Primary bending stress

(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 self-limiting. 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 4-120.1 of the Code, "Classification

of Stresses for Some Typical Cases," is shown here as Table 2-1. 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

stresses at each point of interest in the vessel in their respective classes,

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.,