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Ductile material engineering stress-strain curve

  1. Aug 5, 2012 #1
    I understand that the engineering stress-strain curve of different material under tension test is different, but for the sake of simplicity the scope of this discussion will be for a general ductile material. If you have to pick a specify example, I think perhaps you can use steel or iron.

    There are various reference book out there that explains the stress-strain curve, but all of them vary from each other slightly. So I am a bit confuse and I would like to get some insight from here.

    There are many different points in a stress-strain curve, so I will list down in order base on my understanding:

    1) Proportional Limit
    2) Elastic Limit
    3) Yield Point
    4) Ultimate strength point
    5) Fracture

    Questions:

    (i) Is the point of proportional limit same as the point of linearity limit?

    (ii) If my understanding is correct, elastic limit is the greatest stress the material can take without permanent deformation. Yield point is the point where the material starts to experience plastic deformation. So, is plastic deformation not exactly the same as permanent deformation? Is that means there are many kinds of permanent deformation and plastic deformation just one kind of permanent deformation?

    (iii) Does the nonlinearity starts at the elastic limit, yield point or the proportional limit?

    I have more questions actually, but it is more suitable to be asked after the above three have been answered. So I looking forward to your opinion. Any help is greatly appreciated.
     
  2. jcsd
  3. Aug 6, 2012 #2
    (1) Yes

    (2) Yes, review the definition of permanent set and strain hardening.

    (3) The limit of proportionality

    You got 3 out of 3, well done and keep asking questions

    :biggrin:
     
  4. Aug 8, 2012 #3
    Thanks for the encouragement :blushing:
    Some follow up questions:

    (2.1) If plastic deformation is only a kind of permanent deformation, then prior to the yield point, what kind of permanent deformation is taking place?

    (2.2) I do not have the background on these topics (permanent set and strain hardening). Can you give me some insights?

    (2.3) Since plastic deformation is happens at the end of the stress-strain curve, can we say that the plastic deformation is the ultimate, "last thing to happen", in the world of permanent deformation? Mean there is no other kind of permanent deformation occurs after plastic deformation happens in a material.

    (3.1) If the proportional limit is the end of linearity and beginning of nonlinearity, why there are resources which take yield point as the beginning of nonlinearity? Which one is actually correct?
     
  5. Aug 8, 2012 #4
    Did you look them up?

    First a note on the word 'elastic'.

    In principle, Elastic Behaviour means that if you load a specimen it will deform at least some of this deformation will be restored if you then remove the load.

    It does not mean that deformation is linearly proportional to load. This is linear elastic or hookean behaviour which behaviour also means that the whole of the deformation is recovered.

    So beyond the limit of proportionality unloading still causes recovery of the deformation.
    However beyond the proportionality limit it is found that on unloading some deformation remains.

    This deformation is known as permanent set have a look at this thread.

    https://www.physicsforums.com/showthread.php?t=479133&highlight=permanent+set

    As regards plasticity look here

    https://www.physicsforums.com/showthread.php?t=395692

    Strain hardening is an acknowledgement that once the yield point has been passed it requires an increase in stress to further deform the specimen. This increase is known as strain hardening or work hardening. The material does indeed become harder and stronger under plastic flow as the crystal structure is rearranged. The work is the extra work under the curved section of the stress strain curve.
     
    Last edited: Aug 8, 2012
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