7. D2 o - applied stress in kilopounds (1000 lb) per square inch (ksi) used for fracture calculations oys - ;;!:;f stress of hull material, either aluminum or 0 e VII.D.2: - applied stress in psi from hoop stress equations for cylindrical section GENERAL REMARKS It is desired to evaluate the relative merits of aluminum versus steel with regard to the possibility of assumed surface flaws propagating to critical length before penetrating the material thickness. Figure 7.Dl illustrates the growth of a surface flaw from some initial depth first through the plate thickness and then along the plate as a through-crack. It is undesirable to allow such surface flaws to reach critical length before penetrating the plate thickness for then no warning precedes the unstable fracture of the plate . If the crack propagates stably for some distance after penetrating the plate thickness (Figure 7.Dl,C), the escape of gas can be detected and located as is described in Chapter VIII. Aluminum and steel have very different properties relating to crack propagation and it is important to determine their relative effectiveness in meeting this Leak Before Break condition. This analysis will assume an initial semicircular surface flaw with depth, a, equal to half length, C. VII.D.3: ANALYSIS From (7.Dl) and (7.D2) ,obtain the formulas for the stress intensity factor, K 1 , for both a surface flaw and a through thickness flaw. Through crack: Surface crack:
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