Effects of Loading and Environmental Variables

If NDI techniques can detect initial cracks as small as 10 in., then using Ui = 10 in., the estimated resulting fatigue life would be about 5 x 10 cycles. If, on the other hand, the initial crack size must be estimated from proof testing, say at 150 ksi, the assumed initial crack size for use in the fatigue analysis must be equal to the critical size for the proof stress (cr r), Eqn. (7.14). 

The service life is then reduced to about 6 x 10 cycles. This example illustrates the importance of avoiding, or reducing the size of initial defects in the structure, and the need for improved methods for nondestructive inspection. 

It has been shown that fatigue life is influenced by the fatigue crack growth kinetics and is reflected through changes in A and n in the power-law representation, for 

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