4. The appearance of cracks in the acrylic plastic bearing surface at 

 the penetrations is not a sign of impending implosion, as none of the models 

 imploded during cycling, although the cracks became visible many cycles 

 before the termination of the cycling program. 



5. Pressure cycling capsules to a 500-psi pressure level does not appear 

 to induce irreversible strains in the acrylic plastic, except probably locally on 

 the steel polar closure— acrylic plastic interface. Both the decrease in hull 

 volume, as measured by displacement of water, and the strains on the interior 

 surface of the hull, as measured by electric resistance strain gages, have shown 

 that even after a 120-hour pressure cycle the acrylic plastic hull returns to its 

 original dimensions providing sufficient time is given for its relaxation at 



pressure. This indicates that both the typical instantaneous and time- 

 dependent strains generated in the acrylic plastic hull under a 500-psi loading 

 in the 60 to 70°F temperature range are viscoelastic (Figures 97 and 98). 





















Notes: 



1. Dimensions! 

 15-in. OD x 

 - 2. Temperature 



3. Material — gr 



4. Loading eye 

 at a 1 00-psi/ 

 for 122hr;d 

 min rate; rel; 



jf sphere — 

 14-in. ID 











ade G Plexiglas 

 e — pressurized to 

 min rate, held at 5( 

 epressurized at 10 

 xed at psi for 1 2 



500 psi 

 )0psi 

 )-psi/ 

 2hr 









2 4 6 8 10 



Time (days) 



Figure 97. Change in displacement of a 15-inch-diameter capsule during a 



typical pressure cycle to 500 psi with subsequent relaxation period. 



6. The acrylic plastic models were as viscoelastic at the conclusion of 

 their cyclic testing programs as they were at their beginning. This is based on 

 the observation that when the pressure in a given pressure vessel containing a 

 capsule was raised to 520 psi and then the pressurization was terminated, the 

 pressure dropped a certain amount in the isolated vessel during a 24-hour 

 period due to a decrease in volume of the capsule. Since the decrease of 



132 



