Keywords: ANSYS, BCLAMB, finite elements, circular cylinder, buckling, axisymmetric yield, composites, external pressure, submarines.

This paper describes a number of investigations, both experimental and theoretical, into the collapse of thirty two composite tubes, each of circular cross-section, under the application of external hydrostatic pressure. These investigations were on the collapse of fibre reinforced plastic tube specimens, sixteen of which had been manufactured from fifteen carbon layers, and sixteen of which were manufactured from nine E-Glass layers. This work is of much importance for deep diving submarines; especially for large diameter submarines, which are required to conquer the Mariana Trench. The theoretical work was carried out using separate finite element computer programs, as follows:

1. The 'in house' finite element computer program, called 'The Buckling of Composite Layer Analysis Method' (BCLAMB). This program used 2, 3 and 4 Gauss points, to test for numerical instability, arising from 'locking'.
2. Another finite element computer program, called 'Poseidon'; which was created to overcome the limitations of BCLAMB; namely the fact that only twenty layers of fibres were permissible with BCLAMB, for each specimen.
3. Lastly, the commercially available finite computer program ANSYS, using Shell 93 and Shell 99 elements. This program had the significant advantage of providing graphical displays.

At this point in time, it is convenient to mention that the British Standard 5500 code (PD 5500), does not have the capability of being able to analyse multi-layered composite shells, buckling under external hydrostatic pressure, nor does the literature published by the principal United Kingdom (UK) classification society, namely Lloyds of London. Hence, the work presented in this paper is both novel, and of use to industries, concerned with external pressure vessel construction, using composite materials; especially the resulting design charts provided in this paper, for the first time. The experimental investigations carried out illustrated that the composite tube specimens tested, behaved in a manner comparable to isotropic and composite models previously tested. Moreover, it was determined that the models suffered failure at changes of the composite lay-up, as a consequence of the manufacturing process used for the models in question. The oceans cover some 71% of the Earth's surface, and large quantities of precious metals and minerals, together with large quantities of very valuable methane hydrates have been found in the deep oceans [1], and this is the reason why the present study has been conducted.

1

G.R. Dickens, C.K. Paull, P. Wallace, "Direct measurement of in-situ quantities in a large gas-hydrate reservoir", Nature, 385, 30 January 1997. doi:10.1038/385426a0

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