Thermal cracks and damage evolution in nano-gradient polycrystalline copper under shock loading

Nano-gradient metallic structures, owing to their exceptional mechanical properties, have wide applications in industrial fields. However, research on the propagation behavior of stress waves and the damage/failure mechanisms in nano-gradient metals under shock loading remains limited. In this work, large-scale molecular dynamics simulations are employed to study the wave propagation, reflection, and superposition behavior of nano-gradient polycrystalline copper under shock loadings, the shock front, reflected waves, and release waves are analyzed. The results reveal that the linear Hugoniot relationship, observed in metallic systems with uniform grain sizes, also holds for nano-gradient metallic systems. When the impact piston velocity reaches its critical value, large-volume cracks form at the intersection of the main reflected wave and the release wave. Crack formation reduces the energy dissipation ratio of the system. The gradient polycrystalline copper exhibits the highest dissipation ratio when the minimum grain size is 30 Å and the volume fraction of small size grains is 71%. Analysis of instantaneous atomic heatmaps under shock loading shows that in gradient polycrystalline copper with a small grain size (10 Å), thermal cracks initiate from the central region. Conversely, in gradient polycrystalline copper with a large grain size (30 Å), thermal cracks initiate from the edges of the structures.