Fatigue failure is a type of material failure that occurs after repeated stress cycles, even if the stress levels are below the material's ultimate strength. This gradual weakening can lead to cracks and eventual breakage, often without any visible signs of damage until it's too late. It is commonly observed in metals, plastics, and composites used in various applications, such as bridges, aircraft, and machinery. The process of fatigue failure typically involves three stages: crack initiation, crack propagation, and final fracture. Factors like the material's properties, surface conditions, and environmental influences can affect the fatigue life. Understanding fatigue failure is crucial for engineers to design safer and more reliable structures and components.