Maximum permissible glass content

Discussion in 'Class Societies' started by Deep6lue, Sep 10, 2019.

  1. Deep6lue
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    Deep6lue Junior Member

    Hi

    Does achieving a high glass content, say 70% by weight, have a negative effect on any of the mechanical properties of the laminate? I'm aware that a higher glass content improves mechanical properties in general. But is there a max. limit beyond which it is bad for the structure?

    Is there a max. permissible glass content value in any of the Class Societies' Rules (didn't find any in my search)?

    TY
     
  2. TANSL
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    TANSL Senior Member

    In my opinion, it is not that the greater or lesser% by weight of glass is good or bad. The strength of the laminate increases as the percentage of glass increases. The problem is that with an insufficient amount of resin the laminate may not be sufficiently locked.
    On the other hand, in practice figures greater than 65% glass are difficult to obtain.
     
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  3. fallguy
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    fallguy Senior Member

    A dry laminate is a fear in my own build.

    Testing showed I was at 63-67% glass in my bulkhead sampling.

    One guy said I was dry another guy said I was perfect on destructive testing. I kept going because I could not delaminate the matrix from the core, but the matrix could be pulled apart strand by strand, layer by layer. The 'perfect' guy said that was proper.

    Still makes me nervous. Wish I had run at 60%..less microvoids..

    70 percent will be trouble I'd say...unless single use and not many miles
     
    Last edited: Sep 11, 2019
  4. rxcomposite
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    rxcomposite Senior Member

    That is a difficult question to answer. Different resins have different densities and so does the fiber. Calculations using the weight method is an inaccurate way. The better and more accurate method is to use Fiber Volume ratio rather than weight ratio. Then you have to deal with fiber packing efficiency due to type of weave or none of it. CSM has a very low efficiency while Uni has good efficiency because the fibers nest more closely together. Then you have to deal with the laminating process. Chopper gun, Vacuum bagged, Resin infusion, Autoclaved?
     
  5. gonzo
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    gonzo Senior Member

    Higher glass content does not necessarily improve mechanical properties. A resin-rich laminate with a softer resin can be tougher. Low resin laminates may have higher tensile strength for their weight, but are also more brittle.
     
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  6. upchurchmr
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    upchurchmr Senior Member

    Depends upon the property you want.
    BUT, my old aerospace job targeted 67% fiber volume since more decreased strength due to not enough epoxy holding the fibers.

    But you can probably find a different number depending on the fiber and glue and the desired property.
    You ought to look at the volume fraction when a bunch of circles are close packed. Meaning how much space is left for glue when the fibers touch. Rite around 67% I believe. That is for unidirectional fibers - not a weave.
     
  7. Eric ruttan
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    Eric ruttan Junior Member

    fibers are round. Draw circles on a page. Pack those circles as tight as possible. Now calculate the free space. Thats your max fiber tratio. Probably ~89 %.
    Now imagine a 3d model with fibers both ways. Max ratio is about 78%.
    Given high end aerospace never achieve these ratios, they are theory.
     
  8. rxcomposite
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    rxcomposite Senior Member

    Correct. But the fibers are sometimes just 7 micrometers in diameter and about 25mm in length. It is bunched together to form a filament called tow. The tow can be flattened out during weaving increasing its packing efficiency factor. Standard prepreg technology sell their product at Fiber Volume 60% carbon, 50% glass. A little bit higher glass content can be ordered for autoclave use but that is special.
     

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  9. Eric ruttan
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    Eric ruttan Junior Member

    I am not sure I understand anything you said.

    But infusion will always get a better fiber ratio.
     
  10. rxcomposite
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    rxcomposite Senior Member

    Yes, always. Prepregs has a higher fiber ratio and is controlled in manufacturing by the supplier. Requires pressure oven to melt and cure the resin integrated in the cloth.

    You did not look at the attached image. It is exactly as you said, only illustrated.
     
  11. Eric ruttan
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    Eric ruttan Junior Member

    I did look at the attachment. Example D is a more realistic version of uni stacking.

    The other examples I do not understand.

    But you keep saying prepreg has the best ratios. It doesn't. Vacuum infusion does.
     
  12. rxcomposite
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    rxcomposite Senior Member

    What? Where did you get your data? You say you do not understand what I am saying yet you have opinion. You said you looked at the attachment but I can see in the post that it has no view before you responded.
     
    Last edited: Sep 14, 2019
  13. Eric ruttan
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    Eric ruttan Junior Member

    I do not know about the views. But I have a copy of that file on my phone. Perhaps you should ask an admin?

    Perhaps it was misleading to suggest you can get better ratios with infusion. If you are putting the part in a billion dollar autoclave, then, ya, you get good stuff.

    But if you are doing 'not aerospace' budget stuff, you can get near perfect ratios, very low voids, and perfect fiber orientation with infusion. And save a billion dollars.

    My data is from conversations with composite engineers and industry observations. Boeing and Airbus are all moving to infusion. All the windmill guys infuse. Infusion is where all the action is.
     
  14. fallguy
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    fallguy Senior Member

    There is a real problem with that number. It would be starved. If not; it would be the goal and never is...not because humans cannot achieve it, but because it is not desired.

    If you have a citation from professional literature; please give it. All my reading has never suggested those kinds of numbers would be successful.
     

  15. fallguy
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    fallguy Senior Member

    It could be your weaving theory.

    The fibers are never packed that tightly.
     
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