Mechanical Fatigue

Mechanical fatigue, caused by cyclic stresses, is likely to occur when the vessel has experienced at least 400 stress (pressure) fluctuations that exceed 15% of the maximum allowable design stress for the material of construction (or design pressure for the vessel).

Cracks attributable to mechanical fatigue usually originate on the surface of a pressure vessel at locations of relatively high stress concentration. Nozzles and the toes of fillet welds are likely places for fatigue cracks to occur. Pad-reinforced nozzles and partial penetration welds are more susceptible to fatigue than are integrally reinforced nozzles and full penetration welds. Fatigue cracks can also originate at the stress concentration associated with internal fabrication flaws.

Magnetic particle examination (MT) or dye-penetrant examination (PT) can be used to detect mechanical fatigue cracks that originate on the surface subjected to cyclic stresses. Both the O.D. and I.D. surfaces should be examined at locations of relatively high stress concentration. Ultrasonic examination (UT) using shear wave procedures can be used to determine the depth of fatigue cracks that originate at the surface. A properly planned in-service inspection program will allow the surface fatigue cracks to be detected before they propagate to a size that jeopardizes the integrity and reliability of the vessel. Such a program also enables scheduling of repairs to avoid any unnecessary interruption to production. UT can also be used to detect and size the more insidious fatigue cracks that originate at internal flaws, or at surfaces that are not accessible for MT or PT examination.

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