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 (a) However, this curve gives a single definite value of
k- = 0.5625. According to the report, Loading Characteristics of Air
Blasts from Detonating Charges by Granstrom, this value is assumed to be
valid for scaled distances greater than 15.2 ft/lb1/3. A plot of Equation
(14a) is given in Figure 7b. For values of k- not equal to 0.5625, the
suggested curve should be adjusted such that the area under it equals the
negative phase impulse. The value of k- is defined as follows:
EQUATION:
k- = i-s/P-sot-o
(14b)
(b) Figure 7b is applicable to both incident and reflected
pressures for incident pressure of approximately 10 psi or less.
5.
PARTIALLY VENTED EXPLOSIONS.
a. Definition. Gas pressure pulses developed from partially vented
explosions in cubicles can be far more damaging than the shock pulse,
depending on the duration of the gas pulse, tg, relative to the duration
of the shock pulse, to. If tg/to < 1, the explosion is classified as
a fully vented explosion and the gas pulse, if any, can be neglected in the
design of the cubicle. For tg/to > 1, the explosion is classified as a
partially vented explosion and both the gas and shock pulses must be
considered in the design of the cubicle.
b. Interior Blast Loading. Information pertaining to the blast
environment from partially confined explosions is not available in NAVFAC
P-397. Results of the study done by Keenan and Tancreto, Blast Environment
from Fully and Partially Vented Explosions in Cubicles, are presented in the
following paragraphs:
(1) Peak Gas Pressure. When the openings in a cubicle-type
structure are small compared to the total surface area, the gas pressure
duration (resulting from an internal explosion) is often large relative to
the fundamental period (TN) of the walls and roof of the cubicle.
(a) The maximum mean pressure pmo is recommended in NAVFAC
P-397 to be used as the basis for design and Figure 4-65 of referenced
manual illustrates the relationship between pmo and the charge-to-volume
ratio.
(b) Figures 8 and 9 show the variation of the peak gas
pressure with charge-to-volume ratio for a partially vented explosion in a
4-wall cubicle. Both figures (for TNT and Composition B) were constructed
based upon experiments performed for relatively small vent areas with the
charges located at the geometric centers of the cubicles.
(2) Duration of Gas and Shock Pressures. The relationship between
the duration of the positive pressure in a cubicle and vent area can be
expressed as:
EQUATION:
tg/W1/3 = 2.26(AW1/3/V) -0.86 for A/V2/3 < 0.21
(15)
EQUATION:
to/W1/3 = 0.664(AW1/3/V) -1.14 for A/V2/3 > 0.60
(16)
2.08-21
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