Aluminum tubing compression properties and grade.

Discussion in 'Materials' started by raf pali, Jun 19, 2018.

  1. raf pali
    Joined: Mar 2012
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    Location: Australia

    raf pali Junior Member

    Hello all,
    I need to compare the compression capability of Al tubing.
    Specs require 2M long x 130mm Dia x 6mm wall (6.5f x 5" x 1/4").
    BUT, where I am, (Central Queensland Au) can only find 150mm Dia x 3mm Wall (6" Dia x 1/8 Wall).
    My question is, would they be similar in end to end compression capability?
    The tubing is to be used as a compression post under the mast.
    Also, what grade Al masts extrusions are made of please?
    Thank you.
     
  2. Ad Hoc
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    Ad Hoc Naval Architect

    You really need to provide a tad more info.
    Is it fixed at both ends or is it free to rotate at each end....and then is it welded at both ends?...or is it pin jointed to rotate?

    But basically no.
    The radius of gyration and hence the slenderness ratio of the second tube is roughly 15% less than the first tube. Which means the allowable stress to buckling will be less by approximately 10%.
    Also the slenderness ratio is very different between fixed end and pin ended...hence the question.

    The grade would normally be 6082 - T6
     
    raf pali likes this.
  3. raf pali
    Joined: Mar 2012
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    raf pali Junior Member

    Hello Ad Hoc, thank you so much for your input.
    To explain the function of this Al tubing and the way it attaches to the mast, is rather complex and needs a new thread coz, is a all new sailing catamaran structure and rigging system I designed.
    I was to open the thread for discussion as soon as the engineering shop delivers the bits and pieces and the all is put together. This wont be to long, providing I can find the tubing.
    To give an idea without going into details now, is about revolutionizing a "tube catamaran" structure (a cat held together with Al tube beams) and achieving: 1, independent pitching; 2, rid of compression post under must grazing the water surface; 3, make the beams nonstructural. Al this by adding a "truss" across the central cross section to hold the hulls together. the Al tubing are for the making of the "truss"
     
  4. baeckmo
    Joined: Jun 2009
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    baeckmo Hydrodynamics

    There is contradiction in your statement of scope; first you say "a cat held together with Al tube beams", then you state "make the beams nonstructural". If you are aiming at a 3D framework design with components in "pure" compression and tension, then the end conditions for the compressed details are the limiting factors. As the buckling strength is a balance between load, length, radius of gyration, end conditions and the modulus of elasticity of the material, your preliminary question has too many degrees of freedom to be answered right now. If you are looking for the lightest compressed components, steel may be a better choice due to its much higher E modulus, btw.

    Please do not take offence, but your question reveals that you are probably not quite familiar with the basic principles for strength calculations regarding framework structures. If you really want expertise input from forum members here, you have to share relevant info.
     

  5. raf pali
    Joined: Mar 2012
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    raf pali Junior Member

    No offense taken, I have limits of knowledge in this regard. That's why I'm asking.
    The beams role will change when the new frame is in place. The flexion strength will no longer be required. Beams will only keep the hull parallel and hold the weight of the deck.
     
    Last edited: Jun 21, 2018
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