with respect to increases in the t/d ratio. This is understandable because the 

 strength is based solely on physically forcing the material through the flange 

 opening. Ultimate strength tests give an indication of the catastrophic load 

 but do not indicate at what pressure the viewport fails to fulfill its function 

 of providing visibility. 



The design curves in this report, however, are based on the visibility 

 aspect of the viewports and show that the operating pressure or depth is 

 limited by the yield strength of the material. As exhibited by Figures 8 

 through 13, the peak effective stress asymptotically approaches a finite 

 maximum value as the t/d ratio is increased. 



No extensive creep data exist at temperatures other than room 

 temperature. One can, however, get a comparison between 35 and 70°F 

 tests on viewports by comparing experimental displacement results of 

 the t/d = 0.45 viewport in the main test, and the t/d = 0.45 viewport with 

 0-ring in Appendix A. The displacement at 70°F with a time period of 

 500 hours was 7% greater than the displacement at 35°F for the same length 

 of time, and after 4,800 hours the displacement was 20% greater. 



Figure 1 7 together with Table 3 compare the analytical results of 

 this study with the present viewport designs in operational submersibles. A 

 load duration time of 24 hours was chosen to include the emergency time 

 of an average vehicle. From Figure 17 it appears that many of the present 

 designs are conservative, which was probably due to lack of structural analysis 

 at the time of design. 



FINDINGS 



Within the scope of this investigation, the following findings appear 

 to be valid: 



1. Based on both experimental and analytical results, the strength of a 

 viewport increases with an increase in either the t/d ratio and/or the included 

 angle for limits of 0.25 to 1.75 and 60° to 120°, respectively. 



2. The maximum effective stress for a tensile region always occurs at the 

 center of the low-pressure face with a fixed boundary condition. 



3. The maximum effective stress for a compressive region always occurs at 

 the corner of the low-pressure face with a free boundary condition. 



25 



