Stress Analysis of a Spherical Acrylic Pressure Hull

This study was initiated to: (1) perform an analytical structural analysis on an acrylic pressure hull, (2) compare the analytical results with all available experimental results, and (3) present an operating-depth curve. The design analyzed was a pressure hull incorporating twelve spherical pentagons of acrylic plastic bonded together to form a sphere with an outside diameter of 66 in. and a wall thickness of 2.5 in. Conical steel penetrations were located at the two poles. The experimental results were obtained from strain-gage data from two independent pressure tests to 500 psi on two acrylic hulls of the same design. The analysis on the structure was performed with a finite-element computer code with particular emphasis on the acrylic-steel boundary. The boundary conditions at the acrylic-steel interface were two extreme cases: perfectly fixed and perfectly free. A time-dependent yield-failure criterion for acrylic plastic was combined with the structural analysis to provide an operating-depth curve as a function of both time and temperature. Comparison of analytical and experimental results indicated excellent agreement. At a temperature of 70 deg F and a maximum of 50 hr load duration, the acrylic hull can operate to 1000 ft with a safety factor of 1.5 based on yield and a safety factor of 2.6 based on collapse.