Fatigue cracks refer to cracks that develop in materials under the repeated application of cyclic loading and unloading. These cracks are particularly common in materials subjected to dynamic loads or stress, such as in metal structures in aircraft, bridges, and pipelines. Fatigue cracks can lead to catastrophic failure if not detected and repaired in a timely manner. Research in this area focuses on understanding the mechanisms of fatigue crack growth, developing methods for crack detection and monitoring, and designing materials and structures that are more resistant to fatigue cracking. Techniques such as fatigue testing, fractography analysis, and finite element modeling are commonly used in fatigue crack research.