How do you recognize fatigue cracks in steel and why are they so critical?

Fatigue cracks in steel are recognized by their smooth, often shell-like appearance and their tendency to originate at points of stress concentration, such as weld toes, sharp corners, or bolt holes. They typically propagate slowly over time under repeated cycles of loading, even at stress levels below the steel's yield strength. These cracks are critical because they can lead to sudden and catastrophic failure of steel components without any prior warning. Unlike static failure, which occurs when the load exceeds the material's strength, fatigue failure is a progressive process that weakens the steel over time, eventually leading to fracture. Regular inspection and non-destructive testing are essential to detect fatigue cracks early and prevent structural collapse.