Shock-induced shear bands in an energetic molecular crystal: Application of shock-front absorbing boundary conditions to molecular dynamics simulations

Abstract

W91INF-05-1-0265 REPORT NUMBER REPORT ~'u~mER 48101-EG-MUR The views, opinions andlor findings contained in this report are those of the author(s) and should not be construed as an official Department of the Army position, policy or decision, unless so designated by other documentation. Approved for public release; federal purpose rights The response of the energetic molecular crystal cyclotrimethylene trinitramine (RDX) to the propagation of planar shock waves nonnal to (100) has been studied using large-scale molecular dynamics simulations that employ an accurate and transferable nonreactive potential. The propagation of the shock waves was simulated using nonequilibrium molecular dynamics. Shear bands were nucleated during shocks with a particle velocity of 1.0 km s-I and corresponding Rankine-Hugoniot shock pressun of9.7 GPa. These defects propagate into the compressed material at 45° to (100) in the (010) zone. The shear bands e\'oh'e slowly compared to the time scales routinely accessible to nonequilibrium molecular dynamics toward a liquidlike state as a result of viscous heating. A recently developed shock-front absorbing boundary condition [AV. Bolesta et al. , Phys. Rev. B 76, 224108 (2007)] was applied to the simulation cells at the moment of maximum compression to sustain the shock-compressed state. Molecular dynamics simulations were then employed to study the temIXlral and structural evolution of the shock-induced shear bands toward a steady-fluctuating state. Owing to the intense, viscous flow-driven heati ng within the shear bands. these defects can be considered to be homogeneously nucleated hot spots.

Investigation of Fundamental Processes and Crystal-Level Defect Structures in Metal-Loaded High-Explosive Materials under Dynamic Thermo-Mechanical Loads and their Relationships to Impact Survivability of Munitions (Thrust 4, Topic J)

“…The initial temperature was T = 300 K. Molecules on the surface of the void prior to shock arrival are colored red. The observation of the nucleation and growth of nanoscale shear bands in the material to the left of the void, and symmetric collapse of the void with small jets meeting near the stagnation plane at angles approximately ±45° relative to the direction of shock propagation (blue incursions into the red region in the right-hand panel), are both consistent with expectations 13,22 based on previous simulation results for this shock orientation and impact strength. In the coming year we will perform detailed studies of void collapse for multiple impact speeds, void configurations, and crystal diameters.…”

A perspective on modeling the multiscale response of energetic materials

“…We attempted to reproduce structural features of RDX behind the shock front, as captured in an earlier MD study 18 [and shown in Fig. 4(a)].…”

Analysis of thermomechanical response of polycrystalline HMX under impact loading through mesoscale simulations

“…These hotspots in energetic materials form through mechanisms such as compression of voids, friction between crack surfaces inside the material, and localized shear deformation. [1][2][3][4][5] Heating due to each of these mechanisms depends on the histories of the states of local stress and rate of deformation in the material. Existing literature examines the behavior of energetic crystals and polymer-bonded explosives (PBXs) in two general forms: as single crystals with anisotropic material properties, [6][7][8][9][10][11][12][13][14][15][16][17][18][19][20][21][22] and as heterogeneous collections of energetic grains with isotropic material properties.…”

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We investigate the response of polycrystalline HMX 3,5,3,5, under impact loading through a 3-dimensional mesoscale model that explicitly accounts for anisotropic elasticity, crystalline plasticity, and heat conduction. This model is used to quantify the variability in temperature and stress fields due to random distributions of the orientations of crystalline grains in HMX under the loading scenarios considered.

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