Page 153 - Introduction to Naval Architecture

STRENGTH 139

Corresponding relationships can be worked out for material added
below the neutral axis. If the new material is added above the upper
deck then the maximum stress will occur in it rather than in the main
deck. It can be shown that in this case there is a minimum area that
must be added at any given height in order to reduce the stress in the
deck..

STRESS CONCENTRATIONS

So far only the general stresses in a structure have been considered.
There are several reasons why local stresses may exceed considerably
those in the vicinity. The design may introduce points at which the
loads in a large structural element are led into a relatively small
member. It is useful in looking at a structure to consider where the load
in a member can go next. If there is no natural, and even, 'flow' then
a concentration of stress can occur. Some such details are bound to
arise at times, in way of large deck openings for instance, or where the
superstructure ends. In such cases the designer must take care to
minimize the stress concentration. Well rounded corners to hatch
openings are essential and added thickness of plating abreast the
hatches reduces the stress for a given load. The magnitude of this effect
can be illustrated by the case of an elliptical hole in an infinitely wide
plate subject to uniform tensile stress across the width. If the long axis
of the ellipse is 2 a and the minor axis is 2b, then with the long axis
across the plate the stresses at the ends of the long axis will be
augmented by a factor [1 + (2a/b)]. If the hole is circular this
concentration factor becomes 3. There will be a compressive stress at
the ends of the minor axis equal in magnitude to the tensile stress in
the plate. In practice there is little advantage in giving a hatch corner
a radius of more than about 15 per cent of the the hatch width. The
side of the hatch should be aligned with the direction of stress
otherwise there could be a further stress penalty of about 25 per
cent,
Apart from design features built into the ship, stress concentrations
can be introduced as the ship is built. Structural members may not be
accurately aligned either side of a bulkhead or floor. This is why
important members are made continuous and less important members
are made intercostal, that is they are cut and secured either side of the
continuous member. Other concentrations are occasioned by defects in
the welding and other forming processes. Provided the size of these
defects is not large, local redistribution of stresses can occur due to
yielding of the material. However large defects, found perhaps as a
result of radiographic inspection, should be repaired.