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Homework Help: Finding the maximum load of a bar from the yield stress

  1. Nov 15, 2012 #1
    1. The problem statement, all variables and given/known data
    I have a bar, 500mm long (0.05m) 10mm wide (0.01m) and 3mm deep (0.003 m)

    The bat has a load applied in the centre of 30n

    I know the maximum yield stress is 150 mpa 150x10^6

    How do I calculate the maximum load the bar can take?

    2. Relevant equations

    I used stress = m y / I

    3. The attempt at a solution

    I calculated the bending moment to be 3.75 at 250mm (.25m) using a shear bending diagram,

    I then reversed the equation above to get:

    150x10^6 = 0.0015m/2.25x10^-11

    Which gives m as : (150x10^6 x 2.25x10^-11) / 0.0015

    M = 2.25

    I know the bending moment is at .25 m

    So divide 2.25 / .25

    I get 9

    The answer can't be 9n as the question uses 30n

    Can someone please show me how to solve this?.

    Thanks in advance x
  2. jcsd
  3. Nov 15, 2012 #2


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    How is the bar supported? Are the two ends simply supported or fixed, etc?

    The type of supports affect the maximum bending moment, which will affect your maximum load.
  4. Nov 16, 2012 #3
    Sorry, the bar is simply supported at either end
  5. Nov 17, 2012 #4


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    For a simply supported beam, the maximum bending moment occurs at the center and is given by

    Mmax =PL/4 where P= applied load and L= length of beam
  6. Nov 17, 2012 #5
    So I can then use this to find the max load from the yield stress??
    If so, how?
  7. Nov 17, 2012 #6


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    Use your first equation and rearrange for M.

    σ = My/I
  8. Nov 18, 2012 #7
    thank you for this...
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