In magnetic particle inspection, fatigue cracks are identified when the discontinuity is located in which area of the part?

In magnetic particle inspection, fatigue cracks are identified when the discontinuity is located in which area of the part?

• A. The discontinuity pattern is straight
• B. The discontinuity is found in a nonstressed area of the part
• C. The discontinuity is found in a highly stressed area of the part
• D. The discontinuity is in a surface layer only

Answer: (C)

Fatigue cracks arise where the material experiences the greatest cyclic stress, so they’re most likely to be found in highly stressed areas of the part—around holes, sharp corners, fillets, or other stress concentrators where loading is intense. In magnetic particle inspection, such a crack disturbs the magnetic field along its length, creating a noticeable indication. That makes locating a discontinuity in a highly stressed region the most telling sign of a fatigue crack. If the flaw were in a nonstressed area, fatigue initiation would be far less likely, and a flaw limited to just a surface layer isn’t specific to fatigue. The pattern of the discontinuity alone (for example, being straight) isn’t enough to identify fatigue, whereas its presence in a high-stress region makes fatigue the most plausible origin.

Fatigue cracks arise where the material experiences the greatest cyclic stress, so they’re most likely to be found in highly stressed areas of the part—around holes, sharp corners, fillets, or other stress concentrators where loading is intense. In magnetic particle inspection, such a crack disturbs the magnetic field along its length, creating a noticeable indication. That makes locating a discontinuity in a highly stressed region the most telling sign of a fatigue crack. If the flaw were in a nonstressed area, fatigue initiation would be far less likely, and a flaw limited to just a surface layer isn’t specific to fatigue. The pattern of the discontinuity alone (for example, being straight) isn’t enough to identify fatigue, whereas its presence in a high-stress region makes fatigue the most plausible origin.