High strain rate behavior of spark plasma sintered magnesium aluminate spinel

The high strain rate behavior of Spark Plasma Sintered (SPS) magnesium aluminate spinel (spinel) disc-shaped samples was studied under parallel (planar impact) and divergent (spherical impact) flow conditions. The results of planar impact experiments allowed determining the principal Hugoniot up to 40-GPa compressive stress. The Hugoniot Elastic Limit (HEL) ranged from 10.8 to 12.2 GPa and increases slightly with the magnitude of the impact. The spall strength determined in low-velocity impact experiments has a 0.63-GPa maximum at an impact stress of 1.7 GPa and vanishes completely as the impact stress exceeds 2.1 GPa. The spherical impact tests (convex copper impactors with radii of curvature from 45 to 550 mm and velocities from 290 to 570 m/s) were carried out in parallel with AUTODYN-2D numerical simulations of the experimentally recorded velocity histories. The results allowed establishing the compressive failure threshold surface of spinel in the principal stress space. At a transverse stress of 2.4 GPa, the failure threshold undergoes a change of the dominant mode of the inelastic deformation from brittle cracking to some form of ductile yielding.