Shear localization in high-strain-rate deformation of granular alumina

Dynamic deformation of densified granular alumina of two different particle sizes was investigated by the radial symmetric collapse of a thick-walled cylinder. The densified granular alumina was used to model the flow in ballistic impact and penetration of fragmented ceramic armor. Shear localization was a well developed deformation mode at an overall radial strain of ∼0.2–0.4 and strain rate of 104 s−1. The following qualitative features of shear bands were established:• Shear bands have clear boundaries and their thickness does not depend on the initial particle size and has a typical value ∼10 μm.• The structure of the shear bands was dependent on initial particle size, suggesting differences in the mechanisms of flow. For the ∼4 μm alumina, comminution (break-up) and softening of particles were observed. For the ∼0.4 μm particles, a peculiar structure consisting of a central crack with two lateral cracks was formed.• Distributions of shear bands and displacement magnitudes were dependent on initial particle size.The observed differences in powder behavior are associated with different mechanisms of powder repacking. For large particles (∼4 μm), additional hardening resulting from microfracture and subsequent repacking of different size particles in the powder takes place. The small-sized (∼0.4 μm) ceramic does not go through the particle fracturing stage and the hardening is due to “classical” repacking.