Deformation and melting of steel projectiles in hypervelocity cratering experiments

Abstract– We carried out hypervelocity cratering experiments with steel projectiles and sandstone targets to investigate the structural and mineralogical changes that occur upon impact in the projectile and target. The masses of coherent projectile relics that were recovered in different experiments ranged between 58% and 92% of their initial projectile masses. A significant trend between impact energy, the presence of water in the target, and the mass of projectile relics could not be found. However, projectile fragmentation seems to be enhanced if the target contains substantial amounts of water. Two experiments that were performed with 1 cm sized steel projectiles impacting at 3400 and 5300 m s−1 vertically onto dry Seeberger sandstone were investigated in detail. The recovered projectiles are intensely plastically deformed. Deformation mechanisms include dislocation glide and dislocation creep. The latter led to the formation of subgrains and micrometer-sized dynamically recrystallized grains. In case of the 5300 m s−1 impact experiment, this deformation is followed by grain annealing. In addition, brittle fracturing and friction-controlled melting at the surface along with melting and boiling of iron and silica were observed in both experiments. We estimated that heating and melting of the projectile impacting at 5300 m s−1 consumed 4.4% of the total impact energy and was converted into thermal energy and heat of fusion. Beside the formation of centimeter-sized projectile relics, projectile matter is distributed in the ejecta as spherules, unmelted fragments, and intermingled iron-silica aggregates.