Multi-scale Computing On The Response Characteristics Of Energetic Materials At Extreme Conditions

The response of energetic materials at extreme conditions is an instantaneous phenomenon,and no matter how short the time and space scales, it still has a rich connotation of physics and chemistry, and contains self-excitation- growth- propagation rules. Many of the classic theories for energetic materials at extreme conditions have been set up in fields of impact dynamics and detonation, which involves chemical reaction kinetics mechanism for the above problems and the coupling mechanisms that remain to be desired. Molecular dynamics method which is capable of multi-scale simulations can be used to dig into molecular/atomic level to describe the response of energetic materials at extreme conditions. This paper focuses on the problems of the physical and chemical response of the energetic materials under conditions of high temperature, impact, collision by parallel molecular dynamics algorithm, at the same time combined with reactive force field(Reax FF) which is based on first principles, and, aims to reveal the chemical kinetics mechanism contained in the rapid destruction of energetic materials, and tries to find the new phenomenon and the problem and at the same time to give a reasonable explanation. The main results are as follows:Reaction kinetics mechanism on pyrolysis of CL20-TNT co-crystal: To analyze the reaction kinetics mechanism on conditions of high temperatures, we set up different densities of CL20/TNT co-crystal supercells(1.435g/cm3, 1.910g/cm3, 2.621 g/cm3), and apply high temperature condition on the whole system(2000K, 2500 K, 3000K), and, the energy barriers of the CL20/TNT for thermal decomposition decrease as the densities increase, that is 50.92kcal/mol, 44.06 kcal/mol, 42.403kcal/mol respectively. Under the same temperature conditions, total intracrystalline CL20 attenuation rate is greater than that of TNT, and with the increase of the density of co-crystal, the attenuation rate of TNT molecules obviously accelerates, and at the same time, the decomposition of the TNT inhibits CL20 decomposition.NO2 is the primary product of high temperature thermal decomposition of the co-crystal, and the final products is CO2, H2 O and N2.Spatiotemporal behaviors of propagation of thermal shock in CL20/TNT co-crystal energetic materials induced by local zones of high temperature: Thermal shock mechanical wave induced by local hot regions which makes the particles in cold region with directional displacement, kinetic energies of the particles are converted into heat followed by the process of reciprocal motion, and the particle velocity magnitude is in hundred meters per second. The asymmetry phenomenon of density, particle velocity, the average displacement profile in cold region developed gradually. Heat transfer process tends to further induce CL20/TNT decomposition after the thermal shock mechanical wave attenuation. Finally the decomposition rate closely depends on the temperature at two ends.Shock initiation mechanism of the CL20/TNT co-crystal: The shock wave velocities at 6, 7, 8, 9, 10km/s are loaded by axial compression, and the minimum shock wave velocity of exciting the chemical reaction of CL20/TNT is determined to be of 7km/s, and the impact initiation pressure is 24.56 GPa and the Us-Up relation is Us=1.34634Up+ 4.45004. In addition, with the shock wave at 9, 10km/s, a large amount of carbon atoms emerge in the process of shock initiation.Shock induced RDX chemical reaction and hot spot formation: The range of particle velocity 1-4km/s is given by the method of plane direct impact loading. When the particle velocities are at 1, 2km/s, almost no chemical reaction occurs, with the higher particle velocities of 3, 4km/s, the product identification analysis shows that the number of RDX molecules is significantly decreased, chemical reactions indeed occurr, and the Us-Up relationship is Us=1.79947Up+3.04138.In addition, with the increase of impact velocity, the physical parameters in the wavefront become very steep, and, the densities at the rear of the wavefront increase at varying degrees. From hot spot formation and growth process, as well as the interaction between shock wave and rectangular cavity, we found that when the impact velocity is at 2 km/s, the temperature of the hot spots is located between 1000 K to 3800 K, and when the impact speed goes up to 4 km/s, the area temperature is in the range of 4000 K to11000K.In addition, the process of bending wave front gradually attenuates to the plane shock wave, which is caused by the rare faction wave stretching.Shock wave propagation and spallation induced by the water molecule group hypervelocity impact on the RDX: when water molecule group hypervelocity impact on the RDX is at the speed conditions of 2 to 12 km/s, the shock waves propagate in the RDX in the shape of gradual attenuation at various shock wave velocity conditions, and, when the impact velocity is higher than 8km/s, shock excitation induces RDX decomposition at collision region. Shock wave arriving at the free interface to form the rarefaction wave and stretch RDX fracture, and the fracture form closely depends on the impact velocity. Under the low speed impact, the temperature of the typical cracked layer increases, under the high speed crash, the fracture is of the micro-spallation.