Breaks in corecell

Discussion in 'Fiberglass and Composite Boat Building' started by fallguy, Oct 21, 2017.

  1. fallguy
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    fallguy Senior Member

    where do you allow breaks in corecell in components like bulkheads or decks?

    Obviously a long panel requires breaks, but would you, example, allow a break in a bulkhead?

    If a bulkhead is over the size of the panel size; do you care which way the edges meet or does it not matter?

    The reason I ask is just to understand if others have build standards they follow or if it is really anything goes.

    If you had a demand for a 4' wide bulkhead, for example, would you ever consider making it from pieces, or is there a structural deficit at the bondline?

    It seems like this should be in the manufacturers processing guides, but I did not find it.

    Certainly bulkheads are built that require larger dimensions than core would provide, so is there a requirement when doing so? Or just builder preference, etc.

    When you build a floor in a house, for example; best practices dictates avoiding having four corners of separate panels ever meet, for example.

    Thanks.
     
  2. Mr Efficiency
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    Mr Efficiency Senior Member

    It doesn't matter that I can see, strengthwise, as you know sandwich panels are sometimes made from small individual core squares to conform to complex shapes, with good results. You can glue the butts if you want to, but structurally it isn't necessary.
     
  3. ondarvr
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    ondarvr Senior Member

    Cosmetics are more of an issue with seams in the core than strength, although in some laminates you may slightly shave down the core at a joint so extra fabric can be positioned over the joint so there's no build up, this also helps to hide the seam. Think dry wall in your house.
     
  4. fallguy
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    fallguy Senior Member

    Right, I understand the rebates-I just cut 70' linear feet of it an hour ago, but can you join bulkhead panels just within the lamination or would a rebate and reinforcement be the standard? So far I have been unable to really see any strength affect of the bondlines when we move long panels. Sounds like it is anything goes. In which case, I can bond two 20" x 8' panels the long way to make 40" and vac bag and it is functionally the same thing as a 40" x 8' when done.

    Funny there isn't any standards on it because at least intuitively it seems to have higher likelihood to shear.
     
  5. ondarvr
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    ondarvr Senior Member

    I rarely see the rebates being done, 99.999% of the time they butt the core and leave it as is, then glass the whole thing, most cores are light not strong, so you're not being cheated out of any structural integrity. The resin or putty that fills the gap is strong enough, and even where it's a dry gap it tends to not fail.
     
  6. Mr Efficiency
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    Mr Efficiency Senior Member

    Shear in which direction ?
     
  7. fallguy
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    fallguy Senior Member

    Shear at the bondline. The bondline would be pure epoxy and glass each side and the area next to the bondline would be core. Am I wrong? I don't mind, but if you shear test a panel; doesn't it shear at the bondline? Or is it so close to the same; that is irrelevant?
     
  8. Mr Efficiency
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    Mr Efficiency Senior Member

    Should not have any effect.
     
  9. rxcomposite
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    rxcomposite Senior Member

    Never join cores where the bulkhead will be or any major structural frames/stiffeners because it is the point that receives greatest shear stress. When joining panels, the recommended taper ratio is 1:6 minimum and the joint should be at around 1/4 of the span between bulkheads/frames/stiffeners. The 1/4 span rule is also where you cut the drain holes as it is a point of lesser stress.

    An exception to the rule when the panel is joined together in way of the bulkhead, the 45 degree joint is reinforced by at least two layers of csm between the joints leveling up at least an inch to the surface. The joint thickness is covered by an LR rule that I will have to find in my files later.
     

    Attached Files:

  10. fallguy
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    fallguy Senior Member

    RX, Thanks. I figured there must be a rule or standard somewhere.

    That looks like it could be rather difficult to pull off well on the vac table; worse by hand. Mostly because you'd probably end up with a step if the panels are too close and an air gap if the panels were too far apart, but probably doable with care and some precision. And, that angle is not something to do by hand if you don't want air bubbles, so it would be saw work.

    I have to admit, I'm a little unclear as to where this applies. I read it a few times, but I'm not sure it said this method was needed for hull panel joins. It said beneath primarily longitudinal members, which I assume means hull bottom where shear from say a fall into the trough or from pounding would be serious.

    If you interpret it differently and that it would apply to anytime you max out the core dimension, let me know.

    I think I will use this method in a few of my deck joins where I anticipate a lot of walking, but maybe it is overkill. Just hate to have me and the Mrs. dancing on the deck and hear a big crack!
     
  11. rxcomposite
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    rxcomposite Senior Member

    For your use, you can just glue the edges of the core and position the glue line so that it does not line up with the bulkhead. You can use a bandsaw, tilted so than you can bevel the foam edge. I use greater than 45 deg.

    The shear tie is a must if the core joint falls in line with the bulkhead. Just avoid it with careful planning.

    The need for the shear tie usually comes up during calculations. Usually when joint/bulkhead is a lifting/chock point. It will not show in standard panel pressure calculations. You use it when you specify where the lifting straps will be placed.
     
    Last edited: Oct 23, 2017
  12. fallguy
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    fallguy Senior Member

    Thanks again. I will also make sure to be careful of this at the beam intersections or other lift points.
     
  13. ondarvr
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    ondarvr Senior Member

    Good diagram

    Only a very small number of my customers do it that way, most just butt the foam.

    On curved shapes, most stuff, the foam is slit and on a scrim, so every inch or two you have a but joint anyhow.
     
  14. jorgepease
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    jorgepease Senior Member

    I like to chamfer the edges of core about 3/4 of the way down so you get an easy to fill V but you still have the flat spot to butt them tightly. Then I fill the v with thickened epoxy. At that point the joint is as strong or stronger than the core, just try tearing it apart. Also I offset core sheets just because it helps with lining things up. I put a piece of tape along the chamfer when I fill, the thickness makes it so I only have to give one pass when I fill a little sanding and you don't see any seam.
     

  15. Mr Efficiency
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    Mr Efficiency Senior Member

    I am not really understanding the need for that joint.
     
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