1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Finding elongation of bar and maximum tensile stress

  1. Jan 30, 2013 #1
    1. The problem statement, all variables and given/known data

    L=52 in
    A=2.76 in^2
    E=10.4*10^6 psi

    2. Relevant equations


    3. The attempt at a solution

    4) σAB = (3P)/A
    δAB=(3PL)/(6AE) → δAB=(PL)/(2AE)
    solving for P
    P=[0.17*2*2.76*(10.4*106)]/52 → P=187680 lb → P=187.7 kip

    5) Because AB and CD are in tension i did this...
    solving for P and using 5000psi for σmax i get
    P=-5000*2.76 → P=13800 lb → P=13.8kip

    I tried looking for an example in the book to follow, but they were completely different. I hope i didn't mess up too bad.
    Last edited: Jan 30, 2013
  2. jcsd
  3. Jan 30, 2013 #2
    Don't add the stresses
  4. Jan 30, 2013 #3
    So i should only do one of them? σmaxAB
    or do the entire bar?
    Can I get a better hint than that?
  5. Jan 31, 2013 #4
    I confirmed that I did part 4 right. I still need help with part 5. Anyone?
  6. Jan 31, 2013 #5
    I made a mistake on part 5. I plugged in the value of σBC (-2P/A) in place for σAB (3P/A) in σmaxABCD

    Now i get σmax=3450 lb or 3.45 kip. But still don't know if it's right.
  7. Jan 31, 2013 #6
    Part 5:

    You correctly determined the stresses in the 3 sections of the bar.

    AB = 3P/A
    BC = -2P/A
    CD = P/A

    So, you already know that the maximum tensile stress in the bar is 3P/A. This cannot exceed 5000 psi = 5 ksi

  8. Jan 31, 2013 #7
    So what I'm getting from this is that the maximum tension occurs at AB so I only set σmax=σAB and don't add them with the other member in tension.

    Thanks :)
  9. Jan 31, 2013 #8
    Yes. That's right. The other sections will be less prone to failure.
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook