A design procedure for large spherical dames under external pressure

Large, thin-walled, spherical domes are favored as view windows in underwater leisure vehicles. Such domes are made of polycarbonate material for clarity. Also, their design and construction are governed by the applicable ASME Code [1]. At the basis of the said ASME standard are small aperture windows subjected to high pressure, such as those used in very deep sea research vehicles. However, the leisure vehicles are different both in size and the applied external pressure. Such domes need to be designed against buckling. Buckling of apparently symmetrical shapes can be unsymmetrical due to thickness variations, property variations, orientations, non-linear behavior of the material, hydraulic gradients of the pressure over the dome surface, and fluctuations of external pressure due to waves. Furthermore, the theoretical mode shapes for perfect, thin walled, symmetrical shells can themselves be unsymmetrical. Thus, one needs to recognize the shell buckling in unsymmetrical mode, at a pressure far less than the critical pressure predicted by linear elastic theories; also, the corresponding mode shape does not necessarily relate to the lowest mode. In this paper, an extra-code viewpoint is discussed for the design of large view diameter domes for functionality. An example is used to simulate the buckling process by the finite element method. Lastly, a design procedure is proposed which essentially assumes the experimentally observed lower buckling pressures as the basis rather than the linear elastic buckling formulas.