On the dynamic crushing of open-cell aluminum foams
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This study was designed to examine the effect of impact velocity on the crushing behavior of open-cell aluminum foam over a range of velocities similar to what would be encountered for impact mitigation and blast protection applications. An experimental set-up was designed, fabricated and validated for studying the crushing response of cellular materials at high velocities. It consists of a gas gun, a pressure bar, high-speed data acquisition and high-speed imaging. The facility uses high-speed video images of the crushing event synchronized to force measurements with a pressure bar at one end of the foam to examine the dynamic stress and deformation history of foam specimens. Ten pores per inch open-cell Al-6101-T6 Doucel foam cylindrical specimens with a relative density of about 0.085 were impacted in the rise direction at velocities ranging from 21.6 to 127 m/s. The experimental results show that for impact speeds greater than about 40 m/s crushing of the foam occurred through a shock front. Furthermore, the experiments show an increase in the densification strain, average stress in the crushed region and shock velocity with increasing impact velocity, whereas the stress in the uncrushed region appears to be insensitive to velocity. A method of determining the states across a shock front was derived from shock equations by enforcing conservation of mass and momentum. This was verified through a combination of experiments and direct measurements. The use of high-speed imaging and pressure bar measurements allowed this derivation to be independent of any constitutive model and showed that the assumptions in the commonly used rigid-perfectly-plastic-locking model are not applicable for dynamic impacts. A shock Hugoniot was generated from the test data to characterize the impact response of the foam.