For the purpose of stress evaluation referred to in 4.5.1.4, stress categories are defined
in this section.
4.13.1 Normal stress is the
component of stress normal to the plane of reference.
4.13.2 Membrane stress is
the component of normal stress which is uniformly distributed and
equal to the average value of the stress across the thickness of the
section under consideration.
4.13.3 Bending stress is the
variable stress across the thickness of the section under consideration,
after the subtraction of the membrane stress.
4.13.4 Shear stress is the
component of the stress acting in the plane of reference.
4.13.5 Primary stress is a
stress produced by the imposed loading and which is necessary to balance
the external forces and moments. The basic characteristic of a primary
stress is that it is not self-limiting. Primary stresses which considerably
exceed the yield strength will result in failure or at least in gross
deformations.
4.13.6 Primary general membrane
stress is a primary membrane stress which is so distributed in the
structure that no redistribution of load occurs as a result of yielding.
4.13.7 Primary local membrane
stress arises where a membrane stress produced by pressure or other
mechanical loading and associated with a primary or a discontinuity
effect produces excessive distortion in the transfer of loads for
other portions of the structure. Such a stress is classified as a
primary local membrane stress although it has some characteristics
of a secondary stress. A stress region may be considered as local
if:
where:
|
S
1
|
= |
distance in the meridional direction over which the equivalent
stress exceeds 1.1 f
|
|
S
2
|
= |
distance in the meridional direction to another region where
the limits for primary general membrane stress are exceeded |
|
R
|
= |
mean
radius of the vessel |
|
t
|
= |
wall
thickness of the vessel at the location where the primary general
membrane stress limit is exceeded |
|
f
|
= |
allowable
primary general membrane stress. |
4.13.8 Secondary stress is
a normal stress or shear stress developed by constraints of adjacent
parts or by self-constraint of a structure. The basic characteristic
of a secondary stress is that it is self-limiting. Local yielding
and minor distortions can satisfy the conditions which cause the stress
to occur.