2) The low modulus of elasticity and plas- 

 tic flow characteristic permit localized 

 yielding and redistribution of stresses. 



3) The plastic flow in the form of extrusion 

 and extensive fracturing provides warn- 

 ing of impending failure sufficiently in 

 advance to terminate a dive without the 

 viewport imploding. 



4) Another report (23) states that before 

 failing plastic becomes translucent, but 

 this was not reported in the NCEL stud- 

 ies. 



For such reasons acrylic plastic is the 

 prime viewport material in submersibles. 

 That plastics will remain in this exclusive 

 position is difficult to predict. According to 

 Edgerton (24), glass has the advantage of 

 less optical distortion than plastic, and from 

 Figure 5.3, it is clear that glass offers the 

 best W/D ratio. The present difficulty in 

 working with glass primarily reflects its 

 newness as a candidate for pressure hulls. 

 Acrylic plastic also offered initial difficulties, 

 but years of research and testing have 

 brought it to the point where it will advance 

 from a Category III to a Category II mate- 

 rial (see Chap. 13) after long-term loading 

 tests, now in progress, are completed to de- 

 termine its creep and fatigue characteristics. 

 If the need for glass as a pressure hull, 

 viewport or other component material be- 



comes pressing, it is likely that the technol- 

 ogy will evolve to overcome its present defi- 

 ciencies. 



Three configurations are used to join 

 acrylic plastic viewports to the pressure hull: 

 Flat circular discs, truncated cones and wrap- 

 around windows such as in Martine's SUB- 

 MANAUT. Considerable testing and evalua- 

 tion of the first two forms (Fig. 5.14 & 5.15) 

 have been conducted on acrylic viewports at 

 the U.S. Naval Civil Engineering Laboratory 

 (25, 26). No studies are reported for the char- 

 acteristics of the wrap-around variety. 



Flat Viewports: The results of the NCEL 

 studies show that the windows should be 

 sealed in a flange cavity by means of a 

 radially compressed "O" ring contained in a 

 circumferential groove midway between the 

 viewport's parallel faces. The viewport may 

 bear directly on the steel seat or on a V32- 

 inch-thick neoprene gasket. If no gasket is 

 used, a liberal coating of silicone grease ap- 

 plied to the flange may suffice. To hold the 

 viewport a retaining ring resting on a V4- 

 inch rubber gasket of 60- to 100-durometer 

 hardness is recommended. The viewport 

 should have a maximum diameter to minor 

 opening ratio of 1.5 and a radial clearance of 

 0.005 inch or less between viewport edge and 

 flange cavity (Fig. 5.14). Reference (25) rec- 

 ommends that a safety factor of 4 be used at 



RETAINING 

 RING 



NSERT 



VIEWPORT 



Fig. 5.14 Flat acrylic plastic viewport. 

 260 



