Keywords: crashworthiness, foam-filled columns, finite element modelling, multi-cell modelling, unit-cell models, quasi-static.

The improvement of the crashworthiness of automobiles cannot be overestimated. An efficient crashworthy design system will: (i) dissipate the kinetic energy of the impact in a controllable manner, (ii) retain a survival space for the occupants, and (iii) minimise the forces and accelerations experienced by the occupants. Metallic foams and foam-filled structures possess an excellent potential for enhancing vehicle crashworthiness have due to their superior specific mechanical properties and low weight. Furthermore, the presence of the foam as a filler material in thin-walled extrusions leads to a modification of the mode of collapse of the column, thereby increasing the energy absorption.

The mechanical behaviour of metallic foams and foam-filled structures has frequently been studied using oversimplifying assumptions and methods, which overlook the cellular nature of the material and assume full periodicity of the foam, thus ignoring edge and non-uniform cell-size effects. In this paper, we address these issues and present a multi-cell finite element approach for modelling the quasi-static collapse of aluminium foam-filled structures. A representative unit-cell model recently introduced in the literature [1] is employed to construct multi-cell models of the foam-filler, thus accurately represent the actual foam structure [2]. The work was carried out using LS-DYNA explicit finite element solver. The effect of the foam density, the column wall thickness, and the column cross section dimensions on the load-deformation characteristics and the crush behaviour of the composite foam filled structure is examined and evaluated.

The results revealed that the developed three-dimensional finite element model was able to accurately capture the mode of collapse of quasi-statically and dynamically loaded foam-filled box columns. The column walls and the foam filler were modelled using shell elements, and the column wall material was modelled using an elasto-plastic material law with isotropic hardening and the von-Mises yield criterion. The box column was given an initial imperfection in the form of a cosine shaped trigger in order to initiate the mode of collapse observed in testing. The effect of the impact speed, column wall thickness, column width and foam density on the collapse of foam-filled box columns were explored. From several FE results the following conclusions can be made. The first is that the collapse load increases with foam density, which is directly proportional to its density. For the foams examined in this study, the 10% foam showed the largest increase in the specific energy absorption, which indicates that there is a minimum foam density for which foam-filling is effective. The largest change in the mean load and specific energy absorption occurred with the thinnest tube, with the width being held constant. These results can be readily used in crashworthy design calculations.

1

S.A. Meguid, S.S. Cheon, N. El-Abbasi, "FE modelling of deformation localization in metallic foams", Finite Elements in Analysis and Design, 38, 631-643, 2002. doi:10.1016/S0168-874X(01)00096-8

2

M.S. Attia, J.C. Stranart, S.A. Meguid, "Influence of Cellular Imperfections on Mechanical Response of Metallic Foams", International Journal of Mechanical Sciences, submitted, 2004.

purchase the full-text of this paper (price £20)

go to the previous paper
go to the next paper
return to the table of contents
return to the book description
purchase this book (price £135 +P&P)