Zhang Yaping, Liu Hai, Yang Zhen, Li Qikai, He Yuanhang
State Key Laboratory of Explosion Science and Technology, Beijing Institute of Technology, Beijing 100081, China.
Hypervelocity Aerodynamic Institute, China Aerodynamic Research and Development Center, Mianyang 621000, China.
ACS Omega. 2019 May 2;4(5):8031-8038. doi: 10.1021/acsomega.9b00589. eCollection 2019 May 31.
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, 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 = 4 km·s; hot spot is weak to some extent for = 2 km·s. 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, while the tensile wave induces the microsplit region in RDX crystals in the case of = 4 km·s. Chemical reactions are studied for Rankine-Hugoniot shock pressures = 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 HO, CO, NO, and so forth, around the microsplit regions for the strong shock. The ruptures of N-NO 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.
含能材料在冲击载荷作用下的初始反应机理以及晶体特性在冲击起爆和感度方面的作用仍不明确。在本文中，我们报道了对含有一个立方体形空隙的1,3,5 - 三硝基 - 1,3,5 - 三嗪（RDX）晶体进行冲击起爆的反应分子动力学模拟。研究发现，冲击诱导的空隙坍塌、热点形成与生长以及层裂均依赖于冲击速度。对于2 km·s和4 km·s的冲击速度，空隙坍塌时间分别为1.5 ps和0.7 ps。结果表明，初始热点形成包括两个步骤：一是由局部塑性变形引起的温度升高，二是空隙坍塌过程中上下游粒子碰撞导致的温度上升。热点是否会继续生长或熄灭取决于局部物理和化学反应引起的能量释放与各种热耗散机制之间的敏感平衡。在我们的模拟中，对于 = 4 km·s，热点会生长；对于 = 2 km·s，热点在一定程度上较弱。冲击波到达自由表面后反射的拉伸波导致层裂，这取决于初始冲击速度。对于2 km·s的冲击速度会发生典型的层裂，而在 = 4 km·s的情况下，拉伸波在RDX晶体中诱导出微裂区域。针对兰金 - 于戈尼奥冲击压力 = 14.4、57.8 GPa研究了化学反应。对于弱冲击，在层裂区域附近RDX分子几乎没有分解反应。相反，对于强冲击，在微裂区域周围有大量小分子产物，如HO、CO、NO等。N - NO键的断裂是受冲击RDX晶体的主要初始反应机理，且不受冲击强度影响，而微裂会减缓RDX的分解速率。本文的工作有助于深入理解含空隙的含能材料在冲击载荷作用下的热点火、热点生长及其他物理和化学现象。
