Page 185 - Introduction to Naval Architecture

STRENGTH 17:

Figure 7.23 Load and strength distributions

are defined by the mean and variance of the distributions. They can
then express the safety in terms of a load factor based on the average
load and strength. This may be modified by another factor represent-
ing a judgement of the consequences of failure.
Having ascertained that the structure is adequate in terms of
ultimate strength, the designer must look at the fatigue strength. Again
use is made of the stressing under the various weather conditions the
ship is expected to meet. This will yield the number of occasions the
stress can be expected to exceed certain values. Most fatigue data for
steels relate to constant amplitude tests so the designer needs to be able
to relate the varying loads to this standard data as was discussed
earlier.

SUMMARY

It has been shown how the vertical bending moments and shearing
forces a ship experiences in still water and in waves can be assessed
together with a limited discussion on horizontal bending and torsion of
the main hull. This vertical loading was used, with estimates of the hull
modulus, to deduce the stresses and deflections of the hull. The ability
of the various structural elements to carry load before and after
buckling was looked at leading to an ultimate load carrying capability.
It has been suggested that the structure should be so designed that the
maximum bending moment it can withstand is likely to be experienced
only once in the life of the ship. Thus the chances of the hull failing
from direct overloading are minimized. Failure, if it occurs, is much
more likely to be due to a combination of fatigue and corrosion. These
two cumulative failure mechanisms have been outlined. Associated with
fatigue is the behaviour of steels in the presence of crack-like defects
which act as stress concentrations and may cause brittle fracture below
certain temperatures and at high strain rates. This highlighted the