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SHOCK INITIATION PROPERTIES
4.2 Small-, and Large-Scale Gap Thicknesses. Gap tests are explosive shock
tests. A standard donor explosive produces a shock pressure of uniform magnitude
which is transmitted to the test explosive through an attenuating inert barrier or
gap. By varying the thickness of the barrier between the donor and test (acceptor)
explosives, one can determine the barrier thickness required to inhibit detonation in
the test explosive half the time (G,,).
A variety of gap tests have been used to qualitatively measure the shock wave
amplitude required to initiate detonation in explosives. LASL has used two test
configurations that differ only in scale. The diameter of the cylindrical acceptor
charge in the small-scale test is 12.7 mm; that in the large-scale test is 41.3 mm. An
explosive w!hose detonation failure diameter is near to or greater than the diameter
of the acceptor charge cannot be tested in the small-scale test so the large-scale test
is used. Figures 4.10 and 4.11 show the configuration of both gap tests. The test
procedure is to fire a few preliminary shots to determine the spacer thickness that
allows detonation in the test explosive. Shots are fired with the spacer thickness
alternately increased and decreased until the spacer thickness that allows detona-
tion in the acceptor explosive in half of the trials is determined. A deep, sharply
defined dent in the steel witness plate indicates that the test explosive detonated.
!
Y DETONATOR + l/2- BY I,? in.
/BOOSTER PELLET
r‘
PLASTIC HOLDER
Fig. 4:.10. Large-scale gap
test assembly.
Fig. 4.11. Small-scale gap
test assembly.
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