A History of Research and a Review of Recent Developments

210
The effects of explosive loading
hold, and fell from the sky. The loss of this aircraft has led to research to
investigate the blast forces on aircraft stiffened sheeting and structural members
after the blast has travelled through various irregular geometries of spaces,
conduits and ducts on its way from the hold to the vulnerable zones of the
airframe or control equipment. The geometry of the problem lends itself to a
finite element solution, but the basic variations of blast pressure with distance
and configurations still employ the fundamental science discussed in the later
sections of Chapter 5.
Turning now to ship structures, it is well known that below the waterline
marine vessels are particularly vulnerable to torpedoes and mines, and above the
waterline to shells, bombs and guided missiles. Much research has been devoted
to these problems, and in addition there was considerable activity in the 1950s
and 1960s to examine the response of naval vessels to nuclear blast. The work of
Hopkinson and others on anti-torpedo blister structures during the First World
War has already been mentioned, and at the more recent end of the research
spectrum it is noticed that new designs of warships incorporate structural features
that make them less easy to detect by guided bombs and search/strike missiles.
The costly attack/defence development battle is never ending.
The analysis of ship structures under blast loading is very complex, and
much of the knowledge used by designers is drawn from large-scale testing. In
a paper given in 1988, Charles Smith [8.28] described blast testing on deck, side
and bow components, and made the point that in designing warships to withstand
blast elastic design is generally too conservative, and that fairly large inelastic
deformations are tolerable as long as the protection of internal systems can be
maintained. He showed photographs of internal blast damage caused by guided
missiles, superstructure damage caused by air blast, and the breaking of a ships
back amidships by hull bending excited by a vibratory or ‘whipping’ response
of the hull to a non-contact underwater explosion. The latter problem was
examined in a paper by Hicks in 1986 [8.29], and by Jinhua and Zhang Qiyong
in 1984 [8.30]. The latter authors derived formulae for dynamic bending moments
which were shown to agree well with full-scale experiments in China. In these
tests charges varying in weight between 200 kg and 1000 kg of TNT were
detonated at depths between 7.5 and 40 m, and at distances from the vessels
between 7.5 m and 100m. The amplitudes of the modes of vibration were not
unexpectedly found to depend on the length along the vessel of the point of
action of the explosion. If this point was near the middle cross sections the 1st
mode of vibration was predictably the prime mode. If the point of action was
too near the quarter point the 2nd mode of vibration dominated.
Probably the most interesting paper referring to the history of underwater
explosion research in the ship structure field was published in 1961, and was
written by A.H.Kiel [8.31]. He referred first to the history of systematic testing,
beginning with the first reported tests in the USA in 1881 (Abbott, [8.32]),
and to the development between the two world wars of the side protection
systems against torpedoes used in the battle ships Midway (USA), Hood (UK),