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Homework Help: Stress-strain diagram using deformation ?

  1. Mar 15, 2008 #1
    1. The problem statement, all variables and given/known data

    I have a lab report where I have to plot a stress-strain diagram for 2 beams. We apply an increasing load at midspan and record the deflection. I have to find the strain so I can plot the diagram. Also, the diagram willl be use to find the modulus of elasticity so I can use the stress = Ee

    2. Relevant equations

    I cannot use stress = Ee since the modulus need to be find. I know that strain = (l-lo)/lo but since it is not en elongation but a bending I don't know what's the relation between strain and deflection

    3. The attempt at a solution

    I was thinking about using trigonometry to find the change in length but I wouldn't be accurate and the error on the length will be to big to use. It's a lab report so usually it shouldn't be to complicated. I look on the internet, in all my books and didn't not find any relation between strain and deflection.

    Thank you
    Alex B.
  2. jcsd
  3. Mar 16, 2008 #2


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    Science Advisor
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    Gold Member

    Hi Alex B., welcome to PF. I suggest you find any mechanics of materials book (Beer and Johnston is a classic) and look at the chapter on bending. You will find many useful relationships between stress and strain for a bending beam.
  4. Mar 16, 2008 #3
    Thanks Mapes.

    I found the formula i need 1/p(rho) = M(x)/EI then I find my E, then use O=Ee to find my strain.
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