Size effects on the wave propagation and deformation pattern in copper nanobars under symmetric longitudinal impact loading

Molecular dynamics simulations were performed to study the influence of system size on wave propagation and deformation patterns in 〈1 0 0〉/{1 0 0} copper nanobars with square cross-section under symmetric longitudinal impact loading. Nanobars of longitudinal length 100a with cross-sectional edge lengths h = 10a, 20a, and 40a were impacted on both ends by flyers of size 20a × h × h, where a is the Cu unit cell length, and impact speed 500 m s−1. For reference, quasi-infinite slab samples with periodic cross-sectional edge lengths 10a and 40a were also studied. It was found that the wave propagation speed increases with increasing cross-sectional area and eventually approaches the value obtained for a quasi-infinite sample. Extensive plasticity occurs across the entire length of the nanobars, whereas the quasi-infinite samples remain in the elastic regime and exhibit a vibrating (ringing) behaviour. The deformation pattern in the nanobars is strongly dependent on the cross-sectional area. For the nanobar with h = 10a the material fully reorients from 〈1 0 0〉/{1 0 0} to 〈1 1 0〉/{1 1 1} with few stacking faults and twins. Material in the nanobar with h = 20a does not reorient completely; the local crystal deformation is mediated mainly by a partial dislocation activity leading to predominantly non-intersecting stacking faults and twins. Nanobars with h = 40a exhibit behaviour similar to that for the h = 20a case but with greater propensity for intersecting stacking faults.