Low-velocity impact indentation rate sensitivity of aluminium foams

In this work, closed cell aluminium foams of different densities and correspondingly different cell structures were evaluated to characterise their impact indentation rate sensitivity i.e. increased indentation resistance with an increase in drop velocity. Impact indentation tests were carried out using an instrumented INSTRON 9350 drop weight impact tester. Drop velocity was varied from 1.81 to 6.85 m/s with corresponding impact energy varying between 9-120 J. The dynamic transmitted force (DTF) was taken as the measure of resistance to impact indentation and found to have a strong dependency on drop velocity and foam density. The foams with less than 0.53 g/cc density showed noticeable rate sensitivity, while the foams of higher density were rate insensitive. The dependence of rate sensitivity on foam density under impact indentation was related to the differences in cell structure for the low and high-density foams and the consequent differences in the mechanics of deformation. A hypothesis was proposed wherein the plateau borders of the hexagonal shaped cells (as seen in 2D sections) of the low-density foams, which were statistically aligned with the direction of loading, were envisaged to undergo inertia-induced compression before bending resulting in the enhancement of DTF at higher drop velocities. On the other hand, in the high-density foams with their characteristic circular shaped cells (as seen in 2D sections), deformation was only by plateau border bending and the rate sensitivity was negligible. The densified cell zone developing beneath the indenter stabilised the constraining forces, leading to rate-independent DTF.