Figure 80. Simplified polar penetration closure for 15-inch-diameter capsule 

 that failed by plastic instability and yielding at 1,800 psi of short- 

 term hydrostatic loading. 



1. Acrylic plastic capsules under external hydrostatic pressure 

 exhibit time-dependent strains even at hydrostatic pressures as low as 

 250 psi (one-half the design pressure). The time-dependent strain rate 

 at sustained long-term hydrostatic loading is a function of hydrostatic 

 pressure: higher pressures generate higher strain rates. The time-dependent 

 strain rate is also an inverse function of loading duration: the strain rate is 

 largest immediately after load application and decreases in magnitude with 

 duration of loading (Figures 84 through 86). 



2. The magnitude of strain in a NEMO model after a given duration 

 of sustained loading varies linearly with the magnitude of applied loading 

 from to 800 psi. This relationship becomes (Figure 85) markedly nonlinear 

 at hydrostatic loadings above 800 psi for room temperature ambient test con- 

 ditions. Although long-term tests were not conducted at other ambient 

 temperatures besides room temperature, it can be postulated on the basis 



of research conducted by others with acrylic plastic tensile test specimens 

 that both the magnitude of strain and the magnitude of hydrostatic pressure 

 at which the linear relationship ceases will vary with the temperature. High 

 temperatures will cause larger strains for the same pressure and duration of 

 loading, while low temperatures will be accompanied by low strains under 

 long-term loading. 



120 



