Keywords: circular cylinder, buckling, axisymmetric yield, composite, external pressure, ANSYS.

This paper describes an experimental and an analytical investigation into the collapse of 44 circular cylindrical composite tubes under external hydrostatic pressure. The results for 22 of these tubes were from a previous investigation and the results for a further 22 models are reported for the first time in the present paper. 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°/90°/0°/90°/0°; the carbon fibres were laid lengthwise (0°) and the E-glass fibres circumferentially (90°). The theoretical investigations were carried out using a simple solution for isotropic materials, namely a well-known formula by "von Mises", together with numerical solutions based on ANSYS.

The experimental investigations showed that the composite specimens behaved similarly to isotropic materials previously tested, in that the short vessels collapsed through axisymmetric deformation while the longer tubes collapsed through non-symmetric bifurcation buckling. Furthermore it was discovered that the specimens failed at changes of the composite lay-up due to the manufacturing process of these specimens. These changes seem to be the weak points of the specimens.

For the theoretical investigations two different types of material properties were used to analyse the composite. These were calculated properties derived from the properties of the single layers given by the manufacturer and also the experimentally obtained properties.

Two different approaches were chosen for the investigation of the theoretical buckling pressures, of the previously analysed models, namely a program called "MisesNP", based on a well-known formula by von Mises for single layer isotropic materials, and two finite element analyses using the computer package called "ANSYS". These latter analyses simulated the composite with a single layer orthotrophic element (Shell93) and also with a multi-layer element (Shell99). The results from Shell93 and Shell99 agreed with each other but, in general, their predictions were higher than the analytical solution by von Mises.

A large submarine can only dive to a depth of about 400m (1312 ft), but the deepest part of the oceans is 29 times deeper than this. The reason why a large submarine cannot dive to the average depth of the oceans is that the material used for their pressure hulls is high-tensile steel. That is, as the submarine dives deeper and deeper into the oceans, the external hydrostatic pressure increases, so that the wall thickness has to be increased. Eventually, the wall thickness becomes so large that the vessel has no reserve buoyancy and will sink like a stone to the very bottom of the ocean. Ross [1] has found that for a submarine of internal diameter 10m (32.81ft) and constructed in high-tensile (HY80) steel, the thickness of its hull will be 2.3 m (7.58ft), if it is to be designed to dive to the bottom of the Mariana's Trench! The only way to overcome this problem is to use a material with a higher strength: weight ratio than high-strength metals and this is the reason why the current work is presented.

1

Ross, C.T.F., Pressure Vessels: External Pressure Technology, Horwood Publishing Limited, Chichester, 2001.

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