Compaction waves in granular HMX

Abstract

DISCLAIMERPortions of this document may be illegible in electronic image products. Images are produced from the best available original document. Piston driven compaction waves in granular HMX are simulated with a two-dimensional continuum mechanics code in which individual grains are resolved. The constitutive properties of the grains are modeled with a hydrostatic pressure and a simple elastic-plastic model for the shear stress. Parameters are chosen to correspond to inert HMX. For a tightly packed random grain distribution (with initial porosity of 19%) we varied the piston velocity to obtain weak partly compacted waves and stronger fully compacted waves. The average stress and wave speed are compatible with the porous Hugoniot locus for uniaxial strain. However, the heterogeneities give rise to stress concentrations, which lead to localized plastic flow. For weak waves, plastic deformation is the dominant dissipative mechanism and leads to d i s persed waves that spread out in time. In addition to dispersion, the granular heterogeneities give rise to subgrain spatial variation in the thermodynamic variables. The peaks in the temperature fluctuations, known as hot spots, are in the range such that they are the critical factor €or initiation sensitivity. LA-13546-MS

Modeling and analysis of reactive compaction for granular energetic solids

“…A continuous, dispersed wave structure is predicted due to compaction induced mechanical dissipation. Similar low speed, dispersed structures have been experimentally observed [16,21] and predicted by detailed meso-scale numerical simulations [17] for granular HMX. Smooth increases from their initial values are predicted for all quantities through the compaction zone.…”

“…Applied bulk loads are transmitted at the mesoscale by intergranular contact. Experiments [20] and numerical simulations [17] bulk load is not uniformly transmitted through the meso-structure, but is preferentially transmitted along certain paths that form stress chains; grains not involved in these chains remain mostly unstressed and inactive. It is reasonable then to expect that grain size, shape, and packing arrangement may have a large effect on the meso-scale stress state.…”

“…Though there can exist substantial stress bridging between grains at the meso-scale [17,20], resulting in spatially 3-D flow, the spatially averaged response can be approximated as 1-D provided that appropriate boundary conditions are specified. Stress bridging is, however, an important issue concerning energy localization that will be discussed later.…”

A particle-level set-based sharp interface cartesian grid method for impact, penetration, and void collapse

“…The simulation is carried out for three different initial piston velocities: 50 m/s, 200 m/s, and 500 m/s, using a 200x400 mesh. The void collapse phenomenon has implications for the initiation process in energetic materials (Bowden and Yoffe, 1952, Carroll and Holt, 1972, Kang et al, 1992, Menikoff and Kober, 1999. Here we examine the behavior of the void in copper, for which the material parameters are well characterized.…”

“…First the material is evolved by a Lagrangian step which deforms nodes to new positions and then the field is mapped back to the fixed Eulerian mesh and the new interfaces reconstructed using Young's reconstruction. This approach has been used to good effect in the solution of mesoscale response of energetic materials in shock compression (Menikoff and Kober, 1999) and void collapse (Cooper et al, 2000).…”