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Composite bars

  1. Aug 1, 2016 #1
    1. The problem statement, all variables and given/known data
    φ is the angle of twist , i dont understand why the angle of twist must be the same for 2 bars ...

    2. Relevant equations


    3. The attempt at a solution
    They are made of different material , how could the angle of twist be the same for 2 bars ?
     

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  2. jcsd
  3. Aug 1, 2016 #2

    BvU

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    For a composite shaft, it is useful if the shaft pieces do not come apart at junctions (like point B) :smile: . That means ##\phi_1 = \phi_2## !
     
  4. Aug 1, 2016 #3
    Why?
     
  5. Aug 1, 2016 #4
    why the twisting angle are the same? they are different materials, when same twisting moment applied to them , both of them will have different twisting angle,right?
     
  6. Aug 1, 2016 #5

    BvU

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    What about T = T1 + T2 + T3 + .... ?

    Can you check how 'twisting moment' is defined ?
     
  7. Aug 1, 2016 #6
    ok,i noticed that the torsion(moment ) applied on both bar are not the same,but why are the twisting angle same?
     
  8. Aug 1, 2016 #7

    BvU

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    If they are not the same, then the parts of the shaft have rotated wrt one another. That's not good at all.
    Are we talking about the same ##\phi## here ?
    Work out (7.10) and (7.11) for your composite bar ABC. Surely, at point B you want ##\phi## from bar AB to be the same as ##\phi## from bar BC at point B ?

    I think that's what he means: at point B ##\phi## from bar AB is ##\phi_1## and ##\phi## from bar BC is ##\phi_2##.
     
  9. Aug 1, 2016 #8
    so, the torsion here don't allow the parts of the shaft have rotated wrt one another?
    It's not stated in the question, how do we know that?
     
  10. Aug 1, 2016 #9

    BvU

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    They mention a (one) composite shaft - not two separate shafts that happen to be in each other's neigborhood. . So they are welded (or glued or screwed, or ..) together.
     
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