Investigation of slow cook-off characteristics of self-igniting energetic materials

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2022-2-08
Kaya, Zekeriya Taner
In this study slow heating response of small-scale test items and full-scale munitions due to thermal decomposition of explosives was investigated. Thermal decomposition parameters of the four most prevalent explosive formulations (from P1 to P4) used in defense industry were determined using non-isothermal thermogravimetric analysis and differential scanning calorimetry. Considering these parameters, an igniter formulation was developed to burn the explosive formulations in a predefined temperature before the undesired deflagration and detonation in slow heating test. A small-scale heating chamber was designed according to slow heating test standard requirements to determine the slow heating response (ignition temperature and ignition time) of the explosive formulations and of the developed igniter for 5, 15 and 25 °C/h heating rates in forced convection conditions. Coupling of igniter with P1 explosive configuration led to controlled burn response between 142.5-153.6 °C before the violent response of bare explosive formulation configurations (P1 to P4 without igniter) between 174.8-217.2°C at 5, 15 and 25°C/h heating rates. Cook-off temperature of the igniter, P1 (64% RDX by weight) and P2 (87% HMX by weight) explosive rose with increasing heating rate (from 5 to 25°C/h), whereas cook-off temperature of the P3 (45% HMX by weight) and P4 (20% RDX by weight) explosive decreased with increasing heating rate (from 5 to 25°C/h). Cook-off temperature trend observed for different explosive formulations was attributed to ratio of heat generation due to thermal decomposition in explosive formulations to heat dissipation. This ratio is higher for the formulations that contains more energetic material such as P1 (64% RDX by weight) and P2 (87% HMX by weight). Numerical simulations of small-scale slow heating tests carried out with ANSYS Fluent software were validated by comparing the experimental temperature measurements with calculated results at different heating rates. Maximum temperature difference between experiments and simulations for cook-off temperature was calculated as 0.2, 0.4, and 1.2% for Igniter, P1 and P4 explosives and 1.8-7.5% for P3 and P2 explosives for suggested 15°C/h heating rate in literature. In the second part of the study, a large-scale slow heating test with full-scale munition was conducted in a heating chamber under forced convection conditions. Flow in the heating chamber and around the full-scale munition coupled with the heat transfer in the munition was also modeled with ANSYS Fluent software. Cook-off temperature of P1 explosive was measured as 173.4°C in full-scale test. Cook-off temperature and cook-off time were calculated with 2.7% and 2.2% error in numerical simulations in addition to correct calculation of temperature profile and ignition point. Developed methodology for determining the response of munitions against slow-heating threat by computer modeling is expected to facilitate the munition design phase and reduce the test risks and test costs at TUBITAK SAGE and domestic defense industry.

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Citation Formats
Z. T. Kaya, “Investigation of slow cook-off characteristics of self-igniting energetic materials,” M.S. - Master of Science, Middle East Technical University, 2022.