Abstract:
Taylor tests suggesting a high-velocity impact of specimens on a rigid anvil are widely used to study dynamic response of various metallic materials. In order to extend our previous experimental investigations and to reveal the features of Taylor tests at nanoscale, here we perform molecular dynamics simulations for single crystalline and polycrystalline copper considering both classical cylindrical samples and profiled ones with the reduced head part. The sample shapes are similar to that used in experiments, but the total length is 80 or 200 nm in contrast to 40-mm-length samples in the experiments. In spite of five orders of magnitude difference in length, the MD-calculated curves of the normalized length after impact versus the impact velocity are similar to that experimentally measured for millimeter samples, but, as expected, reveal much higher strength of nanoscale samples. Both single crystalline and polycrystalline samples are subjected to loss of the initial cylindrical symmetry in the course of impact. In the case of single crystal, this is due to activation of certain slip systems. In the case of polycrystals, this is connected with the anisotropy of individual grains and is clearly evident when the profiled part of the sample contains only a few grains in the thickness direction. This loss of the symmetry correlates with the distortion of the shape of the profiled samples in our experiments with annealed coarse-grained copper, in which the largest grains reach millimeter sizes. Thus, the symmetry distortion observed in the experiment has a fundamental reason of the deformation localization.
Keywords:Taylor impact test, nanoscale sample, molecular dynamics, dynamic plastic deformation, copper, single crystal, polycrystal, loss of symmetry, plastic flow localization.
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