Joint between Aluminium and Composite Sandwich

Discussion in 'Boatbuilding' started by Cacciatore, May 9, 2020.

  1. Cacciatore
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    Cacciatore Junior Member

    Hi Guys , someone have experience with the joint between Aluminium vs a Composite Sandwich . I would like to replace Aluminium shell and stiffeners with a Fibercarbon Sandwich structure to keep the VCG at acceptable value .
    It is a superstructure lenghthening so please consider that i have to make the side and the above deck in a composite material . I undestand that I have to consider thermal load on the joint but some would like to see some pictures to have an overview.
     
  2. Mr Efficiency
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    Mr Efficiency Senior Member

    You could consider the possibilities of the alloy transitioning to the composite panel, via an extrusion the composite panel slots into. The extrusion would need first to be welded to the alloy structure.
     
  3. gonzo
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    gonzo Senior Member

    It is impossible to answer correctly without knowing what the structure looks like. However, there are many adhesives that can be used.
     
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  4. fallguy
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    fallguy Senior Member

    pay attention to Gonzo here; dealing with a horizontal and vertical intersection with even similar materials that may move differently or expand diffferently is best done with adhesives

    obviously fiberglass and cores are often glass taped in opposing planes, but an adhesive with a mechanical fastening seems best to me; the mechanical fastening is to ensure proper placement in a dry fitting before adhesives are applied

    The post is largely out of my wheelhouse, but I know what I would try. If you can glass it all together, it is something I would not be familiar with, but I would also be surprised if it were not done; especially in aeronautics.

    if nothing else; perhaps my idiot post will bring some experts onboard familiar

    I am sort of assuming you want a foam superstructure on an aluminum deck...

    if you are extending; then are you extending foam onto an all aluminum house and deck?
     
  5. gonzo
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    gonzo Senior Member

    Using only adhesives with a cored laminate makes the core the limiting factor when the panel is pulled off. I think that mechanical fasteners/adhesive is probably the best way to go.
     
  6. Eric ruttan
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    Eric ruttan Senior Member

    You may want to look at composite plane building yt vids. Rutan and others had embedded aluminum plate to make hard points for engine mounts, ect.

    There were details. And they were important.

    Basically ruttan replaced the core with a large enough plate to distribute the loads. The layups went over the core and over the plate, in a contiguous fasion.

    This gave a hard point to mout whatever. Note that was 50 years ago.
     
  7. Ad Hoc
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    Ad Hoc Naval Architect

    The key issues you need to address are: the load transfer (load paths), the relative rates of strain (and thus what the stress will be) between each mating surface and the deflection at the joints from the different EI between the ally and composite structure.
     
  8. fallguy
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    fallguy Senior Member

    He did say fibercarbon sandwich.

    That alone could be trouble here!
     
  9. Ad Hoc
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    Ad Hoc Naval Architect

    Not sure what you're implying here?
    We've done CF sandwich panels like this before... everything in design is all about mitigation.
     
  10. fallguy
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    fallguy Senior Member

    what is the core and carbon for cf sandwich?
     
  11. Ad Hoc
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    Ad Hoc Naval Architect

    Still not 100% sure I follow what you're asking?

    But we used divinycell H80 for the foam core.
     
  12. fallguy
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    fallguy Senior Member

    Isn't a divinycel h80 and carbon sheathing something of a technical misfit? i.e. Doesn't the carbon fail faster at elongation? Seems a point harped on rather lots here. I suppose for a superstructure it can result in ultralight without concerns about impacts.
     
  13. Ad Hoc
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    Ad Hoc Naval Architect

    This is the part I'm trying to understand in your Q.

    Depends what the CF is supposed to be doing.. and what the core, at that time, is also supposed to be doing.

    Thus, what is the load case?
     
  14. fallguy
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    fallguy Senior Member

    I suppose a superstructure say pilothaus vs a hull and pick a simple vessel say a 12m planing hull. Is this enough? So used to them being automatically considered a misfit...but glad to get corrected
     

  15. Ad Hoc
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    Ad Hoc Naval Architect

    FG

    The mistake you're making is that your starting position is based upon an assumption. The assumption based on "another" design and its its own set of criteria, not this design.
    Everything must be designed for its purpose.

    You wouldn't say don't use aluminium, because it flexes too much... but it does and we do.... you wouldn't say, don't use steel... because it rusts and its too heavy.... but it does and we do.

    The objective of design, especially structural design, is based upon knowing the applied loads and the response to the load by the structure that is to support/shirk said load.
    For example, if the structure fails with skins of 1mm, would you say, the material is rubbish, or, would you merely increase the thickness of the skins until it passes?...or change the material properties...or a combo of the two?

    Without knowing the inputs we can never say what the output will be, until we design it accordingly and mitigate any unwanted effects.
    The key is understanding the EI... thus the stiffness and deflection of structure under an applied load. The material properties is the "E" and the stiffness is the "I".
    That's it.
     
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