Precursor amplitudes in LiF from shocks propagating in 〈111〉 directions

Abstract

Precursor amplitudes have been measured for impact-produced shock waves traveling in 〈111〉 directions in LiF. Impact pressures range from 30 to 98 kbars and precursor amplitudes for the larger pressures are about 60 kbars for crystals containing approximately 100 ppm Mg and about 50 kbars for undoped crystals. The greatest impact pressure at which elastic response is preserved is about 40 kbars. No precursor decay is evident from the measurements, which include sample thicknesses between 0.3 and 5 mm. Mean precursor decay rates inferred for the first 1/2 mm of travel range up to about 100 kbars/mm. Primary slip systems are not active in this geometry and the shear stress on secondary systems ranges up to 38 kbars at the highest impact pressure. This is still well below theoretical shear strength. Mechanisms for the inferred decay are not identified.

Analysis of nonlinear elastic aspects of precursor attenuation in shock-compressed metallic crystals

“…Corresponding error bars are not shown since these would be smaller than the data points themselves in the figure and would not be visible. Uncertainties in data for Cu and W are not reported in [13], but contemporary experimental data for LiF [19,21] suggest uncertainties in the measured HEL ranging from 1% to 10% and uncertainty in the impact stress on the order of 2%. Because the experimental data are sparse, little constraint on model predictions is enabled, and the apparent agreement of the nonlinear model with the data confirms only that the calculations are physically reasonable but does not provide strong validation of the theory.…”

“…The linear form in (2.7) was applied to interpret precursor decay data from shocked polycrystals [42] and single crystals [13]; a nonlinear form analogous to (2.6) was applied to evaluate precursor decay data from single crystals [19][20][21]. Alternative derivations of equations identical to or very similar to (2.4)-(2.7) can be found in [14,15,18,20,21,39].…”

“…The pre-stressed state corresponds to a condition of static equilibrium and homogeneous deformation, and will later serve as an idealization of the region just behind the precursor front at a fixed instant in time. The identical idealization, though unstated, was invoked in prior work [18,19,21] wherein third-order Lagrangian elasticity was used to compute c l . It is most reasonable for steady shocks wherein particle velocity behind the precursor is constant, since deformation gradients and equations of motion are invariant upon superposition of a constant background velocity field (i.e., rigid body translation) in the pre-stressed state.…”

“…. The factor of 1/J transforms the wave velocity a relative to the compressed state at density ρ to c l relative to the natural state at density ρ 0 [18,21].…”

“…Importance of consideration of elastic nonlinearity, especially effects of stress on effective shear stiffness, for correctly predicting precursor behavior have been noted [14,15]. The importance of nonlinear elastic effects on weak shock wave structure was first quantified in experiments on rock materials [17], and further examined in the context of ductile nonmetallic crystals (specifically, lithium fluoride [18][19][20][21]).…”