7. 49 hull thicknesses. It is apparent that there are several uncertainties in this analysis regarding the initial crack size likely to be encountered as well as the load history seen by the structure. These uncertainties threaten the accuracy of a safe life prediction based upon the above analysis. In order to provide for the long-term stru_ctural integrity of the colony it was finally decided to design the hull so that the 6K 1 imposed upon the hull would be less than the threshold value required to grow cracks. That such a threshold stress intensity factor exists has been the subject of some debate, but a review of the charts of (7.13) seems to indicate that a 6K 1 of approximately 5 ksi/in is suffic.iently low so as to preclude fatigue crack growth. We obtain then: Since 60 3 . 54 60 6o(aJ 112 3 . 54 w172 o = 351./42 ksi o (a) Introducing a standard stress concentration factor of 3 for fillets and welds, obtain 3o 0 3\i2 ksi (a) o = 111.,2 ksi o (a) Thus, the mean stress level at which initial cracks will not_propagate is given by the above equation where a is the expected initial crack size.
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