Final answer:
Under magnetic particle inspection, a fatigue crack is identified when the discontinuity is found in a highly stressed area of the part. The identification considers the pattern and location of the discontinuity related to the structure's stress distribution and material properties.
Step-by-step explanation:
Under magnetic particle inspection, a part will be identified as having a fatigue crack under the condition that the discontinuity is found in a highly stressed area of the part. Fatigue cracks are a type of structure defect that occurs due to repeated loading cycles and often takes place in regions experiencing high levels of stress. Visual indications of fatigue cracks under magnetic particle inspection might include irregular or branching lines of collected magnetic particles, indicating the presence of surface disruptions aligned with stress concentrations.
Line defects, such as dislocations, can lead to the formation of fatigue cracks because they weaken the structure of materials along a one-dimensional space. When materials are subjected to repeated stress cycles, these defects can grow and ultimately form cracks. Notably, line dislocations and movements are related to a material's plastic behavior, which can be functionally useful but also a pathway to material failure under cyclical stress.
It is crucial to understand that the presence of a discontinuity itself in a material does not confirm a fatigue crack. The pattern, location, and analysis of the discontinuity relative to the material's grain structure and stress distribution are necessary to identify the nature of the defect.