Strong joins: composite to aluminum

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

  1. fallguy
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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).
     
  8. fallguy
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    fallguy Senior Member

    This is a valid continuation of the thread.

    Does anyone know how I can attach a flat aluminum plate to the t track of a S830 mast section?

    I don't know even the name of tracks that go into the s830 mast. So hard to look it up on sparcraft's site.
     
  9. Ad Hoc
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    Ad Hoc Naval Architect

    Like all things...it depends!

    What is the amount of contact area you have....is it to support any load...what is it being attached too.. etc etc.. what grade of ally and what grade of S830
    A picture answers a million questions :D
     
  10. brendan gardam
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    brendan gardam Senior Member

  11. fallguy
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    fallguy Senior Member

    0272161C-F199-4688-8503-4C8701E767F5.jpeg

    I want to know what kind of hardware works in the track of my section. I can't find it online, probably because I don't know the richtig namens, let alone German grammar!

    I have attached a portion of Wood's drawing. Unfortunately, the locker as drawn is a bit off for aesthetics. And I want the locker to be only 16" wide; not 22". And 4" of the locker will rest over the cockpit sole and be cleated to it. 6" will hang over the aft edge of the beam and that must be closed off to the sea.

    I plan to make a solid piece of glass and simply need to know what type of hardware to buy for the track.

    Here is a crude drawing over the provided drawings.
     
  12. fallguy
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    fallguy Senior Member

    Can someone tell me what the hardware is that goes in here?

    I tried a company out east and they were about as customer friendly as 'tell us what you need via email'. And since I don't know even the name of it; I can't leave an email even! 6D1B4F27-B23B-4C48-BE0D-B80BE1F94C28.jpeg
     
  13. fallguy
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    fallguy Senior Member

    E7FF9A03-9C0D-4195-865F-8A54C9EBD9E2.jpeg
    Here is a crude mockup; dimensionally incorrect. The steel is like the glass. I want to bolt it to studs in the track and then screw it to a cleat on the locker edge to close it off.
     
  14. jehardiman
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    jehardiman Senior Member

    If that is a mast section, the bolt is in the sail track. That particular sail track looks like it can take either a bolt rope or a sail track lug/slug/slide. If you are using the sail track to structurally attach the section (which seems to be the drift of this thread) then what you want is a T-Bolt, though you made need a custom one.
     
    Ad Hoc likes this.

  15. fallguy
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    fallguy Senior Member

    I may make the glass section an angle to give it some stiffness. This will require screws in the aft facing section versus hiding them, but people will sit in here and two downriggers home here; so stiffness might be kind of nice. I can't really laminate the stuff together because it won't fit then and it is a bit of a puzzle piece since the boat is a take apart boat.
     
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