A History of Research and a Review of Recent Developments

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Structural loading from local explosions
gauge installation procedures in rock and soil has been discussed by Florence,
Keough and Mak [5.7]. In particular they investigated theoretically the effect
of bonded and unbonded interfaces between the gauge and the surrounding
soil. They assumed that a bonded interface would transmit compression and
shear, and an unbonded interface compressive loads only. Their calculations
show that lack of interface bonding produces a non-uniform stress distribution
at the gauge which produces inaccuracies at low stresses. At higher stresses
plastic flow in the soil increases, and causes the stress distribution to be more
uniform. This improves accuracy.
5.3 CONCENTRATED EXTERNAL LOADS ON UNDERWATER
STRUCTURES
Much of the load-measuring research in this field has been conducted by naval
research teams interested in the response of ship and submarine structures to
local explosions from mines and torpedoes. Minesweepers and similar vessels
are likely to suffer shock loading from the explosion of underwater charges at
what are called ‘intermediate stand-offs’, which can induce flexural motion
or ‘whipping’ of the hull as well as local rupture and damage. We have already
examined the characteristics of bubble pulsations and shock waves at long
distance, but now we must evaluate the intermediate stand-off effects. Hicks
[5.8], in discussing explosion-induced hull whipping, suggests that British naval
scientists are interested in peak instantaneous pressures in the 100 to 400 bar
range, with durations between 300 and 600 microseconds. He concludes that
with the acoustic velocity of sound in water at 1500m/s, the shock wave occupies
a thin shell of water that might be about 1 metre in thickness.
Hicks investigated how far the formulae for loading from distant explosions
can be applied to close explosions. In the latter case there would seem to be
a loss of accuracy for explosions near the bow and stern, where divergent
flow around the end of the vessel might introduce non-uniformity into the
analysis. Hicks, however, concluded that the accuracy of the distant-flow
approximations for close point sources was extraordinarily good, and he
assumed that the distant-flow analysis was adequate for normal stand-offs.
For very close stand-offs the pressure bubble is likely to completely engulf
the hull and this distorts the analysis too much for the distant-flow equations
to be applicable.
Loading from intermediate stand-offs has also been investigated recently
by Haxton and Haywood [5.9], who took as their model an underwater cylinder
subjected to a pressure pulse originating from a point distant about one radius
from the wall of the cylinder. It was assumed that the peak instantaneous
pressure from the explosion varied inversely with distance, and that the radial
component of the incident particle velocity could be neglected in the shadow
of the cylinder. Referring to Figure 5.7, the peak instantaneous pressure depends
on the radius Rt and the angle θt, and the point of grazing incidence, beyond