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Help on stress

  1. Mar 17, 2008 #1
    I'm having problem knowing how to work out this question, can anyone point me in the correct direction.

    14mm diameter steel rod carries a load of 3.08kN in tension. calculate the stress.?

    I ended up with this:
    Stress = Force/Area.
    Stress = 3.08/154 = Answer = 0.02N/mm²

    if the rod is 2.5m long and the modulus of elasticity is 200GN/m2
    how much will it extend?

    so far i have know idea of the formula or how to work this out?
    can anyone help please

    I also have one other question related to stress that is

    A brass rod measures 10mm in dia, and 150mm length. The rod extends 0.3mm when subjected to a load. given that brass E=85GN/m2
    Calculate the stress in the material and the magnitude of the load:
    if you could help with the formula that would be a big help.
    I'm not able to find any thing on the net and the books i got from the library are over the top in calculations
    please help
    many thanks
  2. jcsd
  3. Mar 17, 2008 #2


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    Hi donkey131, welcome to PF. For a rod undergoing axial loading, stress [tex]\sigma[/itex] and strain [tex]\epsilon[/itex] are related by the equation [tex]\sigma=E\epsilon[/itex].

    Make sure to check your units in your calculations.
  4. Mar 17, 2008 #3
    Another formula that you will need is [tex]\epsilon[/tex]= [l(final)-l(initial)]/l(initial)
  5. Mar 17, 2008 #4
    so how do i now find the Magnitude.
  6. Mar 17, 2008 #5
    not sure about this as im trying to teach my self, it an NVQ course. NOt good teaching your self something you don't know.

    Calculate the stress in the material and the magnitude.
    10mm = Diameter steel rod
    ?? = Area of the rod
    3.08kN = Tension
    ?? = Stress
    ?? = Strain
    150mm = Length
    0.3mm = Extends
    85 GN/m² = Elasticity

    Area = pi x radus² There for: Area = Pi (3.142) x 2.5² (6.25) = 19.637mm² (19.64)

    Strain = extension (change in Length)/Original Length
    Strain = 0.3/150 = 0.002

    There for we now can find the Stress:
    Stress = Strain x Elasticity
    Stress = 0.002 x 85 =0.17N/m²

    Answer = 0.17N/m²

    so how do i find Magnitude:?????
  7. Mar 17, 2008 #6


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    The magnitude is just the absolute value of the number. If the stress is -5 N/m², for example, we describe it as a compressive stress with a magnitude of 5 N/m².

    Again, recheck the units in your calculations - I see several errors.
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