What type of cracks develop generally at stress concentration sites?

Fatigue cracks generally develop at stress concentration sites due to the cyclic loading and unloading of materials over time. When materials are subjected to repeated stress, small flaws or defects present in the material can become sources of fatigue, exacerbated by high-stress regions where the material is under greater strain. Over time, the repeated application of load leads to the progressive crack growth initiated from these stress concentration points.

In contrast, grinding cracks are associated with heat generated during the grinding process, leading to thermal stresses, rather than cycling stress patterns. Surface checks are typically more superficial quality issues that are not primarily associated with stress concentration sites in the same manner. Creep cracks tend to occur under constant stress at elevated temperatures over long periods, differing from the cyclic nature of stress seen with fatigue cracks. Thus, the association of fatigue cracks with stress concentration is a key reason why this option is the correct choice.