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Homework Help: Find n and K value? (sheet metal question!)

  1. May 1, 2016 #1
    • Thread moved from the technical forums, so no HH Template is shown.
    Hello! Can someone please tell me where I'm messing up in this problem?

    At 4% and 8% elongation, the loads on a tensile test-piece of half-hard aluminium alloy are 1.59 kN and 1.66 kN respectively. The test-piece has an initial width of 10 mm, thickness of 1.4 mm and gauge length of 50 mm. Determine the K and n values.

    So, first I calculated the engineering stress at the second point:
    1.66/(10*1.4) = .11857

    Then the engineering strain at that point: (54-50)/50 = .08

    The true stress is then: .11857(1+.08) = .128056

    The true strain is then: ln(1+.08) = .07696

    For for the first point, I did the same thing:
    Engineering Stress: 1.59/(10*1.4) = .011357
    Engineering Strain: (52-50)/50 = 0.4
    True stress: .11357(1+.04) = .0118
    True strain: ln(1+.04)=.03922

    Then, I tried to calculate the n-value by:
    ln(.128056)-ln(.0118) / ln(.07696)-ln(.03922) = 3.5371
    ln(true stress)/ln(true strain)

    The book says the answer is supposed to be 0.12, but I'm not sure what I'm doing wrong?

    Thank you VERY much for your help!
  2. jcsd
  3. May 2, 2016 #2
    Your engineering stress for the first point is off by a factor of 10.
  4. May 2, 2016 #3
    Thank you!! I didn't even realize I did that! I really appreciate it!!
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