7. 4 3 construction. This scheme is shown in Figure 7.15. The plates bear the major portion of the applied load while the stiffeners serve both to add bending stiffness and to stop cracks from propagating across the joints between plate rows. VII.5.2.1: Leak Before Break Design Considerations: In order to provide an efficient, economical system for the detection and repair of damage that might be incurred by the hull, it is necessary to impose a condition upon the hull design. Simply stated, the pressure difference supported by a plate member must change by a detectable amount before the member fractures unstably. The discussion of Section VIII.8.6 describes in detail a proposed inspection scheme which utilizes this leak before break assumption. If the imposition of this constraint seeMs severe in light of the fact that the hull is designed as a Fail Safe system, it should be noted that the damage scenario assumed in the Fail Safe analysis consists of a corner joint failure causing the loss of four lm x lm plates. This damaged area is assumed to be surrounded by intact plates and stiffeners. If the structural members surrounding a damaged area are themselves near failure, the Fail Safety of the structure cannot be guaranteed. Thus, it is necessary to detect and repair flaws before a large number of failed or nearly failed members can accumulate. The discussion presented below will describe how a design plate thickness might be calculated so that the leak before break requirement is met. As such, it parallels to some degree the analysis of Appendix VII.D, and the symbol definitions used will be identical with those of that Appendix. Further, the stress/thickness relation for the outer hull as developed in Section VII.4.4 and transformed in Appendix VII.D will be used in the example calculation. The stress intensity factor associated with a surface flaw (see Appendix VII.D.3) is given by: where Q is a flaw shape parameter and MK is a deep flaw magnification factor. This analysis will assume the presence of a semicircular surface flaw with depth, a, equal to half length, C.
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