Page 156 - Introduction to Naval Architecture

142 STRENGTH

Figure 7,10 S-N curve

typical welded joints have yielded a series of log-log S-N lines of equal
slope.
The standard data refers to constant amplitude of stressing. Under
these conditions the results are not too sensitive to the mean stress level
provided it is less than the elastic limit. At sea, however, a ship is subject
to varying conditions. This can be treated as a spectrum for loading in
the same way as motions are treated. A transfer function can be used to
relate the stress range under spectrum loading to that under constant
14
amplitude loading. Based on the welded joint tests referred to above ,
it has been suggested that the permissible stress levels, assuming twenty
million cycles as typical for a merchant ship's life, can be taken as four
times that from the constant amplitude tests. This should be associated
with a safety factor of four thirds.

SUPERSTRUCTURES

Superstructures and deckhouses are major discontinuities in the ship
girder. They contribute to the longitudinal strength but will not be folly
efficient in so doing. They should not be ignored as, although this
would 'play safe' in calculating the main hull strength, it would run the
risk that the superstructure itself would not be strong enough to take
the loads imposed on it at sea. Also they are potential sources of stress
concentrations, particularly at their ends. For this reason they should
not be ended close to highly stressed areas such as amidships.