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Another one about Yield Stress...

  1. Dec 16, 2016 #1
    Hi Forum,
    I´m happy I found this forum...
    I hope someone can answer my (simple) question:
    According to this thread https://www.physicsforums.com/threads/calculating-0-2-proof-stress-of-aluminium.206648/ on page 2 the stress/strain diagram is not the same as force/elongation.
    Due to the constant area surface and the constant start length, the graphs look the same.

    Now the question:
    Am I wrong when I use the force/elongation graph for further studies? (e.g. Yiel point 0.2% determination). Or did I miss another influence?

    Background:
    We build bending machines and need to find out the Yield point of mainly Aluminium material. The parameters we have can gather are:
    - Force
    - position (length)
    - Time (Speed)
    - We can input length and areal surface in a programmed mask

    Thank´s a lot and excuse my maybe too simple question...
    Jens
     
  2. jcsd
  3. Dec 16, 2016 #2

    PhanthomJay

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    that is correct, they are not the same
    they both are generally linear, but the slopes of the 2 graphs are quite different, however.
    in determining the yield stress of the material at 0.2 percent strain, you can use the force - elongation graph, and determine the elongation at 0.2% length, then find the force at that elongation. The force must then be divided by the cross section area to get the yield stress at .2 % strain
    I assume you are measuring force and elongation in a tensile testing apparatus? And it is not surface area, it is cross section area you want to use.
     
  4. Dec 19, 2016 #3
    Thank you very much for your reply.
    The test happens in a production machine, where we have the possiblitiy to measure force (calibrated suitable for our needs) and displacement (elongation).
    Of course I refered to cross section area, that had been lost in translation ;-)
    We have input possibilities of the cross section area of the used profile.

    So I can go on with the programming.
    Thanks again
     
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