Page 136 - Introduction to Naval Architecture

STRENGTH

likely to be due to a combination of fatigue and corrosion. These
cumulative failure mechanisms are increasingly determining the ship
structure and its likely useful life span.

MODES OF FAILURE

To provide some logical progression through this difficult topic it is
instructive to consider first the various ways in which a ship's structure
may fail and the possible consequences. Although of rather complex
make-up, the ship is essentially an elastic beam floating on the water
surface and subject to a range of fluctuating and quasi-steady loads.
Those loads will generate bending moments and shear forces which
may act over the ship as a whole or be localized. The former will include
the action of the sea. The latter will include the forces on heavy items
composed of gravity forces and dynamic forces due to the accelerations
imparted by the ship's motion. Then there is the thrust due to the main
propulsion forces.
Failure can be said to occur when the structure can no longer carry
out its intended function. If, in failing, one element merely sheds its
load on to another which can withstand it there is usually no great
safety problem although remedial action may be necessary. If, however,
there is a 'domino' effect and the surrounding structural elements fail
in their turn the result can be loss of the ship. Failure may be due to the
structure:

(1) Becoming distorted due to being strained past the yield point.
This will lead to permanent set and the distortion may lead to
systems being unable to function. For instance, the shafts may be
unable to turn.
(2) Cracking. This occurs when the material can no longer sustain
the load applied and it parts. The loading may exceed the
ultimate strength of the material or, more likely, failure is due to
fatigue of the material.
(3) Instability. Very large deflections can occur under relatively light
loads. In effect the structure behaves like a crippled strut.

The approach, then, to a study of a ship's structural strength is to assess
the overall loading of the hull, determine the likely stresses and strains
this engenders and the ability of the main hull girder to withstand
them. Then local forces can be superimposed on the overall effects to
ensure that individual elements of the structure are adequate and will
continue to play their part in the total structure.