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Homework Help: Stress vs. Strain and finding the permanent set of bar

  1. Jan 20, 2010 #1
    1. The problem statement, all variables and given/known data
    I am told a cirular bar of magnesium alloy is 750 mm long. The bar is loaded in tension to an elongation of 6 mm and then the load is removed.
    I am given this Stress vs. Strain graph which I cant put up here since its in my book. But Stress is on the y- axis and strain is the x-axis. It looks almost like a square root curve.

    2. Relevant equations
    Epsilon= sigma/ E
    Where E is the modulus of elasticity right? and sigma is stress and epsilon is strain

    3. The attempt at a solution
    I took a point on the graph to find E and got 26,666.6667 which Im not sure if I did that right. So once I have E how the heck do I find the permanent set of the bar? Also the second part wants the proportional limit once the bar is reloaded.

    Luckily I have the answers in the back of the book but I want to know how to figure this out badly. turns out the permanent set is 2.97 mm and the proportional limit is 1.8 MPa
  2. jcsd
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