7 . 48 \vhile in the second case the through flaw relation must be used: In both cases l':.o is equal to 0.1 times the mean value of the imposed stress or 0.1 o 0 • In the previous section the values of Q and MK were given as 2.3 and 1.1, respectively. It remains to define the the initial surface flaw that may be assumed. Assumed flaw sizes for various types of aircraft structures are specified in (7.15) and this analysis will use the appropriate standard defined therein. For a fail safe structure the assumed initial flaw is a semicircular surface flaw with a length of 0.1 in and a depth of 0.05 in. The relation defining the number of cycles required to propagate the given initial flaw to a through flaw of half length, a, may now be determined assuming the structure meets the leak before break criterion described in the previous section. A. From initial flaw through material thickness: dN = da 6.27xl0- 8{1.l (o.lo) (;~ 3 ) 112 1.u 2 • 25 N = f t da . I 0.05 7.69xlo-10(oa 112 ) 2 · 25 B. From through flaw of half length t to half length a: dN = da 6 . 27xl08 (o .lorn) 2 · 25 The value of NI+NII may be computed with o as a parameter, and the appropriate o/t relation can then be used to plot out log a - log N curves for various values oft and o 0 • If the number of load cycles seen by the structure per unit time can be estimated (Appendix VII.B), the crack yrowth history of the structure may be predicted for various
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