What condition indicates that a part has a fatigue crack under magnetic particle inspection?

Magnetic particle inspection is a non-destructive testing method used to detect surface and near-surface discontinuities, such as fatigue cracks in ferromagnetic materials. A fatigue crack typically develops in regions that experience tensile stress, where repetitive loading or cyclical forces are applied. Therefore, the identification of a crack in a highly stressed area is a strong indication that the discontinuity is due to fatigue.

When cracks occur in highly stressed areas, they are more likely to propagate due to the significant forces acting on those regions. Such locations can often be found in components subjected to dynamic loading, where fatigue failure is a common concern. The presence of a crack in these areas suggests that the material has experienced stress beyond its yield strength over time, leading to the development of fatigue.

In contrast, cracks located in low-stressed areas typically do not pose a significant risk of fatigue failure, as these areas are not subject to the same level of repetitive loading. Similarly, cracks appearing only in paint or coatings do not indicate structural failure of the base material since they are not related to the material's mechanical properties. Visual surface defects may not necessarily correlate with fatigue cracks either, as they could arise from a variety of other surface irregularities not directly related to stress or fatigue.