ИСТИНА

The molecular dynamics simulations of shock wave loading for metal single crystals are performed to understand the role of temperature and preexisting dislocations on the Hugoniot Elastic Limit (HEL) and spall fracture. It is shown that, in ideal crystals, the elastic precursor exhibits a form of plateau, and the HEL almost does not change with shock propagation distance. However, at higher impacts, the perturbations of an elastic precursor are observed which leads to fluctuation in the HEL value. The temperature dependencies of the HEL are strongly anisotropic. The HEL values tend to decrease with temperature for [110] perfect copper crystals, and to increase with temperature for [111] copper crystals \cite{Bryukhanov2023}. The HEL increases with impact velocity and spreads over a range of 10 GPa between copper crystals of various orientations, attaining its maximal and minimal values for [110] and [112] crystal, respectively. Preexisting dislocations in single crystals allow the HEL to decay much faster than in ideal crystals. At the impact velocity of 500 m/s the order [111] > [110] > [100] of the stress decay is confirmed by comparison with known theoretical results for fcc crystals. The HEL decay in copper crystals slows down with temperature resulting in higher HEL values. In Mo single crystals, the HEL decay does not slow down with temperature which is a consequence of dislocation mobility, which is almost independent on temperature. We show that, in most copper crystals, the presence of dislocations slows down the onset of spall fracture and reduces the rate of void growth \cite{Bryukhanov2022}. However, with an increase of impact velocity, the role of dislocations declines. An exception is [100] crystals, in which dislocations, on the contrary, reduce the spall strength and accelerate the spallation process. The [100] and [104] copper crystals with dislocations have the highest void growth rate and are the most brittle, and the [111] crystals have the lowest growth rate and are the most ductile. It is suggested that the comparison between spall fracture for the crystals with preexisting dislocations and the perfect ones may provide a better understanding of which crystal orientations are more brittle and which are more ductile. The reported study is supported by the Russian Science Foundation (RSF), research project №22-71-00088.