Three-dimensional burning crack dynamics in constrained spherical explosive: visualization analysis and cavity-coupled pressure modeling

Accurate characterization of three-dimensional burning crack propagation remains pivotal yet challenging for energetic material safety, as conventional diagnostics and models inadequately resolve coupled crack-pressure dynamics in confined explosives. This study combines a novel spherical confinement system (with/without sapphire windows) with synchronized high-speed imaging and 3D reconstruction to overcome optical limitations in opaque explosives. Experimental analysis of centrally ignited HMX-based PBX-1 reveals: (1) burning cracks propagate radially with equatorial acceleration and polar deceleration, (2) systematic formation of 3-4 dominant crack branches across geometries, and (3) pressure evolution exhibiting gradual accumulation (subsurface cracking) followed by exponential growth (surface burn-through), with decay governed by cavity expansion. Building on Hill’s framework, we develop a model incorporating cavity volume and fracture toughness criteria, validated against PBX explosive (95% HMX-based) experiments. The model demonstrates improved prediction of pressure trends compared to prior approaches, particularly in resolving laminar-phase accumulation and crack-induced surge transitions. Results establish structural cavity volume as a critical modulator of measured pressure and reveal direction-dependent crack kinematics as fundamental features of constrained combustion. This work provides experimentally validated insights into mechanisms of reaction pressure development and burning cracks pathways during constrained PBX explosive combustion. • 3D crack visualization via sapphire-windowed sphere and high-speed imaging. • Crack velocity asymmetry shows faster equatorial vs polar propagation in HMX. • 3-4 dominate burning cracks always form in explosive combustion reaction. • A new cavity-volume model with fracture criteria validated experimentally.