Papers by Author: Andrew P.F. Little

Abstract: This paper reports on theoretical and experimental investigations into the buckling characteristics of a series of six ring-stiffened circular cylinders that experienced general instability when subjected to external hydrostatic pressure. Each study used between 3-5 designs with the same internal and external diameters, but with different numbers and sizes of ring-stiffeners. Four used designs that were machined to a high degree of precision from steel, while the other two were machined from aluminium alloy. The theoretical investigations focused on obtaining critical buckling pressure values, namely Pcr, for each design from the well-known Kendrick’s Part I and Part III theories, together with an ANSYS finite element prediction. The thinness ratio λ1, which was originally derived by the senior author, was calculated together with a dimensionless quantity called the plastic knockdown factor (PKD), for each model. The plastic knockdown factor was calculated by dividing the theoretical critical buckling pressures Pcr, by the experimental buckling pressures Pexp. The thinness ratio was used because vessels such as these, which have small but significant random out-of-circularity, defy “exact” theoretical analysis and it is because of this that the design charts were produced. Three design charts were constructed by plotting the reciprocal of the thinness ratio (1/ λ1) against the plastic knockdown factor (Pcr / Pexp), using results from Kendrick Part I, Kendrick Part III, and ANSYS. Comparison of the results obtained using Kendrick’s theories and experimentally obtained results was good.

Abstract: The paper describes experimental tests carried out on three ring-stiffened circular conical shells that suffered plastic general instability under uniform external pressure. The cones were carefully machined from EN1A mild steel to a very high degree of precision. The end diameters of the cones, together with their thicknesses were the same, but the size of their ring stiffeners was different for each of the three vessels. In the general instability mode of collapse, the entire ring-shell combination buckles bodily in its flank. The paper also provides three design charts using the results obtained from these three vessels, together with the results obtained for twelve other vessels from other tests. All 15 vessels failed by general instability. One of these design charts was based on conical shell theory and two of the design charts were based on the general instability of ring-stiffened circular cylindrical shells, using Kendrick’s theory, which were made equivalent to ring-stiffened circular conical shells suffering from general instability under uniform external pressure. The design charts allowed the possibility of obtaining plastic knockdown factors, so that the theoretical elastic buckling pressures, for perfect vessels, could be divided by the appropriate plastic knockdown factor, to give the predicted buckling pressure. The theoretical work is based on the solutions of Kendrick, together with the finite element program of Ross, namely RCONEBUR and the commercial finite element package ANSYS. This method can also be used for the design of full-scale vessels.

Abstract: This paper describes an experimental and an analytical and numerical investigation into the buckling behaviour of 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°/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” and also by finite element analyses using ANSYS. The experimental investigations showed that the composite specimens behaved similarly to isotropic materials tested by various other researchers. The specimens failed by the common modes associated with this study, namely due to elastic buckling, inelastic buckling and axisymmetric yield failure. 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 experimentally obtained properties. Two different approaches were chosen for the investigation of the theoretical buckling pressure, 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 famous computer package called “ANSYS”. This latter analyses simulated the composite with a single layer orthotropic element (Shell93) and also with a multi layer element (Shell99). It was found out that the results obtained with ANSYS predicted questionable buckling pressures that could not be reproduced logically. Nevertheless this report provides Design Charts for all approaches and material types. These Design Charts allow the possibility of obtaining a ‘plastic knockdown factor’. The theoretical buckling pressures obtained using MisesNP or ANSYS can then be divided by the plastic knockdown factor, to give predicted buckling pressures. This method can be used for the design of full-scale vessels.