# Material fatigue and stress

1. Jan 4, 2004

### Ashley

Could you please explain what each of the following symbols stand for:
da/dN=A(^K)m

I have worked out that da is the crack length
i have worked out that dN is the number of cycles
Are both of these correct?

Thanks

2. Jan 4, 2004

### jamesrc

What you have is a simplified model of the rate of crack propagation:

$$\frac{da}{dN} = A\Delta K^m$$

2*a is the crack length (not da), N is the number of cycles (not dN).

&Delta;K is the stress intensity factor range:

$$\Delta K = Y\sqrt{\pi a}(\sigma_{\rm max} - \sigma_{\rm min})$$

(Y is a crack geometry factor).

A and m are empirical constants (found by plotting log(da/dN) versus log(&Delta;K)).

3. Jan 5, 2004

### Ashley

Thanx for the help.

4. Jan 5, 2004

### Ashley

One more thing

you said that Y is a crack geometry factor, how do you find the crack geometry factor??

5. Jan 5, 2004

### jamesrc

It depends on the shape/aspect ratio of the crack. Hopefully, there is a table or explanation in your textbook. Here's a page I found with a bunch of them that might help:

http://www.ems.psu.edu/~green/436-8C.pdf