7 . 41 that would remove the life supporting ability of the hull. The procedures used in designing the hull to meet this safe life requirement are discussed in the remainder of this section. VII.5.2: Fracture Mechanics Considerations in Component Design: In previous sections of this chapter the terms ~low ~rack ~rowth (SCG) and Fail Safe have been introduced referring to systems of structural design. So that the impact of Fracture Mechanics on the design process may be clearly explained, the implications of each system will be reiterated here. A Slow Crack Growth structure is characterized by the continuous nature of the path supporting the applied loads. The basic design criteria used in this type of structure is that no flaw initially present may grow to critical size within the required lifetime under the maximum expected load. Alternatively, a Fail Safe structure is designed so as to provide multiple paths for the applied loads. Thus, the design criteria here is that the structure must be able to survive a postulated maximum probable failure by redistributing the applied loads around the failed component. Clearly, the alternate load paths must neither yield nor fracture unstably in the failed condition. Both of these methods require a knowledge of the initial flaws that may be encountered in the material, the approximate load history seen by the structure, and the various material properties associated with crack growth phenomena (7.12) (7.13). In the case of SCG structures, the relation between crack length and the load spectrum encountered must be determined so that critical-length cracks may be avoided. The Fail Safe structure can sustain criticallength cracks if the resulting failure is less severe than that assumed in the design. A critical-length crack occurring in one load path must not propagate unstably across the interface with surrounding load paths. Neither must a normally subcritical length crack occurring in the load path adjacent to the failed section become unstable under the failure induced overload. These conditions may, perhaps, be more clearly seen in Figure 7.14. As has been discussed in Section VII.3.3, the configuration chosen for this analysis is that of Fail Safe Plate/Stiffener
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