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The difference between strain and strain_y

  1. Sep 25, 2013 #1
    1. The problem statement, all variables and given/known data

    I am doing a lab. My textbook defines strain as ε= lf-lo and E = σ/ε. In my lab, E= σyy. εy = ΔLy/lo

    I do not understand how they can both = ε as lf-lo is completely different from ΔLy/lo. ΔLy is "elongation at yeild" according to my lab. Why are they both labelled E if they are different? The y subscript, according to the lab, has to do with the yield point. I was under the impression that there was only 1 elastic modulus as after the yield point, stress and strain are no longer proportional and plastic deformation occurs.

    2. Relevant equations

    See above.


    3. The attempt at a solution

    I can only conclude I am either missing something or the equations mean the same thing, somehow.
     
  2. jcsd
  3. Sep 26, 2013 #2

    PhanthomJay

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    that is elongation, not strain... Did you copy this down incorrectly?
    correct. Lf -Lo = ΔL = elongation, and ΔL/Lo = ε = σ/E
    correct the definition for strain in your text. The y subscripts then refer to specific values of stress and strain etc at yield. Without the subscripts, these are the general equations within the proportional limit.
     
  4. Sep 26, 2013 #3
    Thank you, I understand now.
     
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