Experimental Study on the Reaction Evolution of Pressed PBX Charge under Strong Confinement with Different Gap Conditions

The early-stage reaction evolution of confined explosives under non-shock initiation is critically influenced by the gap between the charge and the confinement structure. However, the quantitative experimental study and understanding remain limited. This study, for the first time under macro-scale engineering assembly gap conditions, experimentally quantifies the effects of gap configurations on convective combustion during the early non-shock initiation of pressed PBX charges under long thick-walled cylinder confinement. Three distinct gap configurations are examined: normal-gap, joint-coating, and radial-sealing. Various synchronized diagnostics including high-speed imaging, photonic Doppler velocimetry (PDV), pressure sensors, and electric probes were used to obtain a comprehensive analysis and comparison. Two key novel findings are reported. First, it is demonstrated that convective combustion through the charge-shell gap dominates the initial reaction propagation. A distinct two-stage pressurization process was identified in both unsealed configurations: an initial slow pressurization stage followed by a fast pressurization stage. Crucially, the duration of the slow stage shortens progressively along the axial direction, which is a direct manifestation of accelerating convective flame spread through the charge-shell gap. In contrast, the radial-sealing condition eliminates this slow pressurization stage, resulting in a direct and rapid pressure rise once the sealing layer fails. Second, it is revealed that gap conditions fundamentally govern the reaction pathway and the resulting structural failure mode—most notably, radial sealing induces preferential mid-shell rupture, indicating an altered spatial distribution of energy release. Although reaction intensities varied to some extent among configurations, the differences were not substantial, and no transition to detonation was observed in any test. These findings provide mechanistic insights and quantitative data for assessing the safety of energetic materials.