300, 600, and 4,000 cycles, respectively, the cause for the greater depth of 

 cracks in the model with only 100 cycles could not be the number of stress 

 cycles as otherwise the model with 4,000 cycles would have had the deepest 

 cracks, while in reality it suffered only slightly from cracking (Figure 96). 





- bearing surface for 



| ■ 





polar steel inserts 



I V 







^ cut through the 

 T^wall of the sphere 







Figure 96. Beveled edge of the acrylic plastic bearing surface for the polar 

 steel insert in a 15-inch-diameter capsule after 4,000 pressure 

 cycles to 500 psi with 1-hour dwell times at 500 psi and pressures. 



Since the model with the 100 cycles was subjected to the longest 

 duration cycles (120 hours at 500 psi) and had the longest cumulative time 

 under pressure ( 1 2,000 hours at 500 psi) as compared to the others with 24-, 

 12-, and 1-hour cycles, respectively, it would appear that in all probability 

 it was either the length of the individual cycle (magnitude of time-dependent 

 strain) or the total cumulative time under pressure rather than the number of 

 cycles that caused this model to have the deepest cracks. It would thus appear 

 that the effect on the acrylic plastic bearing surfaces at penetrations in the 

 capsule of a single long pressure cycle or total cumulative time under load is 

 more severe than the effect of several short cycles of the same pressure level. 



Further, it appears that from the two factors (length of individual 

 cycle and cumulative duration of loading at 500 psi) the length of individual 

 cycle is the more important one. This finding is based on the observation 

 that the cracks were more prominent in Model 8 with 300 cycles and 24-hour 

 individual cycle duration (total cumulative time under 500 psi is 7,200 hours) 

 than in Model 1 1 with 600 cycles and 12-hour duration (total cumulative time 

 of 7,200 hours). Since both models have equal cumulative time under load, 

 while Model 8 has both the longer individual cycle and lesser number of 

 cycles, it could not be either the number of cycles or cumulative time under 

 load that caused it to have more severe cracks than Model 1 1. 



131 



