in the joints between the bottom polar pentagon and the adjacent pentagons. 

 When the NEMO system is submerged more than 20 feet underwater, the 

 upward acting hydrostatic pressure overcomes the downward acting weights 

 of bottom plate and internal equipment relieving all the tensile stresses in the 

 joints between individual spherical pentagons. 



Working Stresses. The selection of a working stress level is always a 

 very crucial engineering decision for internal or external pressure hulls whose 

 catastrophic failure would kill the operating personnel. When the material 

 selected has not been used previously for construction of pressure hulls, 

 selection of a working stress level becomes highly subjective. The designer 

 must draw heavily on experimental data peripheral to the problem at hand 

 and on his engineering judgment. 



Since acrylic plastic pressure hulls had not been designed or built 

 previously for manned operation, no precedent existed on which to draw. 

 There existed, however, two sources of experimental data that impinged on 

 the design of acrylic plastic capsules. One was the body of experiments con- 

 ducted over a period of years for determination of allowable stresses in acrylic 

 plastic windows and canopies for aircraft. 4,5 The other source of data was 

 the evaluation of acrylic plastic windows for undersea applications performed 

 by NCEL and other laboratories. 12 " 15 



Although a wealth of experimental data has been generated by 

 engineers during design and evaluation of aircraft windows and canopies, 

 little of it is directly applicable to the design of an acrylic plastic pressure 

 capsule. The windows and canopies in aircraft are (1) in tension and 

 (2) cannot fail by elastic or plastic instability, while the acrylic plastic 

 capsule is (1) in compression and (2) it generally fails by elastic or plastic 

 instability. However, the aerospace research program provided the following 

 information* relevant to the selection of working stress levels for the acrylic 

 plastic capsule: 



1 . Under short-term tensile cyclic loading at a 6,000-psi stress and a 

 77° temperature level, the life of acrylic plastic is about 10 4 cycles. 



2. Under long-term tension at a 6,000-psi stress and a 77° temperature 

 level, the acrylic plastic will rupture only after about 48 hours of sustained 

 loading. At a 4,000-psi stress level, the material will not fail in less than 

 1,000 hours. 



BasedonMIL-P-5425AandMIL-P-21105C materials. 



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