Formulation of Design Charts for Composite Submarine Pressure Hulls

Abstract

This paper describes an experimental and a theoretical finite element (FE) numerical investigation into the collapse of 44 circular cylindrical composite tubes under external hydrostatic pressure. The results for these 44 tubes are from earlier investigations carried out by Ross et al. (2007, 2008), with the resulting design charts building upon earlier work by Ross et al. (1999) and is now presented in a new format for the first time in this paper. The investigations carried out concentrated upon fiber-reinforced plastic tube models, manufactured from a mixture of three carbon and two E-glass fiber layers. The material lay-up was 0 ° / 90 ° / 0° / 90 ° / 0 ° with the carbon fibers being laid lengthwise (0°) and the E-glass fibers being laid circumferentially (90°). The theoretical investigations were carried out using a simple solution for isotropic materials, namely the well-known von Mises Formula, together with FE solutions of a numerical solution. The earlier investigations also used a numerical solution using the software program ANSYS (Canonsburg, PA), however the results of this work were found to be disappointing. The experimental investigations carried out showed that the composite models behaved similarly to isotropic materials previously tested, such that short models suffered collapse through axisymmetric deformation, whilst the longer tubes collapsed through nonsymmetric bifurcation buckling. Additionally, it was discovered that the models suffered failure at changes of the composite lay-up due to the manufacturing process of these particular models and these changes at which failure occurred give the appearance of being the weak points of the models. To carry out the theoretical investigations, two different types of material properties were used for the composite analysis. First, the material properties of the single layers, as supplied by the manufacturer and second, a set of material properties derived experimentally within the engineering laboratory. So as to verify the previously obtained experimental and theoretical results, the identical approach was chosen, in that MisesNP (Portsmouth, UK) was used, which is a program based on the von Mises Formula for single layer isotropic materials. The second theoretical method used two finite element analyses, using the commercial software package ANSYS. Specifically, the ANSYS analyses simulated the composite with a single layer orthographic element (Shell 93) and secondly with a multilayer element (Shell 99). Although the results from Shell 93 and Shell 99 agreed with each other, their predicted buckling pressures were, in general, higher than those predicted by the von Mises analytical method. With respect to the longer models, the von Mises solution produced a more consistent set of results, with the failure mode being elastic instability; this being highlighted when the experimentally obtained material properties were used. Hence the conclusion drawn was that the results obtained via finite element analyses produced buckling pressures that are questionable. This report provides design charts for all 44 models, using all of the theoretical approaches previously adopted, with both theoretically obtained material properties and those material property values obtained experimentally. The resulting design charts have not been previously published for all 44 models combined. These resulting design charts allow the possibility of obtaining a Plastic Knockdown Factor (PKD) for the models under investigation. Once the theoretical buckling pressures have been obtained, using either MisesNP or ANSYS, the theoretical buckling pressures can then be divided by the PKD via the design charts, so as to obtain the predicted buckling pressures. Work has been carried out by Sonardyne International Limited of Yateley, Hampshire, UK, using the design charts of Ross et al. (2007, 2008), in preference to BS5500 (British Standards Institution 2000), with substantial weight savings being made for their pressure vessels.