R. Meyer J. Köhler A. Homburg Explosives

297 Strength
relevant parameter is its “strength”; here the criterion of the performance
is not so much a high detonation rate as a high gas yield and a
high heat of explosion. If, on the other hand, a strong disintegration
effect in the nearest vicinity of the detonation is required, the most
important parameters are the detonation rate and the density (W Brisance).
A number of conventional tests and calculation methods exist for
determining the comparative performance of different explosives. The
determinations of the detonation rate and density require no conventions,
since they are both specific physical parameter.
Lead block test and ballistic mortar test
Practical tests for comparative strength determination are the lead
block test and the declination of a ballistic mortar. In both cases
relatively small amounts of the explosive (of the order of 10 g) are
initiated by a blasting cap. In the lead block test, the magnitude
measured is the volume of the pear-shaped bulge made in the block
borehole by the sample introduced into it; in the ballistic mortar test
the magnitude which is measured is the deflection angle; this angle is
taken as a measure of the recoil force of a heavy steel weight suspended
as a pendulum bob, after the exploding cartridge has fired a
steel projectile out of a hole made in the bob. The performance of the
explosive being tested is reported as the percentage of that of W Blasting
Gelatin, which is conventionally taken as 100% (For further details
W Ballistic Mortar). In both cases the explosive is enclosed in a confined
space, so that, for all practical purposes, the parameter measured
is the work of decomposition of an explosive in a borehole. The
disadvantage of both methods is that the quantity of the sample used
in the test (exactly or approximately 10 g) is quite small, and for this
reason accurate comparative data can be obtained only with more
sensitive explosives; less sensitive materials require a longer detonation
development distance (W Detonation), within which a considerable
proportion of the 10-g sample does not fully react. Practical methods
for determining the performance of explosives requiring much larger
samples (up to 500 g) include the following.
Jumping mortar test
Two halves with finely ground surfaces fitting exactly onto one another
form a mortar with a borehole. One of the halves is embedded in the
ground at a 45° angle, while the other half is projected like a shot,
when the explosive charge is detonated in the hole; the distance to
which it has been thrown is then determined. A disadvantage of the
method is that when high-brisance explosives are tested, the surfaces
must be reground after each shot. The method gives excellent results
with weaker W Permitted Explosives.