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Stress Strain Curve

  1. Apr 15, 2013 #1
    1. The problem statement, all variables and given/known data

    Explain in terms of stress and strain what happens to the stretched material beyond the ultimate tensile stress.

    2. Relevant equations
    A curve similar to this


    3. The attempt at a solution
    I can see that the curve beyond UTS represents increasing stiffness but i can't really explain why or relate this to strain.( Does the zero gradient at uts mean zero stiffness ?)
     

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  3. Apr 16, 2013 #2

    haruspex

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    Decreasing (and negative)?
    Yes. Stiffness would be the slope, no?
    But the question doesn't mention stiffness. It just asks you to discuss what's happening to the sample in terms of stress and strain.
     
  4. Apr 16, 2013 #3
    So Can I just say that as the strain increases the stress decreases at an increasing rate?
    I thought I had to mention the quantity represented by the slope in these curve-related questions.
    I only said stiffness because I know that the Young Modulus is only related to the linear part of the curve.( Honestly I don't know which name to give the slope in this part)
     
  5. Apr 16, 2013 #4

    haruspex

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    Not sure that makes physical sense. Think of the test set-up. You, the experimenter, supply the stress. The test sample cannot control the stress on it. I would say that beyond the tensile strength (i.e. the peak stress) the stress level required to increase the strain diminishes increasingly rapidly.
    From what I read, referring to stiffness does not solve that. That also refers to elastic deformation. This link is useful: http://www.etomica.org/app/modules/sites/MaterialFracture/Background1.html
     
  6. Apr 17, 2013 #5
     
  7. Apr 17, 2013 #6
    Yes this makes a lot more sense.
    Thanks a lot :)
     
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