Keywords: finite element method, metallic honeycombs, dynamic crushing, energy absorption, medium velocity impact.

Cellular materials, which can be defined as materials with complex internal topologies and an extremely high content of voids ranging from 50% to 90 %, are popular in their use as protective materials against impact as they are lightweight yet able to absorb large amount of impact energy but transmit relatively low and stable impact forces to the protected object. The effectiveness of such materials for protection against impact loading is evident in natural materials such as fruit skins, bones and animal shells. Also, the use of cellular materials as core materials in sandwich panels has been investigated rather extensively.

Current and past research on cellular materials are mostly focused on the behaviour of metallic foams and hexagonal honeycombs with extremely low relative densities of around 10%, which has been shown to be very effective in minimizing impact force. However, despite the considerable effort which has been made to establish the basic understanding of metallic foams and hexagonal honeycombs, it has yet to be proven that they are the most efficient form of cellular material for impact protection. It is obvious that the mechanical responses of such materials are highly dependent on their internal topologies, but the impact resistance of cellular materials with other cell shapes and higher relative densities has not been widely studied. Also, the focus of past studies has only been on the impact force rather than the energy absorption capacity of these materials. Moreover, the base materials of these honeycombs are assumed to be elastic-perfectly plastic in past numerical investigations; the effect of base material ductility on the impact resistance of cellular materials has not been considered so far.

These issues are addressed in this paper, in which finite element simulations are carried out to investigate the effects of cell shape and base material ductility on the impact force and energy absorption capacity of metallic cellular materials under medium velocity impact. The results show that cell shape has a very significant influence on the overall response. At the same relative density, cellular specimens with square cells have the greatest overall stress and energy absorption capacity followed by those with triangular, hexagonal and rhombic cells. For these cellular materials with basic shapes the higher energy absorption capacity typically results from the ability of the cellular materials to sustain a larger overall stress rather than having a greater densification strain.

When the effect of base material ductility is taken into account, the overall stress and energy absorption capacity reduces with lower base material ductility. With lower base material ductility, more localised deformation patterns are observed as a result of less extensive load redistribution. Also, the effect of base material ductility on the overall response of the cellular materials is more significant in specimens with higher relative densities. However, the effect of cell shape on the overall response is not significantly altered even when base material ductility is taken into account because the primary modes of load resistance and energy absorption are unchanged.

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