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Torsion and the modulus of rigidity

  1. Aug 8, 2017 #1
    1. The problem statement, all variables and given/known data
    A specimen tested in a simple torsion testing machine , the specimen was 12.5mm in diameter and the angles were read over a gauge length of 0.75m Determine the modulus of rigidity of the material taking the torque T as 4.5Nm and the angle of twist as 4.0 degrees

    2. Relevant equations
    T/J=τMax/R=Gθ\L

    3. The attempt at a solution
    J=∏(0.025)4/32
    J=3.8349x10^-8

    4.0degress=0.0698132

    4.5/3.8349x10^-8=G(0.0698132)/0.75
    =>G=126x10^7
    Wondering if this looks correct ??
     
    Last edited by a moderator: Aug 8, 2017
  2. jcsd
  3. Aug 8, 2017 #2
    Where does the factor (0.025)^4 come from?

    Don't you want pi*r^4/2 for this?
     
  4. Aug 8, 2017 #3
    So I have for the polar moment of inertia to be ∏d^4/32
    I did just see however I wrote 0.025 instead of 0.0125 which is the 12.5mm in m
     
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