Structural Performance of a Hybrid Sandwich Submersible Pressure Hull : Stress and Stability Analysis of a Stiffened Cylindrical Shell including Through-thickness Shear

An analytical study was conducted on a submersible pressure hull composed of a hybridsandwich cylindrical shell reinforced with circular frames. Unlike most sandwich plates, the hybrid consists of a thin core with thick faces. Hulls fabricated from such plate when employing a visco-elastic core exhibit inherent acoustic damping properties when radiated with external noise. Typically, however, candidate core materials are several orders less stiff than the faces. The study addresses failures due to hydrostatic pressure including yielding and stability modes with regard to the reduced through-thickness shear rigidity. In order to assess the feasibility of the structure, frame size and spacing are optimized for each case. Results on hybrid plate buckling under hydrostatic pressure are verified by experimentation on an unstiffened ring. Finally, approximate relations are derived which consists of traditional equations adapted to use equivalent plate properties. The analytical results indicate that only a modest reduction in diving depth results after introducing cores which promise good acoustic performance. Furthermore, the depth performance can be recovered by redesign of the hull resulting in a weight increase of less than one percent. As the core stiffness is reduced below that of the face, the interframe yield failure mode is most affected followed by general instability and interframe buckling. At lower stiffness ranges, this order reverses. Theoretical through-thickness shear affects were verified with reasonable agreement by conducting a buckling test on a steel ring with an epoxy core. Finally, for some modes, the approximate relations developed agree well with the more rigorous analysis and allow simple and accurate results to be computed. Thesis Supervisor: J. Robert Fricke Title: Assistant Professor