This difference can be explained by the hypothesis that since local 

 flat spots and thin sections of the hull cause the acrylic plastic hull to fail 

 at lower pressure by straining locally at a higher viscoplastic rate than the 

 rest of the hull, the rest of the hull does not have the opportunity to reach 

 the compressive strain level at which implosion by general creep buckling 

 mode occurs. Thus, the difference between the volume decrease under 

 short-term and long-term sustained pressure can serve as an indicator of the 

 severity of local deviations from sphericity and thickness in the acrylic plas- 

 tic hull. The rather small difference between the volume decrease under 

 short-term pressure loading (approximately 1,350 ml) and long-term pressure 

 loading (approximately 1,150 ml) seems to indicate that the severity of local 

 deviations in sphericity and thickness was rather low. This is, of course, sub- 

 stantiated by actual measurements of sphericity deviations which were on the 

 order of 0.010 to 0.030 inch for the majority of the models. 



Cyclic tests were performed on four capsule models to determine the 

 effect of repeated pressurizations on the structural integrity of the acrylic 

 plastic hulls. These tests produced results that are less conclusive than those 

 from short-term and long-term sustained pressure tests, largely because there 

 was only one test specimen for a given test condition and there were only 

 four test conditions (Table 6). Because of these limitations, all the findings 

 address themselves to the effects of three variables only: ( 1 ) number of 

 cycles, (2) duration of pressure cycle, and (3) cumulative time under pres- 

 sure. The effect of pressure magnitude is not discussed because only a single 

 pressure level of 500 psi was used in the cyclic tests. 



The cyclic test findings are: 



1 . None of the model capsules failed during pressure cycling from 

 to 500 psi (1,120 feet). The cyclic history at the termination of the tests 

 was 4,000 cycles for the model with a 1-hour sustained loading during the 

 pressure cycle, 600 cycles for the model with a 12-hour loading, 300 cycles 

 for the model with a 24-hour loading, and 100 cycles for the model with a 

 120-hour loading. 



2. No damage was visible in the model capsules except on the beveled 

 acrylic plastic bearing surfaces in contact with the metal end plates. The 

 damage was in the form of circumferential cracks in plane of the wall at right 

 angles to the bearing surface, similar to those observed in some of the capsules 

 under long-term loading. The cracks were concentrated below the inner half 

 edge of the O-ring groove in the steel end plate. 



3. By comparing the depth of cracks on the bearing surfaces of all the 

 models cycled from to 500 psi, it has been found that the deepest cracks 

 were found in the model subjected to 100 cycles with a 120-hour dwell time 

 at maximum and minimum pressures. Since the other three models completed 



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