7. 52 Currently available high toughness steels such as HY-80, HY-100, and HY-130 have KIC values ranging up to 350 ksi/In. Thus, the upper limit of the mean normal stress obtainable with cu~rent materials is approximately 26,200 psi (1.80xl0 8 N/m 2 ). It should be noted that considerable uncertainty exists regarding the mechanical properties that might be attainable in space processed metals. Due to this uncertainty over the appropriate value of KIC and the inexact determination of K 1 , the results of this section should be considered tentative at best. VII.5.2.4: Major Stiffener Design for Fastener Considerations: In previous discussions the role of the major stiffeners has been explained in a general way. The necessity of providing attachment points for the bulkheads and of positively preventing the growth of damage from one hull section to another inplies the use of a specially designed stiffener, different from that which bounds the lm x lm panels. The discontinuity in the load path resulting from the use of fasteners provides a more certain barrier to crack growth than does the welded joint between panels and minor stiffeners. In Chapter VIII the discussion of fabrication procedures will further demonstrate the utility of a fastened joint between the hull plate and major stiffeners. However, the use of fasteners does involve several disadvantages. Because of unavoidable uncertainty concerning the exact stress distribution about fastener holes, it is customary to multiply the design loads by a fitting factor of 1.20 before computing the stresses. Additional failure modes are also encountered in a fastened joint. These disadvantages are outweighed by those benefits described here and in the following chapter and fasteners are chosen to join the hull plate and major stiffeners. In the design of a fastened joint four basic types of failure must be considered (7.1) (7.17). These failure modes are shown in Figure 7.17 and described below: a. Bolt or rivet shear - failure of the fastener in a plane perpendicular to its axis due to an assumed pure shear loading.
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