Analysis of internal stress and anelasticity in the shock-compressed state from unloading wave data

Abstract

Time-resolved shock-wave measurements have often been used to infer microstructural behavior in crystalline solids. We apply this approach to an interpretation of the release-wave response of an aluminum alloy (6061-T6) as it is dynamically unloaded from a shock-compressed state of 20.7 GPa. The anelastic behavior in the initial portion of the unloading wave is attributed to the accumulation of internal stresses created by the shock process. Specific internal-stress models which are investigated are (i) the double pile-up, (ii) the single pile-up, and (iii) single dislocation loops between pinning points. It is found that the essential characteristics of double and single pile-ups can be represented by a single dislocation between two pinned dislocations of like sign. Calculations of anelastic wave speeds at constant unloading strain rate are then compared with experimental data. The results suggest that the residual internal stress is due to pinned loops of density 10 15m −2, and that the viscous drag coefficient in the shock-compressed state is on the order of 10 −7 MPa s (approximately two orders of magnitude greater than expected under ambient conditions).