When a vessel (or part of a vessel) is subjected to pressure loads that repeatedly cycle from low to high, the fatigue life of the vessel may need to be evaluated. This is outside the scope of Section VIII Division 1, but it can be accomplished with finite element analysis (FEA) and the fatigue evaluation methods given in Division 2 of the code. Fatigue curves (S-N curves) are provided for each material type. If the evaluation shows that the allowable number of cycles are less than the design life of the vessel, then geometry changes may be needed to increase the operating life.
The analysis can also be performed for cyclic external loads, such as piping loads on a nozzle. However, cycling temperatures do not normally need to be considered in the same way, since an increase in temperature does not directly cause stress in a component.
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