Far-field positive phase blast parameter characterisation of small-scale ammonium nitrate based explosives

The behaviour of “ideal” explosives, where the energy release is almost entirely facilitated by the process of detonation, is relatively well known. The mechanism of energy release is relatively simple, and consequently it is usually relatively easy to characterise and predict the properties of blast waves resulting from such explosives. With non-ideal explosives, the situation is more complex. Here, at best only part of the energy release is facilitated by a detonation process, and some or all is released via a secondary, usually slower, reaction. This, in addition to the fact that the rates of reactions are often highly sensitive to the thermodynamic state of the partially and fully-reacted products makes characterisation and prediction of blast waves resulting from non-ideal explosives significantly more challenging.

Ammonium nitrate/fuel oil (ANFO) is such a non-ideal explosive. When initiated, the ammonium nitrate detonates, with the resulting products including a small amount of excess oxygen which is able to react with the fuel oil component. A relatively large amount of data exists from tests involving the detonation of large (>>100kg) ANFO charges. In such tests, the velocity of detonation (VoD) tends towards a limit of around 4.5km/s, and the resulting blast waves can be approximated as being equivalent to those from a TNT charge of 0.8-0.85 times the mass of the ANFO charge. However, significantly less information exists on the mechanisms of energy release and the resulting blast waves from detonations of much smaller ANFO charges.

This paper presents blast experimental work through detonation of small scale ANFO (Ammonium Nitrate 94.5% and Fuel Oil 5.5% mixtures) hemispherical charges to assess the validity of current assumptions made about the non-ideal explosive which are currently integrated within best practice modelling methods. Comparisons will be made to detonation of ideal plastic explosives conducting the same methodology and analyse to validate the present ANFO data.