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CivilComp Proceedings
ISSN 17593433 CCP: 88
PROCEEDINGS OF THE NINTH INTERNATIONAL CONFERENCE ON COMPUTATIONAL STRUCTURES TECHNOLOGY Edited by: B.H.V. Topping and M. Papadrakakis
Paper 141
Plastic General Instability of RingStiffened Conical Shells under External Pressure C.T.F. Ross, A.P.F. Little and G. Andriosopoulos
Department of Mechanical & Design Engineering, University of Portsmouth, United Kingdom C.T.F. Ross, A.P.F. Little, G. Andriosopoulos, "Plastic General Instability of RingStiffened Conical Shells under External Pressure", in B.H.V. Topping, M. Papadrakakis, (Editors), "Proceedings of the Ninth International Conference on Computational Structures Technology", CivilComp Press, Stirlingshire, UK, Paper 141, 2008. doi:10.4203/ccp.88.141
Keywords: general instability, plastic buckling, ringstiffened cones, external pressure, ANSYS, finite elements.
Summary
The paper describes, for the first time, experimental tests carried out on
three ringstiffened circular conical shells that suffered plastic general
instability under uniform external pressure. In this mode, the entire
ringshell combination buckles bodily in its flank. The cones were carefully
machined from EN1A mild steel to a very high degree of precision.
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 fifteen 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 ringstiffened circular cylindrical shells, using Kendrick's theory, which were made equivalent to ringstiffened circular conical shells suffering from general instability under uniform external pressure. The design charts allow for the possibility of obtaining plastic knockdown factors, so that the theoretical elastic buckling pressures for perfect vessels can 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 [1], namely RCONEBUR and the finite element package ANSYS. This method can also be used for the design of fullscale vessels. Submarine pressure hulls usually take the form of ring stiffened circular cylinders and cones blocked off by dome ends. If a long thinwalled circular cylinder or cone is not ringstiffened, its buckling resistance under uniform external pressure is abysmally poor, [1]. Such a vessel may fail by nonsymmetric bifurcation buckling, or shell instability. One method of greatly improving the buckling resistance of such vessels is to ringstiffen them. If, however, the ringstiffeners are not strong enough the entire ringshell combination can collapse due to the application of uniform external pressure. This mode of failure is known as general instability. Exact theoretical analysis of many of these structures has been defied, particularly for the less slender vessels that buckle plastically, as the slightest initial outofcircularity causes the vessels to fail at buckling pressures that are much lower than predicted by theory. The reason for this is partly because the initial outofcircularity is random and difficult to model. In practice this random initial outofcircularity grows nonlinearly with increase in uniform external pressure so that one part of the vessel becomes plastic. When this occurs, the tangent modulus of the vessel in the area that has gone plastic, decreases to quite a small value. This further complicates the deformation, and causes other parts of the structure to go plastic. The situation is then worsened and eventually the vessel suffers sudden and catastrophic failure. The paper presents several such design charts, which can be used for fullscale vessels. References
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