2. Since the low temperatures that commonly exist in the ocean at 

 continental shelf depth make the test results obtained in the laboratory at 

 room temperature (75 to 77°F) conservative for the design of hulls that will 

 operate in the cooler ocean waters, there is no reason to conduct capsule eval- 

 uation tests under simulated ocean temperature, as lower temperatures are 

 operationally much more difficult to maintain over long periods of time in 

 pressure test facilities than room temperature. 



Since the objective of the acrylic plastic pressure capsule program 

 was to provide an operational system for continental shelf depth, the max- 

 imum operational depth had to be in the 600-to- 1,000-foot range. The 

 working stress in the hull had to be selected accordingly either for the 

 600- or the 1,000-foot operational depth. Because the exact relationship 

 between the working stress level and the cyclic life of the acrylic plastic 

 hull was not known, it was decided to select the greater depth of 1 ,000 feet 

 as the maximum operational depth. A 1:3 relationship between the maxi- 

 mum working depth and the implosion depth under short-term loading was 

 selected; implosion depth could be approximated with existing equations. 

 In this manner, if the working stress corresponding to a 1 , 000-foot depth 

 did not cause premature fatigue failure, the hull would be rated for a 

 1,000-foot operational service. If, on the other hand, the working stresses 

 at a 1,000-foot operational depth caused premature cyclic failure (that is, 

 in fewer than 1 ,000 dives to 1 , 000-foot depth) of the hull, then the working 

 stress could be lowered by changing the operational depth of the hull from 

 1 ,000 feet to 600 feet. Since the 600-foot operational depth was also 

 acceptable, such an arrangement was considered to be satisfactory and the 

 working stress level was chosen to be 1/3 of the stress level associated with 

 the calculated implosion pressure of the capsule under short-term loading. 

 The working stress level was chosen on the basis of the relationship with 

 the short-term hydrostatic loading of the capsule to implosion rather than 

 on the basis of relationship with ultimate failure of an acrylic plastic test 

 specimen under uniaxial compression because the fact that plastic insta- 

 bility must be considered in the evaluation of capsule failure. The 

 compressive test of an acrylic plastic specimen does not take this into 

 account. 



In addition to selecting the working stress for the acrylic plastic, 

 the appropriate working stress had to be selected for the metal hatch and 

 bottom insert plate. The problem here was considerably simpler. Metals 

 such as steel, titanium, or aluminum do not exhibit viscoelastic or visco- 

 plastic behavior in the 32 to 120°F temperature range, and the response 

 of metals to different compressive or tensile stress levels is well understood. 

 Thus the selection of working stress here was more of a design than research 

 problem. The metal hatch and polar plate could be designed to fail at higher 



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