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Fatigue of materials problem

  1. Apr 24, 2013 #1
    1. The problem statement, all variables and given/known data
    Compute the crtitical surface edge crack if the fatigue life = 8000000 cycles and maximum tensile stress is 21000 psi. m=3.5 and A= 0.00000000004 in ksi and in. units. Initial crack edge is 0.120 in. Y= 1.15

    2. Relevant equations

    The equation is given in the attached file and a0 is the initial crack length and af is final crack length


    3. The attempt at a solution
    I have tried substitutuing the values in the equation but on substitution and solving i am not getting the correct answer.
    I think i am going wrong in the conversion of ksi to psi

    Please help me out in this
     

    Attached Files:

  2. jcsd
  3. Apr 24, 2013 #2
    can someone please help me out in this.. i am using the correct equation substituting correctly both still not able to get the answer..
     
  4. Apr 24, 2013 #3

    SteamKing

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    The conversion from ksi to psi is simple: 1 ksi = 1000 psi.

    Instead of showing only the raw formula, show your calculations. It may be some simple arithmetic mistake.
     
  5. Apr 24, 2013 #4
    I have been doing it over and over but get the same answer... I want to know conceptually it is just substitution in the equation right?
     
  6. Apr 24, 2013 #5
    I am attaching my substitutions in the formula. When I solve I end up getting a negative value for the crack length
     

    Attached Files:

  7. Apr 25, 2013 #6
    Desperately need a solution to the problem! please somebody help me.. i am substituting the values but still get a negative value!
     
  8. Apr 25, 2013 #7

    SteamKing

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    I can't read your calculation snapshot.
     
  9. Apr 25, 2013 #8
    Oh ok sorry for the poor resolution... basically what I have done is just substituted the values in the equation..
    I have converted 21000 psi into 21 ksi as in the question it says that A = 4.1x10^-11 in ksi and in..
     
  10. Apr 25, 2013 #9
    I have attached my solution attempt.. I have substituted and attempted the solutuion
     

    Attached Files:

  11. Apr 25, 2013 #10

    SteamKing

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    I'm not familiar with this equation, but I would check m, especially the term (-m/2+1) in the denominator. That is one reason you are getting a negative crack length. Can you provide a source for this equation?
     
  12. Apr 25, 2013 #11
    The equation is from foundations of material science and engineering 5th edition. by William F Smith and Javad Hashemi... chapter 7
     
  13. Apr 26, 2013 #12
    Somebody... anybody please help me out in this
     
  14. Apr 28, 2013 #13
    If anyone could please help me out in this. If anyone is familar with the Willaim smith book and knows how to solve the problem please let me know.
     
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