Abstract
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 Up = 4 km·s–1; hot spot is weak to some extent for Up = 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 Up = 4 km·s–1. Chemical reactions are studied for Rankine–Hugoniot shock pressures Ps = 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.
Highlights
Condensed phase energetic materials usually contain defects, making the energetic material heterogeneous, which is beneficial for hot spots formation and detonation under impact loading.[1−5] Hot spots are localized regions of high temperature and high pressure in materials that serve as nucleation sites for initiating and possibly sustaining rapid chemistry
Various mechanisms have been provided to explain the formation mechanism of “hot spots”: void collapse, adiabatic shear bands, fraction, and heating at dislocation pileups.[6−9] Because of the complex physical and chemical coupling effects and the elastoplastic deformation of the energetic material, as well as very short time scale and space scale, the current experimental techniques are difficult to reveal the detailed process of the shock initiation of energetic materials from an atomic and molecular level, while the molecular dynamics (MD) simulation method can be used to reveal the formation process of hot spots and the progress of chemical reactions from the microscopic scale; the latter provides a new way to analyze the physical and chemical processes of energetic materials under shock loading
The internal temperature of the void is higher than 2500 K, and the hot spot where the shape remains radially symmetrical is formed at the end of void collapse
Summary
Condensed phase energetic materials usually contain defects (such as crystal dislocations, shear bands, defects, voids, etc.), making the energetic material heterogeneous, which is beneficial for hot spots formation and detonation under impact loading.[1−5] Hot spots are localized regions of high temperature and high pressure in materials that serve as nucleation sites for initiating and possibly sustaining rapid chemistry. An et al.[19,20] reported on the shock-induced behavior of PBX based on the silapentaerythritol tetranitrate, 1,3,5-trinitroperhydro-1,3,5-triazine (RDX) They found hot spots occur when a plane shock wave propagates to the heterogeneous cross section of a polymer explosive due to the difference in density of the two materials in the polymer. Nomura et al.[23] reported million atoms reactive force field MD simulation of shock initiation of RDX crystal with a nanometer-scale void They indicate that the nanojet formed during void collapse is related to the free volume of the void to enhance the intermolecular collision. The formation of hot spots and shock-induced spallation during the interaction between shock wave and cube void are to be studied in order to explain the detonation mechanism of energetic materials from an atomic level. The work should give insight for thermal ignition and hot spots growth of energetic materials
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