Elucidation of the dynamics for hot-spot initiation at nonuniform interfaces of highly shocked materials

The fundamental processes in shock-induced instabilities of materials remain obscure, particularly for detonation of energetic materials. We simulated these processes at the atomic scale on a realistic model of a polymer-bonded explosive (3,695,375 atoms/cell) and observed that a hot spot forms at the nonuniform interface, arising from shear relaxation that results in shear along the interface that leads to a large temperature increase that persists long after the shock front has passed the interface. For energetic materials this temperature increase is coupled to chemical reactions that lead to detonation. We show that decreasing the density of the binder eliminates the hot spot.

Figure 1

Snapshot of PBX during shock loading at $U_p$ $=$ 2.5 km/s (for 6.0 ps). The shading is based on the total slip in angstroms. This system is 54 nm thick in the shock direction (x), with a period of 27 nm along the sawtooth direction (y), and is uniform with a periodic length of 25 nm in the z direction. The arrow indicates shock direction.

Figure 2

Time evolution of temperature and von Mises shear stress for $U_p$ $=$ 2.5 km/s at various stages during the shock compression. The solid line represents the interface position between the RDX and the binder. Shock wave propagates from left (RDX) to right (binder). (e) 4.5 ps, shock wave has not propagated to polymer; (a),(f) 5.0 ps, shock wave just reaches the interface; (b),(g) 5.5 ps, adiabatic shear localization in a small triangle interface region; (c),(h) 6.0 ps, shock wave has passed through the hot-spot region; (d) 8.0 ps, shock wave has passed through the interface.

Figure 3

2D distribution and time evolution of chemistry (NO${}_2$ formation) for various $U_p$. (a) $U_p$ $=$ 2.5 km/s at t $=$ 6.0 ps: The chemical reaction has occurred mainly in the hot-spot region. (b) $U_p$ $=$ 2.5 km/s at t $=$ 8.0 ps: Additional chemical reactions have occurred and a small amount of NO${}_2$ has diffused to the polymer region (the right region of the solid line in the $xy$ plane). (c) $U_p$ $=$ 3.5 km/s at t $=$ 4.5 ps: The chemical reaction occurs in the shocked regions but much more occur in the hot-spot region. (d) $U_p$ $=$ 3.5 km/s at t $=$ 6.5 ps: We see diffusion of NO${}_2$ into the polymer region.

Figure 4

The dependence of hot-spot formation (temperature and shear stress) on the density of the polymer binder at a shock velocity of 3.5 km/s. The middle (0.95 g/cm${}^3$) is normal density. Thus using of a polymer with half the normal density prevents hot-spot formation. This is due to the improved interface impedance which allows reflective rarefaction wave relaxation. In contrast, increasing the binder density enhances the hot spot due to the reflective shock waves reflected back into the RDX.