What can fatigue in steel be caused by?

Fatigue in steel is primarily caused by the cumulative effects of cyclical loading over time. When a material is subjected to repeated stress variations, even if these stresses are below the material's ultimate tensile strength, microscopic damage can accumulate. This can lead to the initiation of cracks, which can propagate and ultimately result in failure at lower stress levels than the material would normally withstand under static loading.

The persistence of cyclical loads causes continual bending, tensile, or compressive stresses, leading to the characteristic fatigue failure. This mechanism is significant in structural steel design, where dynamic loads—such as those from traffic on bridges or machinery—can result in these fatigue-related issues. Therefore, understanding the effects of cyclical loads and the resulting reduction in strength is critical for engineers to prevent unexpected fractures in steel structures.