Strength of Fiberglass/composite Repairs

Discussion in 'Boat Design' started by Barry, Mar 14, 2023.

  1. Barry
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    Barry Senior Member

    Over the years of participating in this valuable website, I have wondered how strong a fiberglass/composite repair actually offers. While fiberglass strengths were briefly touched on in the Strength of Materials courses that I took, decades ago, our instruction was based on whole beams/plates/structures where the actual glass fibers are the primary and CONTINUOUS load carriers within the binder/resin/fiber structure, with the resin mainly holding the fibers in place.

    There have been many threads where contributors have offered solutions to repair where glass and resins are, for example, merely adhered to a structure and not offer the continuity of the fibers to bridge the gap and transfer the loads, relying on the "glued" joint to carry the loads from section to section.

    To provide a structure for discussion, take a 4 inch x 4 inch beam by 4 feet long, simply supported, cut a 6 inch piece out of the middle, add the 6 inch piece back in, re-epoxied, without any fiber penetration across the original beam joint, how strong would such a joint/beam be as compared to the original.
     
  2. fallguy
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    fallguy Senior Member

    The local expert willing to do the explanation is @rxcomposite .

    But there are so many unknown factors in the description.

    Is the original beam wood or something else? About the only 4x4 structural beam I can imagine is wood and in a marine environment wrapped to prevent rot. If so, the issue is then the strength of the bond versus the strength of the wood. And so many factors such as build quality. Did they precoat the core? Is resin starvation possible?

    In many cases, once someone has boogered up their boat; the best effort is all there is. That fellow who cut is stringer web for the tank comes to mind. He did so against most sage advice. So what now? Don't bother trying to help him avoid catastrophe? Get to a single safety factor? Oftentimes, something beats nothing.
     
  3. Barry
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    Barry Senior Member

    The 4" x 4" x 4' beam is pure composite and the dimensions were just a numbers to provide a focal point. No wood, not any real life reference. The query is to understand the strength of the joint as compared to the original.
    There have been hundreds of threads from learned contributors on how to tab, replace, repair etc. A homogeneous composite beam will have certain tensile and shear strengths, but how much load/stress will the resin/epoxy carry without the
    interconnecting strands of fiberglass/Kevlar etc bridging the joint

    I chose a beam with fixed dimensions as a focal/discussion point only.

    I do understand that you would not merely add in the 6 inch section with the same dimension, more than likely extend new fiber material across the joint to increase strength.
    Similar to fish plating metal butt weld beams
    [​IMG]

    I would not make up a fish plate in this configuration but this concept picture was on the internet
     
    Last edited: Mar 14, 2023
  4. Blueknarr
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    Blueknarr Senior Member

    Simply but gluing the section back in would only be slightly stronger than with the section missing.

    Properly done FG repairs are scarfed. The length of scarf depends on the fiber and resin used originally.

    However, shortcuts or poor craftsmanship will produce a less than ideal result.

    If the same resin and fiber with sufficient scarf is used the repair will have the identical properties as the original construction.
     
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  5. Will Gilmore
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    Will Gilmore Senior Member

    [​IMG]
    A scarf joint is very strong, when done properly in glass, wood, metal, but it requires the removal of extra material.
    In wood, where grain direction is a concern with regard to gluing, it is the best possible joint. But the finger joint is similarly strong without having to remove as much of the remaining sections.

    For a glass joint on something like a 4 x 4 beam, I would want to sand down the ends of the whole beam to something like a 3.75x3.75 beam, for a few inches at the repaired ends, then cut in a piece using a finger joint, finishing with a sleeve wrap to rebuild to 4x4. I can't imagine that won't be like new.

    -Will
     
  6. rxcomposite
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    rxcomposite Senior Member

    Is this a box beam or a solid laminate?
     
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  7. Barry
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    Barry Senior Member

    rx
    This not a real life beam but I was wondering more from an engineering/design viewpoint, if the thread fibers do not cross the interface between the two ends, how much strength is available from only the adhesion of the introduced resin

    I guess the 4 x 4 is large and perhaps I should have chosen say 1 1/2 square tubing with a 3/16 inch wall.
     
  8. Will Gilmore
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    Will Gilmore Senior Member

    Not much. In terms of fiberglass repair, the point of the scarf is to provide that interlocking of fibers.
     
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  9. rxcomposite
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    rxcomposite Senior Member

    Epoxy resin has a minimum of 13.8 N/mm2 shear strength and tensile strength varies from 64 to 74 N/mm2 (depending on source). So if you cut a laminate in half and join them together by epoxy, you will be limited by these values. Even if you scarf it by 45 degrees to increase surface area , the most shear strength you can get is only 1.44x.

    The reason I asked if this is a solid laminate or a box beam is because there are two different set of formulas to be used. 1 1/2" tube seems to be a good number.

    From an engineering point of view, these can be proven and I am sure you are very capable. Needs only the 1st principle of engineering. You need to model a hollow beam first and see what tensile/compressive strength you are getting and the shear strength. Then compare to what you will be getting if you cut it in half and just glue them together. Maybe not just limit yourself to epoxy but consider Industrial adhesive like Hysol or 3M 2216.

    This is an interesting study and will also dig up my data to see.
     
  10. Ad Hoc
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    Ad Hoc Naval Architect

    It depends how good the bond is of the new resin to the original...
    If perfect, it would only be as good as the max shear stress, since the mode of failure would be via shear, not tensile.
    If the bond is not good...then of course it would be much less...as this is where it will fail first....again, via shear.
     
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  11. Will Gilmore
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    Will Gilmore Senior Member

    I would think a standard fiberglass repair scarf would be in excess of 7:1. Wood scarfs are recommended at that angle.
     
  12. fallguy
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    fallguy Senior Member

    7x4" is 28" scarf

    the sample part is 48"

    Cut two 28" scarfs and you are beyond the beam..

    And and always, workmanship becomes a major issue.
     
  13. wet feet
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    wet feet Senior Member

    I'm pretty sure the reference means seven times the skin thickness rather than the beam depth.
     
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  14. Will Gilmore
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    Will Gilmore Senior Member


  15. fallguy
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    fallguy Senior Member

    I thought there was no skin in this repair..
     
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