Improvements to the High Explosives Initiation Time (HEIT) for heterogeneous initiation of explosives

The High Explosives Initiation Time (HEIT) test has been used over the last several years to obtain time-to-explosion versus temperature data for high explosives, providing valuable Arrhenius kinetics data [1]. The test is based on work by Wenograd [2] in which explosives were loaded inside stainless-steel hypodermic needles, then electrically heated on a microsecond-scale to temperatures of the order of 1000 °C. Other laboratory methods used to obtain kinetics data, e.g., differential scanning calorimetry (DSC), are slow and cannot reach these high temperatures.

In the HEIT test the explosive initiation time is obtained using high speed photography and the temperature is obtained by measuring the needle resistance. The test is well-suited to the analysis of explosive safety. The rapid heating of the explosive leads to heterogeneous initiation of the explosives, as reactions begin at the needle wall and propagate inwards. Heterogeneous ignition, rather than homogeneous behavior, is typical of most accident scenarios where initiation occurs before thermal equilibration is established. We will present the Arrhenius kinetics for various explosive materials and show how the kinetics for heterogeneous and homogeneous behaviors differ.

Improved diagnostic techniques are being developed, including the use of fiber-optic coupled photodiodes, which show how ignition occurs within the needle before burst.

1. Tasker, D.G., et al., ‘An updated technique to obtain explosive kinetics data on microsecond timescales’, Rev. Sci. Inst., 2024, 95, pp. 074903

2. Wenograd J., ‘The behaviour of explosives at very high temperatures.’ Transactions of the Faraday Society, 1961 Vol. 57, pp. 1612-1620.