A History of Research and a Review of Recent Developments

Concentrated external explosive loads on surface structures 93
Figure 5.2 Spherical charge exploding close to flat surface (from Henrych, ref. 5.1).
outflowing particles (outburst speed) is u x, as before, Henrych shows that
the specific impulse, i, at point A, is
(5.6)
where W is charge weight, A0=(Vs+ux)/4π, and a is the angle between the
vertical at A and the line joining A to the centre of the charge. V s is the
displacement velocity of the outburst surface.
For a cylindrical charge,
(5.7)
where W c is the mass per unit length of the charge.
In order to find the total impulse on the surface it is necessary to integrate
the specific impulse over the whole area. If the surface is a circular plate, the
extremities of which have a value of a designated a 0, then
I=pA 0W sin 2 a 0.
If the plate has infinite dimensions, so that a 0 approaches p/2, then
I=pA 0W.
(5.8)
(5.9)
To use all the above equations we have to know the value of V s and u x. As
discussed above, u x=(2Q) 1/2 , and V s is the velocity of the shock front. From
experiments it has been reported by Henrych that V s has the following values
in metres/sec for TNT and PETN: 7100 and 8450. The values given for u x for
the same two explosives are 6450 and 7700 metres/sec. Care must be taken
when applying the above analysis that the units are correct.
Explosive charges close to the ground are often assembled in an array, to
distribute total explosive mass over a large area and give some uniformity of