Airblast Equivalent Weight and Yield Determinations Based on Measurements of Energy and
Other Blast Wave Parameters
M. M. Swisdak, Jr.; APT Research, Inc.; Huntsville, AL, USA
L. D. Sadwin; Sadwin Consultancy; Kefar Pines, Israel
Keywords: TNT Equivalency, Energy, Explosive Comparison
Abstract
The airblast Equivalent Weight (EW) behavior of various energetic materials is of primary
importance in the fields of Protective Construction and Explosive Quantitative Risk Assessment
(EQRA). The concept of EW/equivalent yield is reviewed and calculation methodologies are
discussed. A new, simplified methodology is presented for determining the hemispherical TNT
explosion yields based on measured values of incident pressure, time of arrival, positive phase
duration, incident impulse, reflected pressure and reflected impulse.
The energy flux of the blast wave, based on the integral of the square of the overpressure versus
time, is described. For explosions in air, its use has been problematic to evaluate as this integral
must be divided by the characteristic impedance (product of the density and wave velocity) of
the air. The characteristic (shock) impedance of air varies considerably with the shock pressure.
The paper describes and demonstrates the calculation of energy flux measurements from
conventional P-t curves taking into account the variation of characteristic impedance of air as a
function of overpressure along the P-t curve. The energy flux for Modified Friedlander
waveforms is also derived and the results applied to a compilation of ammonium nitrate/fuel oil
(ANFO) airblast data.
Introduction
The airblast characteristics of various explosives have been evaluated by conducting full-scale
and modeled field tests since before World War II. Comparisons between explosives have been
made by performing equivalent weight (EW) analyses, which are only comparative and do not
provide the energy of the blast wave itself. The parameters usually observed (shockwave time of
arrival, peak pressure, positive duration, positive phase impulse) are obtained from
measurements of the overpressure of the blast wave in air versus time (P-t curves). The
equivalent weight or equivalent yield can be determined from any of these airblast parameters. It
should be noted, however, that the equivalent weight is not usually a constant value; rather, it
will vary with the scaled distance (range divided by the cube root of the charge weight).
In addition to comparisons that are made using the usual airblast parameters, such as pressure
and/or impulse, a parameter such as energy flux density could be computed and used. Energy
flux is routinely computed for underwater explosions but has not generally been used for
explosions in air. Later portions of this paper will describe a calculational methodology for
explosions in air and then apply it to a sample problem.
