A hyperbaric vessel is designed to operate for 20 years. It is specified
that during that period it will start up and shut down 3,000 times, and will
reach the following number of steady-state pressures and resulting maximum
stress intensities:
No. of
Operations
Pressure, psi
PL + Pb + Q + F, psi
1000
0 - 1000 - 0
0 - 60,000 - 0
1000
0 - 1000 - 0
0 - 52,000 - 0
500
0 - 700 - 0
0 - 42,000 - 0
500
0 - 500 - 0
0 - 30,000 - 0
(Note that the difference between the two 1000 items could reflect
a difference due to a thermal stress quantity.)
For each of the operations shown above, the figures given in the column
headed by PL + Pb + Q + F, psi identify the "alternating stress intensity
range." The amplitude of each alternating stress is half of this value,
identified by the symbol Sa.
For most of the Code-authorized materials Design Fatigue Curves have been
established.* Figure 5.110.1 of the Code is shown here as Figure 2-8.
This curve and the others like it in this appendix are based upon
experimental fatigue data generated over many years. The values of Sa,
shown here represent amplitudes of alternating stress intensities. (Note
again that this is half the value of the alternating stress intensity range.)
These curves have been adjusted for mean stress values and only the
alternating component of stress need be considered. For any given value of
Sa, there is an acceptable number of cycles of operation and this number
constitutes the design life of the vessel. For instance, referring to Figure
2-8, if a pressure vessel were to operate at a constant Sa value of PL +
Pb + Q + F = (74,000)/2 = 37,000 psi, and the vessel were fabricated from
carbon steel with an ultimate tensile strength of less than 80,000 psi, then
the fatigue life of that vessel would be approximately 10,000 cycles; i.e.,
the load could vary from 0 - 1,000 psig-0, 10,000 times. The Code requires
that all fatigue loadings applicable to the figures shown in Article 5-1, are
to be interpreted from a linear cumulative damage criteria. That is, all
cycles of stress must be accounted for in the total fatigue life of the
vessel. In the example in the above paragraph, 10,000 cycles of operation is
the entire design fatigue life of the structure. Now consider the loading
conditions described above.
* These curves are based on tests which did not include the presence of a
corrosive environment which might accelerate the fatigue failure.
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