Keywords: interactive buckling, axial compression, external pressure, conical shells, eccentric loading, Mylar.

Motivation for the current paper comes from the lack of up to date experimental data for elastic buckling of shells subjected to simultaneous and independent action, of two or more, loads. The case of axial compression and external pressure acting together has not been thoroughly researched in the past. The existing experimental data is 'old', not comprehensively described and sometimes difficult to interpret. With this in mind a test rig for buckling tests of shell components subjected to axial compression (centric or eccentric) and, or external pressure has been developed. Conical shells were chosen to validate its performance. This was partially motivated by parallel research into the combined stability of steel cones, [1,2]. Five cones, of different geometries, were manufactured using Mylar material which over the years has been the backbone of buckling tests within the elastic range. Shape measurements were taken prior to tests. Here, a different approach to all previously known tests was adopted. Cones were sprayed with an ultra-thin layer of conducting carbon in order to avoid mechanical contact between the probe and the shell's wall. Instead, the closure of an electrical circuit served as a measure of the probe-wall contact. It is believed that in this approach there has been no pressure applied on the Mylar wall during the shape measurements. All models had an internal mandrel which supported the shell once it buckled. Once the load was removed the shell returned to the previous state. The use of the mandrel allowed specimens to be re-tested under different combinations of axial compression and external pressure. The paper provides interactive plots for all models.

In practice, the load may be accidentally shifted from the central axis. The effect of such eccentric loading is also discussed in the paper.

On the analysis part, the paper extracts the worst inward and outward dimple-type shape distortions. It has been found that the magnitude of these invisible by naked eye distortions was up to 3.3 times the wall thickness for the case of outward dimples (and 2.1 times for the inward dimples). The finite element calculations have been carried out for the interactive diagrams. Results have been obtained for perfect models. As expected, large differences between theory and experiment exists. But these can be attributed to the existence of geometric shape imperfections. The source of these is also discussed in the paper. One possibility is associated with embedded residual stresses which have developed during the long term storage of Mylar in a roll (approximately ten years).

1

O. Ifayefunmi, "Combined stability of conical shells", PhD Thesis, The University of Liverpool, 2011.

2

J. Blachut, O. Ifayefunmi, "Plastic buckling of conical shells", Journal of Offshore Mechanics and Arctic Engineering, 132(041402), 1-11, 2010. doi:10.1115/1.4001437

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