Observation of fragments from the 1 -inch-thick polar acrylic plastic 

 pentagons confirmed the observations made previously on the 1/2-inch-thick 

 polar pentagons that the magnitude of cracks in the beveled bearing surface 

 pressing against the steel inserts is a function of magnitude of time-dependent 

 strain, not stress level. Thus, for example, the cracks in the fragment 

 (Figure E-3) from the capsule that imploded at 3,000 psi after 1 ,575 min- 

 utes of sustained hydrostatic loading are deeper than those found in the 

 fragment from the capsule that imploded at 4,000 psi after 23 minutes of 

 sustained loading (Figure E-4). 



CONCLUSIONS 



Acrylic plastic spherical hulls of 66-inch diameter, if fabricated from 

 standard nominally 4-inch-thick plate (thickest commercially available grade G 

 Plexiglas plate), merit certification to an operational depth of 1,050 feet if the 

 stress level, fatigue life, and time-dependent failure criteria are similar to those 

 employed for certification of the nominally 2.5-inch-thick acrylic plastic hull 

 to a 600-foot depth. 



RECOMMENDATIONS 



For operational depths either greater or lesser than 600 feet, the 

 t/D ratios shown on Figure E-5 are recommended for manned acrylic plastic 

 capsules. These values of t/D have been calculated using the same criteria of 

 safety established experimentally for the NEMO 600-foot operational depth 

 capsules of this report. When more experimental data become available on 

 the relationship between the t/D ratio and safe operational depth the 

 Figure E-5 will be revised to incorporate the new findings. 



199 



