7 .12 reasonable yield stress of approximately 1.03 x 10 9 N/m£ (150,000 psi) make 4130 an acceptable alternative. Important too is the fact that 4130 contains only about 2% alloying elements (that is, those not available on the Moon). In order to minimize the probability that cracks initiated on one plate will traverse the boundary with a neighboring plate, a strip of crack stopping material will be welded into place at the joint between the standard lm x lm plates (Figure 7.4). The crack stoppers can be formed from either HY-80 or HY-100 steel (7.7). These alloys are designed specifically to possess very high values of fracture toughness (~ 250-300 ksi/in) in addition to tensile strengths of 5.5 x 10 8 N/m 2 (80,000 psi) and 6.9 x 10 8 N/m 2 (100,000 psi), respectively. The limitation of these alloys is that they require large quantities of alloying elements and complex processing. See Table 7.1 for a listing of the mechanical properties and compositions of 4130, HY-80, and HY-100. In concluding the discussion of materials choice it is necessary to explore the possibility that earthbound concepts of the availability and properties of structural alloys may have little relation to reality at LS. It has been assumed to this point that the production of usable alloys from lunar ore can be accomplished in a manner not very different from methods proved on Earth. This assumption of necessity forms the basis for the results of the present analysis,though it has yet to be convincingly verified. If large scale materials processing is indeed practical in the zero-g vacuum of LS, the question remains as to what ranges of mechanical properties might be achieved in the resulting alloy(s). As will become apparent in Section VII.5, the nature of the alloy used in construction of the colony has a profound effect upon the plate thickness required and, consequently, upon the resulting hull mass. Specifically, the material fracture toughness (Klc) affects the hull thickness in a major way in the fulfillment of the several safe-life criteria. It has been suggested that the absence of contaminants in the space environment may allow the production of low carbon alloy steels with very high values of fracture toughness (7.8) . If this is true, a substantial mass savings would then result in the hull. Therefore, it is imperative that the material properties
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