d = diameter of area with a larger 

 radius of curvature than R 



Phase 1 — Small area with 

 larger radius resulting from 

 imperfect fabrication. 



Phase 2 — Under hydrostatic 

 loading both the area as well 

 as the radius of curvature 

 increase at the imperfection. 



Phase 3 — The area of imperfection 

 does not increase any more, only 

 the radius of curvature increases. 



Phase 4 — Area of imperfection 

 remains constant, the direction 

 of the radius of curvature 

 reverses itself, fractures appear. 



Phase 5 — Local penetration of 

 pressure hull takes place. 



Phase 6 — Surface of the hull 

 returns to its original sphericity 

 at zero pressure. 



Figure 89. Mechanism of localized viscoelastic failure in an acrylic plastic hull 

 under sustained external hydrostatic pressure. 



6. The total decrease in volume of model capsules at the moment of 

 their failure under long-term sustained hydrostatic pressure was approximately 

 the same regardless of the sustained hydrostatic pressure level at which they 

 were kept. The total decrease of volume just prior to implosion was some- 

 what less than in short-term pressure tests. The magnitude of volume decrease 

 under long-term hydrostatic pressure was approximately 4% (Figure 86), as 

 compared to 4.8% under short-term pressurization (Figure 79). 



126 



