Hugoniot Elastic Limit of Single-Crystal Sodium Chloride

Abstract

Single-crystal specimens (7- and 12-mm thick) of sodium chloride were impacted with flat-nosed, gas-driven projectiles, and the Hugoniot elastic limit (HEL) was determined by reducing quartz gauge measurements. The HEL for the [100], [110], and [111] crystal directions was 0.26, 0.77, and 7.4 kbar, respectively. Stress—time profiles for specimens shocked in the [100] and [111] direction show evidence of stress relaxation behind the elastic precursor. This phenomenon is more pronounced in 12-mm-thick specimens. The ratio of the resolved shear stress on the active slip systems for uniaxial strain (shock loading) conditions to that for uniaxial stress (static loading) indicates a strain-rate effect. This ratio increases from 3.1 for loading in the [100] direction to 8 for the [110] direction and to 21 for the [111] direction. The anisotropy of the HEL with crystal direction is related to the resolved shear stress on the primary and secondary slip systems in single-crystal sodium chloride. The large HEL for shock loading in the [111] direction is a consequence of the resolved shear stress on the primary slip systems being zero. Thus, for deformation by slip to occur, a secondary slip system (or systems) must be activated which will require a higher resolved shear stress. The experimental data for single crystals of copper and beryllium can also be explained in terms of the resolved shear stress.