Strong joins: composite to aluminum

Discussion in 'Fiberglass and Composite Boat Building' started by fallguy, Mar 19, 2018.

  1. fallguy
    Joined: Dec 2016
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    fallguy Senior Member

    How are the strongest joins made to aluminum?

    We need to tie the base of the cabin and removable cockpit sections to aluminum sailboat mast sections being used as cat beams.

    Is a fiberglass/epoxy bond to aluminum as strong as a mechanical fastening? Or would both be better? Or is the epoxy bond foolish?

    It hasn’t been spec’d yet by NA, so please don’t go there. I am just trying to get a conceptual understanding of what is typical.

    A straight mechanical fastening is also curious as the beams are big 10x6” ovalish shaped with a flat on the bottom. I don’t even know how mechanical fastenings into these large tubes are done; you certainly can’t put a nut behind a bolt. Perhaps a flange of aluminum angle might need be welded?

    And the beams will probably take some stresses and have bending as an attribute; so if you apply epoxy bond to the side of the bending moment; it might like to shear on the bond.

    Thanks for any replies if you have seen it done. We are a good six months away from needing it so I don’t want to bother the na for my curiousity.
     
  2. Steve W
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    Steve W Senior Member

    Most of the mechanical properties data sheets where they give tensile strength etc are done bonding aluminum, the reason of course is that they are a lap joint and need to be fixed in a testing machine and they need the bondline to fail before the tabs that are bolted into the machine, if they used wood or even fiberglass it would possibly/probably fail first and they would never get a reading. So, you need to prep the aluminum the way they do which is usually just the typical, abrade for tooth, wipe down to degrease and the the acid etch 2 step process, alodine and something else, I don't remember what. A few years back the west system folks stopped selling the etch products because when they introduced their G flex toughened epoxy product they found that even without the etch it out performed what they were previously doing with their 105 epoxy. So, while I'm not a big fan of the Gougeons line of brittle epoxies I am of G flex and of their testing of materials. So, I would certainly give their tech department a call and discuss your situation.

    Steve
     
  3. gonzo
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    gonzo Senior Member

    Maybe the easiest solution is to use square or rectangular sections.
     
  4. fallguy
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    fallguy Senior Member

    The sections are already chosen. I found out a couple of things today from a few fellows.

    Gel Magic by systems 3 is the bonding method of choice and an etching process precedes; laminating resin is not used.

    or

    A simple saddle over the beams can also be done with mechanical fastenings.

    This might be enough for me to rest a bit easier.

    Thanks for the replies thus far. Much appreciated!
     
  5. Ad Hoc
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    Ad Hoc Naval Architect

    You're putting the cart before the horse.

    You need to know what loads and what the load paths are before you can attribute any type of material and means of connection.

    We connected 2 GRP hulls together via aluminium box sections. It is easy once you analyse what the loads will be and their load paths. Without which, you are just guessing and no idea if the means, material and method of connection will hold or last.
     
  6. fallguy
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    fallguy Senior Member

    All that is done. The beams are all spec'd. The selected beams just over spec are S830 mast sections from Charleston Spar sitting on a shelf in my boat shop. I just want to know various ways composites can be connected to aluminum for my own knowledge. I do know how the mast sections are connected to the hulls as that is already drawn, but a few of the other details are forthcoming and I just wanted to understand before I get the drawings..but I suppose you have a good point; ultimately, the means of connection are also vital to loading calcs.

    I learned today about saddles over the top and bonding with Gel Magic. I suppose one could also weld angle to the beams, but not sure if this would affect their strength ratings after welding, etc.
     

  7. Ad Hoc
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    Ad Hoc Naval Architect

    Exactly, and that's the point.

    Since if you wish to say, bold the two pieces together, what size bolts?..only your load analysis will tell you. Then, will the bolts rip through the connection?...the material selection of bolts and spacing of the bolts from the free edge will tell you etc etc...as well as will the epoxy/VE/laminate hold up too etc etc

    Very simplistically. Stress = force/area. Thus the area, is whatever you have connected and calculated from the material you are using, the stress experienced by the material in questions and then the force,....aaah..where does that come from...yup..the analysis!

    Can't "design" without knowing the loads and load paths and their effects - it is all connected (excuse the pun).
     
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