7. 56 is that the thickness of a stiffener leg is greater than or equal to the thickness of the plate to which it is attached. VII.5.3: Choice of Design Points: In Sections VII.4.3, VII.4.4, and VII.4.5 the stress/thickness relations are derived for the inner hull, the outer hull, and the bulkheads, respectively. Reproduced below are the relevant plots of log a 0 versus t which define those relations. The design thicknesses may now be chosen using the following rule. The structure will be designed to sustain the most severe of the following loading conditions: a. The mean stress level during normal operation of the colony shall not exceed the threshold stress, assuming an initial flaw size of from 0.1 to 0.2 in (depending upon the capability of inspection techniques). b. The stress level occurring in panels adjacent to those which have failed according to the maximum expected damage scenario shall not exceed the yield stress of the primary structural material (Fail Safe). The uncertainty in the specification regarding a reasonable choice of the initial crack size reflects the sometimes large discrepancies which exist between flaw detection capability attainable in laboratory tests and in actual field service. Chapter VIII will provide further discussion of inspection techniques and capabilities. It should also be noted that the customary stress concentration factor of 3 for holes and fillets was not used in the fail safe specification. It was assumed, somewhat arbitrarily, that the structure could tolerate some local yielding in the event of major damage. In the calculation of the threshold stress a factor of 3 was assumed, however. In each of the figures that follow (renumbered 7.18, 7.19, and 7.20) three points are labeled. A. +the point on the normal stress/thickness curve that corresponds to the threshold stress level computed using an initial crack size of 0.1 in. B. +the point on the normal stress/thickness curve that corresponds to the threshold stress level computed using an initial crack size of 0.2 in.
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