Page 316 - Introduction to Naval Architecture

VIBRATION, NOISE AND SHOCK 301

where:
Wh the charge weight
R is the distance from charge to the keel
0 is the angle between the line joining charge to keel and the
normal to the keel plate.

Since this expression is not non-dimensional and different formula-
tions exist, care is needed when using shock factors. Various explosives
are in use and they are usually related to an equivalent weight of TNT
in deducing shock factors and comparing results of explosive testing. In
addition to the shock factor, the intensity of shock experienced by an
item of equipment depends upon its weight, rigidity, position in the
ship and method of mounting. For critical systems, perhaps one vital to
the safety of the ship, it may be necessary to calculate the shock likely
to be felt at a specific position in a given design. This can be done by
calculation and/or model experiment using methods validated by full
scale trials.
More generally equipments are fitted to more than one design and in
different positions in any one ship so they must be able to cope with a
range of shock conditions. The approach is to design to generalized
shock grade curves. The overall design can be made more robust by
providing shock isolation mounts for sensitive items and by siting
system elements in positions where the structure offers more shock
attenuation. This has the advantages that the item itself does not have
to be so strong and the mounts can assist in attenuating any noise the
equipment produces, reducing its contribution to the underwater
noise signature.
The reaction of equipment to shock is a dynamic problem and the
response will depend upon an item's flexibility. This must be allowed
for in calculating its ability to survive and function. As a guide,
designers should avoid cantilevered components, avoid brittle materi-
als, mount flexibly and ensure that movements in response to the shock
are not impeded by pipe or cable connections or cause impact with
hard structure. It is important to allow for the behaviour of materials
used when subject to high rates of strain. In plating subject to collision
20
or shock loading the maximum strain rates are estimated to lie
1 4
between 2 and 20 sec" . This compares with a strain rate of about 10~
1
sec" when the hull girder is bending under wave action. Some
materials, notably mild steel, exhibit an increase in yield point by a
factor up to two when subject to these high strain rates.