Energy dissipation in polymer-bonded explosives with various levels of constituent plasticity and internal friction

The ignition of energetic materials (EM) under dynamic loading is mainly controlled by localized temperature spikes known as hotspots. Hotspots occur due to several dissipation mechanisms, including viscoplasticity, viscoelasticity, and internal friction along crack surfaces. To analyze the contributions of these mechanisms, we quantify the ignition probability, energy dissipation, damage evolution, and hotspot characteristics of polymer-bonded explosives (PBXs) with various levels of constituent plasticity of the energetic phase and internal crack face friction. Using PBX9501 consisting of HMX (Octahydro-1,3,5,7-Tetranitro-1,2,3,5-Tetrazocine) and Estane as a reference material, we analyze variants of this material with several values of the yield stress of the energetic phase and coefficients of internal crack face friction, while other parameters are kept unchanged. The impact loading involves piston velocities between 200 and 1200 m/s. The analysis uses a Lagrangian cohesive finite element framework that explicitly accounts for finite-strain elastic-viscoplastic deformation of the grains, viscoelastic deformation of the binder, arbitrary crack initiation and propagation in the grains and the binder, debonding between the grains and the binder, contact between internal surfaces, friction and frictional heating along internal surfaces, heat generation resulting from inelastic bulk deformation, and heat conduction. To determine the ignition status of the material or "go" or "no-go" state, we use a criterion based on a criticality threshold obtained from chemical kinetics calculations. For PBX with various levels of HMX plasticity and friction, the probability of ignition, the evolution of dissipation caused by plasticity and friction, the density of cracks, and the locations of cracks are quantified. Results show that samples with higher levels of constituent plasticity (lower yield strengths) or lower levels of internal friction are less likely to ignite. The relative importance of plasticity and friction depends on load intensity, with frictional heating decreasing as load intensity increases. Although the overall viscoplastic heating outweighs the overall frictional heating, friction plays a very important role in hotspot development at all load intensities analyzed, owing to the fact that frictional heating is more localized than viscoplastic heating. The predicted thresholds and ignition probabilities are expressed in a load intensity-load duration relation for PBX with different constituent properties.