Air blast TNT equivalence concept for blast-resistant design

Trinitrotoluene (TNT) equivalence is a well-known concept, which has been extensively employed by the engineering community for the blast-resistant design of structures. It is invoked to predict the blast parameters generated from detonations of other types of high explosives. However, TNT equivalence factors available in the literature usually have significant variability, which may lead to large errors in estimating the blast loads. This study discusses the air blast TNT equivalence factors of seven types of high explosives (Composition A3, Composition B, Composition C4, PBX-9404, PBX-9501, Pentolite and Tetryl) for free air and surface bursts. The authors presented a numerical approach in the previous study, which used a curve fitting method to develop the empirical formulae of TNT equivalence factors. This paper extends the previous study by applying this numerical approach to the high explosives, which were extensively discussed by the blast engineering community in the past. The empirical formulae, which depict the TNT equivalence factors for overpressure and impulse as a function of the scaled distance Z, are developed for each explosive. To demonstrate the full potential of the developed empirical formulae, the results of TNT equivalence factors are verified against data collected from different sources. Two important conclusions are drawn. Firstly, the significant variance of TNT equivalence factors in the near field (Z from 0.1m/kg(1/3) to 1.1-1.7m/kg(1/3)) requires different values depending on the scaled distance, as a constant value is insufficient for this range. Secondly, far-field detonations (Z from 1.1-1.7m/kg(1/3) to 10m/kg(1/3)) allow a constant value of TNT equivalence to be used for each explosive, which is independent of the scaled distance, selected blast parameter for comparison (overpressure or impulse) and burst type (free air or surface burst).