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 where,
tg
=
duration of gas pressure
to
=
duration of shock pressure
A
=
vent area, ft2
W
=
charge weight, lb
V
=
volume of cubicle, ft3
It should be emphasized that Equations (15) and (16) are only
approximate and were based on charges located at the geometric center of the
cubicles. When the other charge locations are involved, Equation (17)
should be used. This equation was recommended in NAVFAC P-397 to be used
when the duration of the shock pressure on the wall of a cubicle is to be
calculated.
EQUATION:
t'o = (tA)F - (tA)N + (to)F
(17)
where,
t'o
= design duration of the positive pressure, ms
(tA)F = time of arrival of the blast wave at the point on the
wall farthest from the charge, RF
(tA)N = time of arrival of the blast wave at the point on the
wall nearest to the charge, RN
(to)F = duration of the blast pressure at the point on the wall
farthest from the charge, RF
The procedure for solving Equation (17) is dealt with in depth in NAVFAC
P-397.
(3) Impulse of Gas Pressure. Keenan and Tancreto performed
experiments to determine the relationship between the scaled peak impulse of
the gas pressure inside a cubicle and the sealed vent area of the cubicle.
Figure 10 illustrates this relationship which can also be described by:
EQUATION:
ig/W1/3 = 569(A/W2/3) -0.78(W/V) -0.38
(18)
for A/V2/3 < 0.21.
Again, the curves in Figure 10 and Equation (10) were derived for a
charge located at the geometric center of the cube. NAVFAC P-397 contains
a series of charts (Figures 4-17 through 4-62) for predicting the average
shock impulse acting on the walls of a cubicle of any shape and size.
c. Blast Environment Outside Cubicle. The entire process of detonation
in a cubicle and gas venting produces a train of shock waves which travel
away from the cubicle and attenuate with distance. At any point outside
the cubicle, the pressure-time history has the characteristics of an
unconfined explosion, except that it contains a number of pronounced
pressure spikes, particularly close to the cubicle. The first pressure
spike constitutes the largest pressure; the number of spikes decreases
with distance and, for a given charge weight, the number of spikes tends
to decrease with increasing bent area.
2.08-24
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