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.