The initial reaction mechanism of energetic materials under impact loading and the role of crystal properties in impact initiation and sensitivity are still unclear. In this paper, we report reactive molecular dynamics simulations of
shock initiation of
1,3,5-trinitroperhydro-1,3,5-triazine (
RDX) crystals containing a cube void.
Shock-induced void collapse, hot spots formation and growth, as well as spalling are revealed to be dependent on the
shock velocity. The void collapse times are 1.5 and 0.7 ps, for the
shock velocity of 2 and 4 km·s-1, respectively. Results indicate that the initial hot spot formation consists of two steps: one is the temperature rise caused by local
plastic deformation and the other is the temperature increase resulting from the collision of upstream and downstream particles during the void collapse. Whether hot spots will continue to grow or quench depends on sensitive balance between energy release caused by local physical and chemical reactions and various heat dissipation mechanisms. In our simulations, hot spot would grow for U p = 4 km·s-1; hot spot is weak to some extent for U
p = 2 km·s-1. The tensile wave reflected by the
shock wave after reaching the free surface causes the spalling, which depends on the initial
shock velocity. Typical spalling occurs for the
shock velocity 2 km·s-1, while the tensile wave induces the microsplit region in
RDX crystals in the case of U p = 4 km·s-1. Chemical reactions are studied for Rankine-Hugoniot
shock pressures P s = 14.4, 57.8 GPa. For the weak
shock, there is almost no decomposition reaction of the
RDX molecules near the spalling region. On the contrary, there are large number of small molecule products, such as H2O, CO2, NO2, and so forth, around the microsplit regions for the strong
shock. The
ruptures of N-NO2 bond are the main initial reaction mechanisms for the shocked
RDX crystal and are not affected by
shock strength, while the microsplit slows down the decomposition rate of
RDX. The work in this paper can shed light on a thorough understanding of thermal ignition, hot spot growth, and other physical and chemical phenomena of energetic materials containing voids under impact loading.