Sandwich construction question

Discussion in 'Fiberglass and Composite Boat Building' started by Cpalm, Feb 8, 2012.

  1. Cpalm
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    Cpalm Junior Member

    What would happen if a third skin were used on a sandwich laminate, in the centre? Essentially dividing the laminate into two cores. Would this add strength or cause unforeseen problems?
     
  2. tunnels

    tunnels Previous Member

    Take two step backwards and give a little more information about where you would want to use it !! and the thicnesses of core you would be wanting to do it with !!
    Foam comes 40mm thick so two layers ?? 80mm plus ??
    Foam also can be 5 mms thick and why would you want to make 10 mm when you can by it off the shelf ??
    Have seen Balsa core 300 mms thick !!
    Bending thin sheets of foam into a curved place you're better to use corebond between the sheets and bag it down .Core bond will and does hold better than just glass and resin !,as it has a slight element of flex that glass and resin dosent have !! Used this once making some floor to ceiling panels for units with big curved ends and got wood veneer applied to the outside seen surface . :p
     
  3. Cpalm
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    Cpalm Junior Member

    You're over complicating this question I'm speaking hypothetically. In a simple panel lets say a rectangular board 4" thick is there a benefit to putting an additional fiber layer in the center, with a core on each side and fiber skin on top of that.

    Is there any strength gain or does it cause other problems?
     
  4. tunnels

    tunnels Previous Member

    If for argumentts sake you wanted to make a table top 100mm thick there wouldnt be any advantage glassing the layers together ! ,just corebond it together . i
    n the case of the table carrying a heavy load still not much advantage as its the outside skins that are doing all the work and the core is only holding the glass skins apart !!
    If it was used in something like a ice breaker type situation where the chances of punching a hole through the panel were very high !,yes a good layer of kevlar between in conjunction with epoxy type resins could make the panel much more robust and water resistant and get you home for repairs .
    But there would be other ways of making a panel stronger rather than doing a layer of glass sandwiched between the 2 layers . It could also be an advantage against the possabilities of core shear where the core actually shears under extreme load !! . But again these are extreme situations not the normal everyday types of situations .The added cost of materials and labour would likely put paid to the idea . But those are just my thoughts .
     
  5. Cpalm
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    Cpalm Junior Member

    Thank you that answers my question nicely! Ive read that chopped strand mat improves bonding between glass layers, would that also be true if the core had to be made from several layers of core material?
     
  6. Eric Sponberg
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    Eric Sponberg Senior Member

    Cpalm,

    In general, no, glass in the middle of the sandwich does not add too much, so it is mostly added weight and cost with no strength benefit. A composite sandwich isolates the load paths to tension in one skin, compression in the opposite skin, and shear through the middle where the core is. The primary load that causes these stresses is pressure that bears on one of the skins normal (perpendicular) to the surface.

    So, to understand how the sandwich works, imagine that you are holding a peanut butter and jelly sandwich in one hand, and you place your other hand on top of the sandwich, and you try to slide the two slices of bread apart. One hand represents tension, the other hand represents compression, and the peanut butter and jelly in the middle are trying to hold the sandwich together--they are experiencing the shear. The more peanut butter and jelly you have (or the thicker the core) the more easily they can resist shear. Also, the denser the peanut butter and jelly (say you took the PB from the bottom of the jar where the oil had separated out), then the more shear strength it has. Same way with the core--the denser it is, the more shear strength it has.

    Glass has tremendous shear strength and stiffness, but because of that, placing it in the middle of the sandwich introduces discontinuities, weight, and cost into the mix. It is better to use a compatible layer or material when stacking up layers of core, such as the core bonding materials that are available--they are tougher and lower strength/stiffness than fiberglass mat and resin. Just plain neat resin or resin with coloidal silica in it is better than fiberglass.

    I hope that helps.

    Eric
     
  7. Cpalm
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    Cpalm Junior Member

    Thank you for the detailed explaination...very helpful
     
  8. david@boatsmith
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    david@boatsmith Senior Member

    that would be a club sandwich, ya
     
  9. Herman
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    Herman Senior Member

    The glass in the middle is on the neutral axis, so does not much in terms of adding stiffness. Better to add the glass on the skins, or use the weight to add more foam (more thickness).

    However, there can be other reasons to add a fabric in the middle. If the foam is not available in that thickness, and infusion is the preferred fabrication technique, a glass in the middle sometimes makes it possible to do all the laminating in 1 step. By impregnating the dry fabric it glues the 2 cores together.

    Also when using 2 layers of Soric, a glass in the middle helps distributing compressive loads over the misaligned cell walls.
     
  10. tunnels

    tunnels Previous Member

    Long time ago i did a little exsperiment with resin and glass bonding of cores and core bond bonding and the core bond won hands down so i have no quarms about the stickability of corebond in a foam to foam situation !!!
    Was always a litttle concerned with mixes have used over the years of the secret recipies each differant place used where iv'e worked .:p:D:p
     
  11. brian eiland
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    brian eiland Senior Member

    Club Sandwich

    Agreed but what if you were building a larger vessel with a honeycomb core such as polypropylene and you were contemplating a double skin sort of like a double planked wood vessel ?? In this case you would need a cell sealing layer between the two sandwich panels.

    sorry got to run at the moment and can't complete this thought
     

  12. brian eiland
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    Location: St Augustine Fl, Thailand

    brian eiland Senior Member

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