Cookoff of Powdered and Pressed Explosives Using a Micromechanics Pressurization Model

Abstract

Cookoff experiments of powdered and pressed TATB‐based plastic bonded explosives (PBXs) have been modeled using a pressure‐dependent universal cookoff model (UCM) in combination with a micromechanics pressurization (MMP) model described in a companion paper. The MMP model is based on the accumulation of decomposition gases at nucleation sites that load the surrounding TATB crystals and binder. This is the first cookoff model to use an analytical mechanics solution for compressibility and thermal expansion to describe internal pressurization caused by both temperature and decomposition occurring within closed‐pore explosives. This approach produces more accurate predictions of ignition time and pressurization within high‐density explosives than simple equation‐of‐state models. The current paper gives details of the reaction chemistry, model parameters, predicted uncertainty, and validation using experiments from multiple laboratories with errors less than 6 %. The UCM/MMP model framework gives more accurate thermal ignition predictions for high density explosives that are initially impermeable to decomposition gases.

A Micromechanics Pressurization Model for Cookoff

“…The molecular weight of the decomposition gas products is determined from the chemical mechanism. Details of the chemistry and mechanisms are beyond the scope of the current paper but are discussed in a companion paper [15] that describes the reaction mechanism and parameters in detail. In the companion paper, the MMP model is applied to both low‐density and high‐density explosives using the mechanism given in Table 1.…”

“…The companion paper [15] presented 8 low‐density (743±3 kg/m 3 or 38 % TMD) and 23 high‐density (1881±5 kg/m 3 or 97 % TMD) SITI experiments using various ullages ranging from 20 % to 75 % of the overall confined volume. Most of these experiments had smooth temperature profiles as shown previously in Figure 6(a) for high‐density SITI experiment 280 that had 25 % ullage.…”

“…The condensed and gas volumes in Eqns. (14)(15)(16)(17) were used to determine the gas volume fraction which is given in Eq. ( 18).…”

“…Uncertainties in the predicted pressure profiles were determined without changing any of the parameters associated with the heat transfer or chemistry model such as thermal conductivity, specific heat, and reaction rates. Overall uncertainty that includes the heat transfer and chemistry parameters are given in a companion paper [15]. The uncertainty discussed in the current section was determined by varying parameters specific to the MMP model with uncertainty ranges in Table 7.…”

“…However, simple solutions to the model are obtained by providing both T and S f . More practical solutions of the model are also provided by simultaneously solving the micromechanics equations with the conductive energy equation and a simple decomposition model for cookoff of the TATB-based explosive as described more fully in a companion paper [15].…”