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Determining yield stress via a bar subjected to tensile force

  1. Jan 6, 2017 #1
    1. The problem statement, all variables and given/known data

    upload_2017-1-6_13-50-58.png
    2. Relevant equations
    All Equations Below

    3. The attempt at a solution
    upload_2017-1-6_13-52-20.png
     

    Attached Files:

  2. jcsd
  3. Jan 6, 2017 #2

    PhanthomJay

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    You are using the wrong area for the cylinder. What should it be?
     
  4. Jan 7, 2017 #3
    Do I use the area of a circle as the force is acting on the circular end? pi r^2
     
  5. Jan 7, 2017 #4
    Of course. The force is equal to the stress times the cross sectional area.
     
  6. Jan 9, 2017 #5
    upload_2017-1-9_11-14-43.png
    Does This look better?
     
  7. Jan 9, 2017 #6
    Once again, 1 m2 is NOT 1000 mm2! And GN m is not a unit of force.
     
  8. Jan 9, 2017 #7
    I should have noticed that given the fact I only did it last week! As for the units, I think GPa is the correct unit to use in my answer? I am getting mixed up by force measured in Newtons and Newton meters as a measurment. I have reworked my answer again?
    upload_2017-1-9_12-55-36.png
     
  9. Jan 9, 2017 #8
    Looks OK, except for the units. GPa is a unit of stress, not force. It would be better to just give the answer in N.
     
    Last edited: Jan 9, 2017
  10. Jan 9, 2017 #9
    That would be 117.6 Pascals then.

    Thanks!
     
  11. Jan 9, 2017 #10
    No it wouldn't. You have a stress in GPa and you multiply it by an area in m2. What are the units of the answer?
     
  12. Jan 9, 2017 #11
    Sorry I did not read the last comment properly,
    I only caught half of it in my emails (Looks OK, except for the units. GPa) and assumed it was ok.

    As my undertanding goes with these units:

    A Pascal is a unit which can be used for internal pressure acting in all directions equally within a material
    A Newton is a unit which can represent force which acts upon a structure or an object
    meters and millimeters is obviously a measurement of distance
    meters^2 is a measurement of 2 dimentional area
    meters^3 is a measurement of 3 dimentional area

    The part where I get mixed up is when I have a GN m^-2

    a Giga Newton is type of unit and a m^-2 is another but when do you choose to drop the distance/ area measurement out of the final answer giving a Force answer in simply Newtons or Giga Newtons rather than Newton meters ect

    So... I am unsure whether my answer should be in Newtons or Newton meters
    As the calculations go I presume they are ok so my attempted answer is:
    upload_2017-1-9_13-57-21.png
     
  13. Jan 9, 2017 #12
    Much better.
     
  14. Jan 9, 2017 #13
    You don't "choose to drop the distance/area measurement", as if this was optional. It disappears as a result of being multiplied by a distance/area measurement. N m-2 * m2 = N.
    Remember, quantity equals number plus unit. Until you get the hang of this I would strongly advise you to write your units in your calculations, e.g.
    Force = 6 x 10-3 GN m-2 * 1.96 x 10-5 m2 = 1.176 x 10-7 GN
    And you should just learn the basic units for common quantities, e.g:
    Force: N = kg m s-2
    Energy: J = N m = kg m2 s-2
    Pressure and Stress: Pa = N m-2
    Strain: m/m = dimensionless (sometimes we use "units" like με = 10-6 m/m)
    etc.
     
  15. Jan 9, 2017 #14
    I think I need to read up some more on common units and measurements, It was introduced into the earlier assignments without covering in the maths bridging course I completed before starting this HNC,. Once again I thank you!
     
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