7. 39 The window is a spherical section with a radius of curvature equal to 100m (see Figure 7.13). The window is placed in the hull in such a manner that the entire window is subjected to compressive stresses only. This takes advantage of the relatively high compressive yield stress of glass to eliminate brittle fracture due to bending or tension. As calculated in Appendix VII.H, the glass need only be 0.589 cm thick including a factor of safety of 2. Since this analysis does not account for bifurcation buckling of the window, an additional factor of safety of 10 is proposed for the individual glass layers. Furthermore, the window should have area density 5000 kg/m 2 to shield the interior of the colony from radiation . Using silica (density 3 g/cm 3 ) requires a window 1.67m thick or 28 6 cm layers. It is impractical to use glass layers spanning the entire 40m diameter so the window is a glass mosaic laid over a steel truss support structure. In this case, the problem of buckling can be solved by suitably sizing the supporting trusses but the detailed design of such a window was not undertaken in this study. Another problem not analyzed in this report is that of transferring the total atmospheric pressure load incident on the window to the circumference of the cutout in the endcap. Some local strengthening would be required to safely accommodate this concentrated load. VII.5: FATIGUE AND OPERATIONAL SAFETY LIMITS VII.5.1: General Remarks: In order to ensure the required operational life of the hull, it is necessary to account not only for the limits of material yield and ultimate strength but also for considerations of crack initiation and propagation. Accidental damage, fatigue, stress corrosion cracking and the inevitable imperfection of the manufacturing process are all means by which flaws may be introduced into a structural component. It is required that such flaws as may be present in the material at the time of construction, or may be initiated during the lifetime of the hull, will not propagate to such a size that catastrophic failure of the hull will result. Here catastrophic failure is assumed to mean any failure
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