A History of Research and a Review of Recent Developments

Introduction xxiii
own versions of the pressure transducer, ranging in size from miniatures with
diameters of a few millimetres to large-scale components with diameters of 10
centimetres and more. These and others have been reviewed in detail by Wilfred
Baker in his book Explosions in Air, published by the University of Texas Press
in 1973 in conjunction with the Southwest Research Institute of the USA. In a
most useful survey he also discussed the design of gauges to measure dynamic
pressures due to blast winds and to record the time of arrival or time of start of
release of blast energy.
The accurate measurement of the characteristics of the dynamic loading
due to explosive shock has, as we have seen, been of great interest to structural
engineers. The instrumentation for a major nuclear or high explosive test can
be astronomically expensive, but unless the pressure/duration characteristics
of a blast wave at various distances from the source and at various parts of a
structure can be measured the calculation of structural response in air, water
or underground cannot be carried out with scientific confidence. Care must
also be taken to distinguish between detonation and deflagration, the absence
of sudden shock, and changes in the shape of the pressure/duration curve.
These factors are pursued later.
It is important in the analysis of loading and response not to overlook the
lack of uniformity in explosive actions. The pressure distribution depends on
the shape of the charge, but even with a perfectly spherical charge under ideal
conditions it would be wrong to assume that the peak pressure at a given
distance from the centre of the sphere was uniform and regular. There are
peaks and troughs in the distribution pattern that make it difficult to treat the
loading analysis as an exact science. Charges of nominally the same weight
and geometry do not necessarily yield similar pressures, durations or impulsive
characteristics. The physical conditions are so variable that great care must
be taken when drawing analytical conclusions, because experimental scatter
in laboratory or field tests is high.
Some of the earliest methods of calculation of structural response were
associated with the assessment of the strength and safety of gun barrels. Cast
guns often burst on discharge because of minute flaws, and in the fifteenth
century in England accidental bursts of military ordnance were considered
important enough to alert the privy council. The idea of proving the strength
of ordnance by ‘proof testing developed in successive centuries, and today we
have ‘proof and experimental establishments’ like that at Shoeburyness as
historic links with earlier days. The structural strength and safety of shotgun
barrels are still tested today by proof testing, where a charge well in excess of
that normally associated with the explosion of a cartridge is fired in the chamber
of the gun.
As the design of guns progressed, early versions of ‘thick cylinder’ theory
began to be used to predict strength, and this was probably the first use of
structural analysis to judge response to shock loading. The earliest guns fired
simple stone or cast-iron cannonballs, or canisters containing grape shot or