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.