7 .10 time after it has penetrated the full plate thickness. The purpose of this requirement is to allow for detection and repair of a damaged plate before it fractures unstably. This will help avert the possibility of an accumulation of failed plates, individually tolerable but dangerous in the aggregate. Presented in Appendix VII.Dis a detailed analysis of the leak-before-break criterion for both aluminum and steel. The comparison is made based on the pressurization and artificial gravity loads only, and a coefficient tis derived. The coefficient tis the ratio of the hull thickness of aluminum to the thickness of steel required to meet the leakbefore-break criterion. In Figure 7.3 a plot oft versus steel thickness is made and it can be seen that for the pressurization loads alone 10 times the required steel thickness is needed for an aluminum structure. This ratio increases to about 15 for the stresses associated with an operational colony. The unwieldly nature of such a thick hull and the cost involved with transporting approximately 3.5 times the material to do the same job signifies that steel will be our main structural material. Once steel was chosen as the working material, the alloy composition had to be picked. The alloy considerations were based on fracture toughness, percentage of alloying materials required from the Earth, and the strength of the alloy. Initially, a new type of alloy called ausformed steel was considered (7.5). This is a low carbon steel with a negligible amount of alloying material needed from the Earth. Ausformed steel possesses an extremely high Klc' equal to 200 ksi/In or better. The material was reported to be very strong with a yield stress of approximately 2.1 x 10 9 N/m 2 . This alloy was a leading contender for the hull structure until further research (7.6) revealed that ausformed steel is extremely orthotropic. The high values of fracture toughness and strength occur only in the direction parallel to the grain's elongation. In the transverse directions the properties do not approach those of the primary direction. It was found difficult to reliably assess the usefulness of this promising alloy within the limits of the present analysis, so a more conservative choice was adopted in the form of AISI 4130 steel. The relatively high Klc of from 70-100 ksi/In and its
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