(c) Primary bending stress. This is a bending stress that is
induced directly by the pressure load acting upon a specific geometry and
is not induced by discontinuity conditions.
(2) Secondary stress. Such a stress is generally imposed by
constraints between adjacent parts of a shell and is selflimiting. That is,
if local yielding takes place as the load is applied, local distortion
relieves the stress in this area and the load is redistributed to the
surrounding material.
(3) Peak stress. Such a stress is imposed by very local geometric
discontinuities such as cracks, grooves, and fillets, and by certain types of
thermal loading conditions.
(4) Code identification.
The Code identifies those five (5)
stresses by the following symbols:
Primary general membrane stress

Pm
Primary local membrane stress

PL
Primary bending stress

Pb
Secondary stress

Q
Peak stress

F
d. Stress classification. Table 4120.1 of the Code, "Classification
of Stresses for Some Typical Cases," is shown here as Table 21. It should
be of assistance in classifying the types of stresses in pressure vessels for
hyperbaric facilities.
In the most general cases of shell analysis, the five types of stresses de
scribed above will contain shear stresses as well. If so, then each group of
stresses must be converted into principal stresses, [sigma]1, [sigma]2, and
[sigma]3. Fortunately, most pressure vessels usually consist of shells of
revolution and, except in the vicinity of penetrations and nonsymmetrical
supports or loads, the stresses as calculated are principal stresses, usually
denoted by [sigma]t, [sigma]*l, or [sigma]r.
In many types of shell analysis, particularly those carried out by computer
methods, the stress determined at any point will be a total stress. The
designer should break down this total stress to the five component types
described.
For each classification of stress as defined above, there may be more than
one set of stresses. This can arise, for instance, if a pressure vessel is
loaded by more than one type of load. One example of this is a pressure
vessel with high internal pressure and having significant thermal gradients
in its walls. The designer may choose to develop the stresses induced by the
thermal gradients separately from the stresses induced by pressure, thus
giving two sets of stresses at any point in the shell.
e.
Determining stress intensities. After grouping the principal
the stresses in each group are added to achieve one set of three stresses for
each class, i.e., [sigma]1, [sigma]2, and [sigma]3. The "stress
intensity" for each class is then calculated as the greatest absolute value
of the differences of any two of the three stresses, i.e.,