Three-stage compaction mechanism of energetic material particles revealed by atomistic simulations

Plastic-bonded explosives derive their performance and safety in part from the mesoscale mechanics of particle contacts formed during pressing. Using large-scale molecular dynamics simulations of cyclotrimethylene trinitramine particle assemblies, this study examines compaction, heating, and post-unloading retained states over a range of particle sizes and volumetric strains. The evolution of pressure, porosity, and bulk modulus reveals a clear three-stage densification sequence, with a characteristic discrete-to-continuous transition near a volumetric strain of 0.45, under an idealized simple-cubic packing. Before this transition, temperature rise is strongly co-localized with strain localization at contact-dominated regions. Beyond this, this correspondence weakens as heat redistribution becomes increasingly effective in the densified solid, although contact-related dissipation remains significant. Upon unloading, the relaxed volumetric strain, recovered density, residual stress, and residual strain all tend to saturate once volumetric strain exceeds about 0.45, indicating diminishing gains in permanent densification at higher compaction levels. Smaller-particle systems retain lower mean residual stresses, associated with differences in interfacial contact statistics and modestly more effective stress relaxation. These results identify a characteristic transition range for the present RDX packing and provide an atomistic basis for balancing densification, thermal response, and retained residual fields in energetic particulate systems.