Quantitative analysis of shear band formation around collapsing pores in shocked energetic organic crystals

Shear bands can play a significant role in energy localization in energetic crystals loaded under the high-pressure, high strain-rate conditions of shock waves. While the origin and growth of shear bands have been well studied and visualized in atomic crystalline solids (metals and alloys), as well as in amorphous materials (e.g., metallic glasses), they are less well understood in reactive low-symmetry organic energetic crystals. Recently developed atomistic-consistent material models for commonly studied energetic crystals, HMX (1,3,5,7-tetranitro-1,3,5,7-tetrazocane) and RDX (1,3,5-trinitro-1,3,5-triazinane), have been shown in meso-scale (continuum) simulations to produce shear bands in good agreement with molecular dynamics (MD). Here, by exercising this atomistic-consistent continuum model, we analyze meso-scale simulation-generated shear band patterns during shock-induced pore collapse in HMX and RDX, spanning a wide range of pore sizes and shock strengths. Quantitative pattern analysis of shear bands is performed to extract key metrics, such as spacings at various instants of pore collapse, shear band propagation velocities, etc. These metrics show good agreement with the corresponding MD data for HMX and RDX; we assess the quantitative characteristics of the simulated shear bands against theoretical scaling relationships for shear bands developed mostly in the context of metals. The characteristics of simulated shear bands, such as spacings and growth rates, are found to align well with thermomechanical instability theory. Shear band growth rates display the expected initial slow incipience, intermediate fast growth, and later fast extinction phases seen in experiments. This work indicates that the theory of shear band formation and growth from the surface of pores and defects appears to hold across solids with quite different molecular arrangements. Valuable insights are obtained that enhance our understanding of the contribution of shear banding mechanisms to energy localization in shocked energetic materials.