The strength of thin-walled structures is usually calculated on the basis of the "effective width" model and their ultimate capacity is evaluated using a reduced or effective cross-section and, additionally, the elastic limit for maximum stress. This approach is currently used in almost all design codes. However, short open-cross section members display a significant post-elastic capacity. It means, that the actual load-carrying capacity of such a member is higher than the ultimate load calculated using the method mentioned above [3]. It concerns both static and dynamic loads. Thus, the evaluation of the collapse (crushing) behaviour of thin-walled elements becomes an important problem in modern engineering analysis and particularly concerns the dynamic crushing of thin-walled structures [1].

This paper deals with dynamic crushing of thin-walled sections, namely the axial or eccentric compressive crushing of columns of open cross-sections (top hat-sections). The term "dynamic crushing" is used in the present paper in the sense of progressive crushing of a structure subject to the impulse of an applied load.

The plastic mechanism analysis (yield line analysis) is performed in order to investigate the crushing behaviour of the member [3]. The strain-rate sensitivity of the material is incorporated into the analysis and the strain rate influence on the structural behaviour is investigated. The Cowper-Symonds constitutive relation is taken into consideration. The strain- hardening effect is neglected, so that in the analytical plastic mechanism solution and in the finite element analysis the rigid- perfectly plastic material is taken into consideration The influence of the strain rate upon the upper bound estimation of the load-carrying capacity (ultimate load) is analysed. The finite element analysis taking into account the strain rate effect is performed. Theoretical results (obtained both from analytical and numerical solutions) are compared with the results of experimental crushing tests carried out on the especially designed experimental stand, where the dynamic crushing load is realized by means of the hydraulic system, as well as with the results of the preliminary quasi-static tests.

Corresponding comparative diagrams of loading paths of thin-walled sections under investigation are presented. Some conclusions and remarks concerning the strain rate influence are made.

1

N. Jones, "Structural Impast.", Cambridge University Press, 2003.

2

M. Kotelko, R. Mania, Z. Kolakowski, "Dynamic crushing of thin-walled profiles - strain rate sensitivity analysis", Proc. of ICTWS 2008 Conference, "Recent Innovations and Developments", M. Mahendran, Editor, Queensland University of Technology, Brisbane, 687-693, June 2008.

3

M. Kotelko, "Load-capacity estimation and collapse analysis of thin-walled beams and columns - recent advances", Special Issue, "Cold formed steel structures: recent research advances in Central and Eastern Europe", D. Dubina, Editor, Thin-Walled Structures, 42(2), 153-175, 2004. doi:10.1016/S0263-8231(03)00055-7

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