Abstract. In practical scenarios, cookoff of explosives is a three-dimensional transient phenomenon where the rate limiting reactions may occur either in the condensed or gas phase. The effects of confinement are more dramatic when the rate-limiting reactions occur in the gas phase. Explosives can be self-confined, where the decomposing gases are contained within non-permeable regions of the explosive, or confined by a metal or composite container. In triaminotrinitrobenzene (TATB) based explosives, self-confinement is prevalent in plastic bonded explosives at full density. The time-to-ignition can be delayed by orders of magnitude if the reactive gases leave the confining apparatus. Delays in ignition can also occur when the confining apparatus has excess gas volume or ullage. Understanding the effects of confinement is required to accurately model explosive cookoff at various scales ranging from small laboratory experiments to large real systems.
IntroductionPredicting the response of energetic material during an accident, such as a fire, is important for high consequence safety analysis, even for insensitive high explosives (IHE). The ignition time, the amount of decomposed gas, the thermo-mechanical sensitization of the degraded energetic material (EM) at ignition, burning of the degraded EM, the possible run-up to detonation and subsequent violence is needed for safety assessments. Even if the EM does not thermally ignite, degraded HE at elevated temperatures are more sensitive to shock initiation than pristine HE. For example, the shock sensitivity of the IHE triaminotrinitrobenzene (TATB) at elevated temperature was shown to be similar to room temperature 1,3,5,7-tetranitro-1,3,5,7-tetrazocine (HMX) [1]. Predictive models must be able to resolve phenomena across ten orders of magnitude in time from hours to sub-microseconds and six orders of magnitude in space from microns to meters. These models should include robust computational tools with solvers for numerically stiff sets of differential equations using adaptive gridding on massively parallel computers. Constitutive models for the EM should consider coupled thermal, mechanical, and chemical phenomena. Constitutive models for confining materials are also needed at elevated temperatures.In the early stages of a cookoff event, the mechanical response and decomposition gas velocities are slower than acoustic wave speeds and can be determined in a quasistatic manner. During this time, heat transfer, decomposition, and gas generation are coupled. Reaction rates are dependent on temperature and, in some cases, pressure. Disparity in thermal expansion between the confinement and the EM can change heat transfer paths as well as available gas volume. The pressurization rate and the degree of confinement influence the probability of deflagration-to-detonation transition (DDT) in the EM.