If your skins are thinner, then you do not have a stiffer section. You may have a slight improvement in the mechanical properties being infused. Its the EI that's important, the E changes (but one needs to test it to establish how much), but the I changes significantly when thinner laminates are used.

    And when too thin, buckling begins to dominate.
     
  8. groper
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    groper Senior Member

    True AH, important to note that the structure E*I dominates as you said earlier.

    To clarify, Tunsten has been building (already built) his own sandwich panel boat from foam core and epoxy e-glass laminates. Referring to this, i dont beleive we are discussing solid laminates here, but rather foam sandwich panels. Thus the "I" is dominated by the foam core thickness and we can basically ignore it for comparison.

    Infusing a solid laminate, for purposes other than specific heavy guage solid laminates such as masts, spars, beam tension/compression caps, composite chainplates etc etc doesnt really hold an advantage over hand laminating as it reduces the thickness which is often required to achieve adequate stiffness - such as a solid glass hull panel for example.

    So Tunsten, you need to clarify what it is your trying to achieve, when considering the strength, stiffness and laminate thickness.

    the mechanical properties of infused laminate VS hand laid can be generally compared by looking at the appendix of the following text;
    http://www.google.com.au/url?sa=t&r...=gnx_0r4Cch7yGnH8xl_2YA&bvm=bv.58187178,d.dGI
     
  9. Herman
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    Herman Senior Member

    I entered biax400 into my Excelsheet, and at 50% Vf (fiber to volume fraction) I get 1,27 mm for 4 layers.

    So your measurement of 1,5mm is not even on the thin side, given the fact that most infused biax400 even goes over 50% Vf when infused. (usually anywhere between 53 and 60%)

    Just to let you know that you need to expect your laminates to go even further down in thickness....

    And yes, loss of thickness can be a problem. Also density increases, as fiber is heavier than resin.
     
  10. Tungsten
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    Tungsten Senior Member

    Yes groper is correct this is for foam core.
    My tests so far have just been glass on a flat surface,today I plan on infusing some foam.

    By going by my previous build stiffness is not a problem.What I'm trying to achieve is a little more strength when it comes to hitting rocks .

    One value I'm missing so I can compare is the outer skin thickness from my last build.This was wet bagged @ 50% vac so I'll have to make up a sample of this also.

    Thanks for the replies.
     
  11. groper
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    groper Senior Member

    Then what you need is not solved via the laminate thickness in isolation.

    You need to look at what is underneath the laminate... The core...

    Consider a thin layer of plywood, say 4mm, then the foam to the inside. You need something with much higher compressive strength to prevent localised impact damage. There are synthetic alternatives to ply but they do get expensive...

    Failing that approach, consider going to a solid glass laminate instead of foam core, and Use a floor stiffening grid to achieve a reasonable stiffness to weight structure...

    Foam sandwich panels at the end of the day, are not a good idea for anywhere which is subjected to impacts...
     
  12. Ad Hoc
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    Ad Hoc Naval Architect

    You wont get that with a sandwich construction.

    Impact loads are all about localised loads, thus only the outer skin pays a part. You end up with a puncture of the outer skin. It is the reason every commercial workboat that needs to consider such events (from debris in the harbour, against the jetty or another boat etc etc) always use single skin. The UK lifeboats, all single skin.

    The vessel I mentioned above, was a workboat. Single skin bottom and sandwich sides.
     
  13. Ilan Voyager
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    Ilan Voyager Senior Member

    All the lifeboats I know are single skinned (except maybe some Norwegian lifeboats for oil platforms, but I'm no sure). In single skin, a good way to get thickness without too much weight is to use a bulk agent like coremat or matline, which are polyester fiber felts and microspheres. 600 kg/m3 instead of 1550 kg/m3, so it's interesting as "poor man's sandwich", rather cheap and do not delaminate. The impact resistance of such panels is rather good, very close to all fiber panels. That works very well on decks for commercial fishing boats and also diving boats where the cylinders and lead weights can destroy a deck in a few weeks.

    Foams, and honeycombs are incompatible with rocks...for example kayaks, like workboats, are single skinned for evident reasons. In fact for leisure boats used and abused on rocks, gravel, concrete and other abrasives the winner is the humble polyethylene.
     
  14. PAR
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    PAR Yacht Designer/Builder

    I've always seen single skin in the heavier duty service laminate schedules and as Ilan mentions, polyester or modi-acrylic bulking fabrics employed to gain bulk/toughness.
     

  15. Tungsten
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    Tungsten Senior Member

    Well I've thought about single skin, some flex to float over shallow bars would be nice,a double floor in the center so no oil canning is felt when standing.
    A male mould covered in plastic could work I think?With all the layers needed infusion would be the only way I would guess.If say a Ve resin is used do you still use matt between layers of glass?(with infusion)

    Thanks for your help.
     
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