~l — I I I I II I 



Notes: 



1. Time is measured from the instant pressurization - 

 has been completed at 100-psi/min rate, and the 

 hull is under constant sustained pressure 



2. Dimensions of spheres — 15-in. OD x 14-in. ID 



3. Temperature — 60 to 70°F 



4. Material — grade G Plexiglas 



1 1 i i i i i 1 1 il i i i i i i ii I i i i i i i ii 



" 1 1 10 10 2 10 J 10 fl 



Time (hours) 

 Figure 87. Implosion pressure as a function of sustained pressure duration at 

 room temperature for 15-inch-diameter capsules. 



3. Long-term sustained hydrostatic loading caused model capsules 

 to fail at a lower pressure than short-term loading. The relationship between 

 the magnitude of pressure and the duration of loading at which the models 

 failed were represented by a linear graph when the test results were plotted 

 on a log-log scale (Figure 87). When the critical pressure versus time relation- 

 ship of Figure 87 was extrapolated, it was found that under a sustained 

 pressure of 500 psi and in the 60 to 70°F ambient temperature range the 

 time-dependent implosion of the model capsule would not take place before 

 at least 1 years of exposure. At higher temperatures, since the time- 

 dependent strain rate is higher, the implosion failure would occur sooner, 

 while at lower temperatures, since the time-dependent strain rate is lower, 

 the failure would occur later. For example, the model capsule under 750- 

 psi long-term pressure loading at 70°F average ambient temperature failed 

 after 366 hours, while another model (10% thicker) under the same magnitude 

 of pressure loading, but at 60°F average temperature, failed after 4,500 hours. 



4. The time-dependent failure of the model capsules was local in 

 nature (Figure 88). It appeared to have been triggered by local deviations 

 from sphericity and uniform wall thickness. Since local decrease in curvature 

 or wall thickness causes a local increase in compressive membrane stresses, 

 the time-dependent strains in this area would be higher also, thus further 



123 



