Composite jointing

Discussion in 'Materials' started by Mick@itc, Sep 8, 2011.

  1. Mick@itc
    Joined: Jan 2011
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    Mick@itc Junior Member

    Hi all
    So heres a question. I have been told that you get make a butt joint as strong as the material either side. Is this true? If yes...how could that be? So say we were joining a vertical hull side made of say 1000g glass then 25 mm core cell then same outside with epoxy and say infused. How would we be able to make a joint as strong as the sides?
    Thanks
    Mick
     
  2. PAR
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    PAR Yacht Designer/Builder

    By feathering the existing 'glass back, pushing the new piece into alignment, also with the 'glass feathered to a dramatic taper, then replacing the missing fabrics (both sides) with more goo and material.
     
  3. Mick@itc
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    Mick@itc Junior Member

    Thanks but...

    Thanks for the reply.
    But would the bond between hardened epoxy and new epoxy not be weaker than the original epoxy?
    I'm struggling to get my head around it unless the epoxy bonds to the same strength as the homogenous material. I'm not for one minute saying you are wrong, it just seems to defy normal material properties. But I guess composite material can not be classed as normal.
    Thanks again...
    Mick
     
  4. PAR
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    PAR Yacht Designer/Builder

    A mechanical bond is weaker then a chemical bond, but it's still strong enough to rip out huge chunks of unmolested 'glass work, in the event of a serious test on the area.
     
  5. Stumble
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    Stumble Senior Member

    Mick,

    Think about it like this. In a composite structure the failure point of the weakest point is where breaks will occur. Assuming epoxy is used, the mechanical bond is still stronger than the material it is bonded too. So the substrate will fail before the glue joint does. It is true that the mechanical bond is weaker than a chemical bond but so what.

    For example, if you take superglue and glue a 2x4 to a piece of sheet metal, then rid the 2x4 off, the failure point is the glue joint. On the other hand if you glue the same 2x4 to a piece of paper, the paper will rip long before the glue joint fails. The same thing happens here but with different materials. Epoxy is just so strong, that even it's weaker bond is still stronger than what it is attached too.
     
  6. rwatson
    Joined: Aug 2007
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    rwatson Senior Member


    Who told you that ?

    A plain butt joint can never be as strong as either side. It might be 'strong enough' but there is no way that a break in the glass strands will be as strong as a continuous layup.

    PAR's commnets on scarf joints or additional layers either side is a way of increasing the strength, if it is required.

    The big question is not if the joint is stronger than continuous layup (which it cant be) , but like Stumble says, it may be 'strong enough' for the job.
     

  7. tunnels

    tunnels Previous Member

    Butt joins ???not even worth wasting your time thinking about it with or without a core
    Scarf joins need to be done from both sides in a cored situation
    Do one side first and when hard and dried ,Then scarf the other side and relay new core and then glass . As already stated joins usually end up stronger than the panel being joined !,also theres a change of stiffenss and a harder place so to speak ,There will be greater flex each side of the joint. Scafs need to be long as possible to do . The thinner the glass lay up the longer the scarf !
    Try repairng a 900 gram layup from the outside without changing the panel flexability to much !! Its not easy believe me !!!. :confused:
     
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