Plastic buckling of axially compressed cones using the equivalent cylinder approach

Abstract

This paper aims to propose an equivalent cylinder radius for the buckling behavior of relatively thick steel conical shells subjected to axial compression. This is both experimental and numerical/theoretical work. Test data on this subject is scarce and the existing design recommendations have not been validated experimentally, especially in the plastic buckling region. In order to authenticate the appropriateness of these existing rules and suggest others, collapse tests were conducted at the University of Liverpool and Universiti Teknikal Malaysia Melaka on cones and their equivalent cylinder counterparts. The first set of experiments consisted of two nominally identical cones and one equivalent cylinder. These mild steel models were CNC-machined, with the cones having the following geometric parameters: r2 = 99 mm, r1 = 49 mm, h = 100 mm, β = 26.56°, wall thickness of 3 mm. The equivalent cylinder was machined using the DNV CV 30.1 equivalent cylinder radius. The results show that the equivalent cylinder overestimates the conical models by 35% and 44% respectively.The second set of four steel models consisted of: two cones having, r2 = 70 mm, r1 = 40 mm, h = 112 mm, β = 15°, wall thickness of 2 mm, and two equivalent cylinders designed according to ‘ECCS 2008 equivalent cylinder radius’. All four shells were manufactured in pairs using the traditional welding process. It has become evident here that the experimental collapse load predicted by the equivalent cylinders underestimated that of the cones by about 9%. Furthermore, numerical calculation indicated that the equivalent cylinder radius, suggested by the ECCS 2008, mimics its conical counterpart with good accuracy for cones with cone angle, 5° ≤ β ≤ 20°. However, for cones with large cone angle, β > 20°, one obtains large discrepancies between the cone and the equivalent cylinder. This highlights the weakness of the ECCS 2008 recommended equivalent cylinder radius. Finally, a new equivalent cylinder radius for axially compressed cones is suggested with better accuracy as compared to the ECCS current recommendation, and this in turn could be useful for design purposes.