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Physics stress homework help

  1. Mar 6, 2007 #1
    1. The problem statement, all variables and given/known data
    A shaft of 0.3 metres long and 45mm diameter twists 2 degrees under a torque of 7kNm.
    On a tensile test the same material extends 0.015mm on a length of 110mm. If the tensile force producing this extension was 500N and the diameter of the test piece was 5mm determine the values of :
    G, E, v, K

    Any help/ guidance would be greatly appreciated

    Kindest Regards
     
  2. jcsd
  3. Mar 6, 2007 #2

    radou

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    Any attempts/elaborations/efforts from your side first would be appreciated even more. :wink:
     
  4. Mar 6, 2007 #3
    My first thoughts on the question were to figure out v then with v= lateral strain/longditudinal strain i can only figure out one of these strains i then got stuck as v is in the other formula. Apoint in the right direction would be helpful

    Regards
     
  5. Mar 6, 2007 #4

    radou

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    If the applied load in the first case causes torsion, then you can use the relation [tex]\phi = \frac{M L}{G I}[/tex], where I is the polar moment of inertia and I = [tex]\frac{d^4 \Pi}{32}[/tex].
     
  6. Mar 6, 2007 #5
    i take it that i re-arrange the formula to get G am I correct

    Regards
     
  7. Mar 6, 2007 #6

    radou

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    Yes, you can calculate G, and I hope it works out.
     
  8. Mar 7, 2007 #7
    Am i correct in saying that M is the bending moment and in this case it is = to 500 Newtons
     
  9. Mar 7, 2007 #8

    radou

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    No, it seems to equal 7 kNm.
     
  10. Mar 7, 2007 #9
    The answer i get is very small 4.22 x10-7

    Is my working out right

    7 x 0.32/ (0.045to the power 4 x pi / 32) x 2
     
  11. Mar 7, 2007 #10

    radou

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    Looks right, except the length of the shaft is 0.3 m
     
  12. Mar 7, 2007 #11
    what is 4.22 x 10 -7 measured in and what forumula do i use next

    Regards
     
  13. Mar 7, 2007 #12

    radou

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    Well, how is the shear modulus G defined?

    Next, you can calculate the module of elasticity E. Further on, there is a neat relation between G, E and v, do you know it?
     
  14. Mar 8, 2007 #13
    yes i do know what it is but the formual has two unknown values in it poisson's ratio and E. I also know that i have to calculate poissons ratio next and i know this is equal to lateral strain / longditudinal strain. But i dont know how to get these values from the question

    Regards
     
  15. Mar 8, 2007 #14

    radou

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    You already calculated G. Now you can calculate E, since you know the force which caused the given extension at the tensile test. Then you can easily calculate v from the mentioned relation.
     
  16. Mar 8, 2007 #15
    I think i am doing something wrong i calculated G which i feel is right with other collegues of mine but am i right in saying E = stress / strain then i get a value of V = .765 which can not be right
     
  17. Mar 8, 2007 #16

    radou

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    Did you turn the torque from [kNm] to [Nm]?
     
  18. Mar 8, 2007 #17
    I have solved that problem now calculating poissons ratio having difficulty transposing the formula to find v
     
  19. Mar 8, 2007 #18
    Thanks for all the help i solved it in the end

    Many thanks
     
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