Effect of initial void ratio on phase transition of confined HMX-based PBX-3 under slow cook-off

In order to understand the effect of initial void ratio on the thermal phase transformation and ignition response characteristics of HMX-based PBX-3 under slow cook-off condition, a series of experiments were designed and conducted on the confined PBX-3 explosives with the initial void ratios of 1.0%, 4.2% and 13.8%. In each test, the PBX-3 sample composed of two cylindrical pieces of explosive, 25 mm in diameter and 10 mm in height for each, was prepared. The temperature was monitored by the five small-size type-K thermocouples, 0.25 mm in width and 0.15 mm in thickness for each, among which four were arranged at the different positions in the interior of the PBX-3 and one was positioned on the surface of the shell. The experimental apparatus was positioned in a slow cook-off chamber with a transparent glass cover. The slow cook-off setup was heated to 150 ℃ within 30 minutes, followed by a 45-minute soak at 150 ℃, and then heated at 0.25 ℃/min until thermal explosion occurred. During the process, the temperatures at different locations inside the explosive and at the surface of the shell were acquired. The process of the HMX phase transition, the mechanisms exhibiting the effect of the initial void ratio on the HMX phase transition and the effect of the HMX phase transition process on the thermal explosion temperature were analyzed in detail. The result shows that the lower the initial void ratio is, the higher the thermal stress the PBX-3 is subjected to when it is heated to the HMX phase transition temperature, which delays the transformation process of β-HMX into δ-HMX during the slow cook-off. Due to the higher thermal sensitivity of δ-HMX, the longer the phase transition process of HMX in the slow cook-off is, the slower the heat accumulation resulting from the δ-HMX exothermic decomposition reaction, and the higher the temperature of the confined shell at the time of thermal explosion.