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Homework Help: Bending Moment question

  1. Jun 15, 2010 #1
    1. The problem statement, all variables and given/known data
    The problem can be found here: 2422006-1.png

    Sorry the picture is so small. I guess you can download it and then zoom in or something. The main question I'm interested in 2b. Calculating the moment.

    position of neutral axis: 37.14mm
    Second moment of area: 2.83 x 10^(-6)


    2. Relevant equations
    The flexure formula.


    3. The attempt at a solution
    My answers for maximum compression : 58.4MPa and maximum tension: 34.5MPa. They're off by about 4Mpa respectively.
     
  2. jcsd
  3. Jun 16, 2010 #2

    PhanthomJay

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    I get your numbers also.
     
  4. Jun 16, 2010 #3
    Sorry I should have made it more obvious. The answers are:

    Stress tensile=30.93 MPa, Stress compressive=-61.99 MPa.

    I can't figure out why I'm wrong.
     
  5. Jun 16, 2010 #4

    PhanthomJay

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    :smile:Me neither,so maybe they're wrong?
     
  6. Jun 16, 2010 #5
    I hope not. It was an exam question. I'd imagine they'd be more careful.
     
  7. Jun 17, 2010 #6
    Your answers are correct for bending alone, but there is an additional 3.6 MPa for the axial compression. In other words apply the combined stress formula N/A +- M/Z
     
  8. Jun 17, 2010 #7

    PhanthomJay

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    Nice catch, thanks, we completely overlooked that.
     
  9. Jun 17, 2010 #8
    Oh of course I can't believe I forgot that. Thankyou so much.
     
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