Keywords: elastic buckling, inelastic buckling, axisymmetric deformation, circular tubes, submarines, carbon and glass fibre composites, external hydrostatic pressure, finite element methods, ANSYS.

This paper describes an experimental and a theoretical investigation into the buckling behaviour of circular cylindrical composite tubes under external hydrostatic pressure. The investigations concentrated on fibre reinforced plastic tube specimens made from a mixture of three carbon and two E-glass fibre layers. The lay-up was 0^o/90^o/0^o/90^o/0; the carbon fibres were laid lengthwise (0^o) and the E-glass fibres circumferentially (90^o). The theoretical investigations were carried out using a simple solution for isotropic materials, namely one by "von Mises" and also by the finite element method using ANSYS.

The experimental investigations showed that the composite specimens behaved similarly to isotropic vessels tested by various other researchers. The specimens failed by the common modes due to elastic buckling, inelastic buckling and axisymmetric yield failure. Although three quarters of our planet is covered with water, only about 0.1% of the oceans' bottoms have been explored. Many scientists are interested in the oceans, and some believe that beneath the deep oceans, which can have a depth of more than 11,000 metres, lie big resources of methane, minerals, etc. Although the scientists changed their opinions about the amount of methane hydrate in the oceans from e.g. 3 x 10¹⁸ m³ to 1-5 x 10¹⁵ m³, a value approximately one order of magnitude lower, these resources are still bigger then the known resources above the deep sea.

Additionally the deep sea offers an excellent place to store carbon dioxide. According to the U.S. Department of the Interior, it is planned to inject 50 to 100 tons of liquid carbon dioxide into waters off Kona, Hawaii. Even the tourist industry would be interested in submersibles that can cope with the deep-sea environments. Future expeditions under the edge of the Arctic or Antarctic and equivalent offer a big business and wait for suitable submersibles.

The main reason why the deep oceans haven't been explored by large submarines is because of the high pressures in the depths of the oceans. The Mariana's trench has a depth of approximately 11.52 km, and causes a hydrostatic pressure of around 1150 bar or 115 MPa.

The shapes and especially the materials of submersibles used today consist partially of technical knowledge that is more than 100 years old. The material used for the pressure hulls of modern underwater vehicles is high-tensile steel. Steel is well known and has been developed for several hundred or even thousands of years, but its strength: weight ratio, even of today's high alloy steels, is too low to build large submersibles for the high pressures in the deep oceans. The required wall thicknesses for these vessels would be so large that the vessels would sink like stones to the very bottoms of the oceans. The strength: weight ratios of steels is not enough to dive into the deepest regions of the oceans; this has been pointed out previously. A new kind of material with a much higher strength: weight ratio than high-tensile steel has to be used.

Thus, new materials like fibre composites are needed for the exploration and utilisation of the deep oceans, which have a much higher strength: weight ratio than steel. It is for this reason that the present study has been made. Additionally, it will be attempted to create Design Charts that help to predict the buckling or other failure of pressure hulls for these composites.

purchase the full-text of this paper (price £20)

go to the previous paper
go to the next paper
return to the table of contents
return to the book description
purchase this book (price £75 +P&P)