# Homework Help: Fatigue of materials problem

1. Apr 24, 2013

### chiraganand

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

#### Attached Files:

• ###### equation.docx
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14.8 KB
Views:
72
2. Apr 24, 2013

### chiraganand

can someone please help me out in this.. i am using the correct equation substituting correctly both still not able to get the answer..

3. Apr 24, 2013

### SteamKing

Staff Emeritus
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.

4. Apr 24, 2013

### chiraganand

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?

5. Apr 24, 2013

### chiraganand

I am attaching my substitutions in the formula. When I solve I end up getting a negative value for the crack length

#### Attached Files:

• ###### 1366825387115.jpg
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12.7 KB
Views:
122
6. Apr 25, 2013

### chiraganand

Desperately need a solution to the problem! please somebody help me.. i am substituting the values but still get a negative value!

7. Apr 25, 2013

### SteamKing

Staff Emeritus

8. Apr 25, 2013

### chiraganand

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..

9. Apr 25, 2013

### chiraganand

I have attached my solution attempt.. I have substituted and attempted the solutuion

#### Attached Files:

• ###### 1366891891221.jpg
File size:
6.3 KB
Views:
131
10. Apr 25, 2013

### SteamKing

Staff Emeritus
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?

11. Apr 25, 2013

### chiraganand

The equation is from foundations of material science and engineering 5th edition. by William F Smith and Javad Hashemi... chapter 7

12. Apr 26, 2013