Flanged Acrylic Plastic Hemispherical Shells for Undersea Systems—Part 2: Static and Cyclic Fatigue Life Under Hydrostatic Loading

Abstract

Over 25 acrylic plastic windows with t/Ri = 0.364 in the shape of hemispherical domes with equatorial flanges have been thermoformed from flat sheets and tested under short term, long term, and cyclic pressure loading at 65–75°F ambient temperature. Two kinds of flanges with O-ring grooves on the bearing surfaces were experimented with: Type 1, a flat lip with a rounded heel and instep, and Type II, a conical lip with a rounded heel. The 14,500 psi short term critical pressure of hemispherical windows with t/Ri = 0.364 was found to be independent of the equatorial flange configuration. Both the static and cyclic fatigue lives of the windows were also found to be independent of equatorial flange configuration. In either case, the maximum acceptable working pressure for 65–75°F temperature range was found to be 1000 psi. Only by elimination of the O-ring groove in the bearing surface of the window flange and the use of a thin neoprene bearing gasket between the arylic flange and the steel is it possible to extend the working pressure to 2000 psi for 65–75°F temperature range. Operating the flanged windows at pressures in excess of the safe working pressures shown above will generate fatigue cracks in the bearing surface of the flange in less than 1000 pressure cycles; at 5000 psi pressure the cyclic fatigue life decreases to less than 100 cycles.