7. D4 Critical stress intensity is the condition brought about by a specific combination of loading and crack length at which the crack propagates unstably and the member fails. The stress intensity factor associated with this condition, KlC' is found for a given loading when the critical crack length, Ac or ac, is substituted into one of the above equations. KlC is essentially independent of the loading and may be considered a material property. Specific values of KlC for aluminum and steel are 22 ksi/In and 70 ksi/In, respectively (7.D2). The case of interest is that in which a= C = t and the desired condition is that the stress intensity factor of the corresponding surface flaw be less than the appropriate Kie· Referring to Figures 7.D2 and 7.D3, note that for a se~icircular crack Q = 2.3 and MK~ 1.1. Th~ values of Q and MK for other types of ~racks could easily be substituted in further analyses but the the present case: l.lcr (;~3)1/2 (1.1) < KlC Setting a t 1.4142a (t) 1 / 2 < KlC Now determine the a-t relation for both aluminum and steel using the outer hull as the example. Recalling the discussions of Sections VII.2.2 and VII.4.4, obtain: For aluminum: 0.098 lb/in 3 0.0269 lb/in 3 For steel: 0.284 lb/in 3 0.0779 lb/in 3
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