14 August 2019

Safer ammunition through FOI’s research

What does the process look like when explosive materials ignite on being heated? FOI is investigating this with the help of cook-off tests. The results from this thermally-induced ignition can be used, among other things, to develop safer munitions.

Photos from an experiment

Small-scale testing apparatus for studying thermally-induced ignition. The metal casing that contains the explosive material is warmed by a rope heater until the reaction occurs. On the right hand side: An example of what remains of the casing after a completed test. Photo: Dan Martelin/FOI.

Unintentional ignition of energetic materials including high explosives, such as TNT and propellants, can cause major material damage and personal injury. For example, serious accidents have occurred when handling ammunition on board American aircraft carriers, and all over the world fires in or near munitions depots have had extensive consequences.

To reduce the risk of this type of event and increase ammunition safety, FOI is conducting research on thermally-induced ignition; that is, on processes where some form of heating induces ignition of an explosive material.

“We investigate, on the one hand, what can trigger unintentional ignition, and produce models to calculate whether – and if it’s the case, when and where – this can occur. On the other hand, we study different methods and technical solutions that reduce the risk of its occurring,” says Rasmus Wedberg, senior scientist in FOI’s Division for Defence & Security, Systems and Technology.

Cook-off testing

In their work to model thermally-induced ignition, Rasmus Wedberg and his colleagues mainly use variations of the standardised slow cook-off test, SCO. The researchers subject the explosive charge to heating at the same time as they study what is happening inside it during the process, and do so by measuring, among other things, temperature, pressure changes and the quantities of decomposition gases that are produced.

“We compare the results with our computational models, to develop and validate them. The models can then be used in developing safer ammunition or when assessing existing ammunition. They can help the Swedish Armed Forces and the Defence Materiel Administration, for example, to judge whether it’s worth investing in insensitive munitions, IM, which often come with a higher price tag.”

Post-ignition phenomena need more research

Rasmus Wedberg sees that there are a number of questions in this field that could be fruitful for future research. For example, until now FOI’s research has focused primarily on high explosives, but unintended ignition of propellants has not been studied as extensively.
“Another question requiring more research is what the consequences of thermally-induced ignition are. Today we can predict whether an explosive material ignites with a particular thermal loading, and when and where it occurs, but it would of course also be relevant to be able to calculate whether the combustion is going to be slow, or whether there will be a detonation after the ignition.”

Fact boxes

The slow cook-off test simulates situations where an explosive material is subjected to gradual heating. This can be done, for example, by placing the ammunition in an oven that is then slowly warmed.

The fast cook-off test assesses the sensitivity of an explosive material when it is subjected to rapid heating, often in the form of an intensive external fire. One way to do this is to place a large oil burner under the munitions.

Munitions that fulfil particular requirements for low sensitivity to different types of external influence, which can include both slow and fast cook-off. In several countries, especially in the NATO sphere, research and development is underway to improve the IM characteristics of munitions.