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Homework Help: Direction of max shear strain

  1. Feb 9, 2015 #1
    1. The problem statement, all variables and given/known data
    Hi Everyone,

    I am going to be doing an experiment soon using strain gauges on a beam and I will have to, among other things, calculate the direction of the maximum shear strain with respect to the axis of the beam. I am trying to find the correct equation to use.
    2. Relevant equations

    3. The attempt at a solution
    I have found this equation in a text book of mine: tan2θ = - (εxx - εyy) / 2εxy. I looks to me like the right one but the text is a bit ambiguous. I know this isn't a very specific question but is this the equation I would need to calculate what I said above? I just want to know roughly what I'm doing before I go to the lab.

  2. jcsd
  3. Feb 9, 2015 #2


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    Staff Emeritus
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    Homework Helper

    The Greek letter ε typically denotes axial strain. The Greek letter γ typically denotes shear strain.

    The shear in your beam is going to depend on the loading and the support conditions.

    It's a good idea to understand an experiment before you perform it. Unfortunately, PF is not set up to teach you what you should know.
  4. Feb 9, 2015 #3
    I assume that you are attaching strain gauges to either the top or the bottom of the beam. Do you know what the principal directions of strain are when a beam is bent?

  5. Feb 10, 2015 #4
    They will be attached to the top of the beam. I'm not sure about the principle directions but there will be a small force pushing the beam directly downwards if that helps.
  6. Feb 10, 2015 #5
    Go back and check your textbook. The principal directions of strain in beam bending are along the beam and across the beam. What does that tell you about the direction of maximum shear strain?

  7. Feb 10, 2015 #6
    As far as I can tell that means that the directions are just at 45o (or 90o on a mohr's cirlce). If that is the case, what is the equation I posted used for?

  8. Feb 10, 2015 #7
    If the components of the stress tensor are expressed with respect to a Cartesian x-y coordinate system, this equation give the angle of the maximum shear stress.

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