The required period of safe damage growth is a function of design category (either slow crack growth or fail safe) and the degree of inspectability as defined in Section 1.3.3.  In order to cover various uncertainties associated with crack growth during service usage that may not be adequately accounted for in the analyses or laboratory test, the structure must withstand a period of service usage longer than the planned inspection interval.  The periods of unrepaired service usage for the inspectability categories is given in JSSG-2006 Table XXXIII and repeated in Table 1.3.5.

Table 1.3.5.  Minimum Periods of Unrepaired Service Usage for In-Service Inspectable Structures