What is fatigue failure and why is it critical in aircraft structures?

Fatigue failure occurs when materials fail under repeated cyclic loading at stress levels below the static yield strength. It is critical in aircraft because structures experience millions of stress cycles during their service life from pressurization, maneuvers, gusts, and ground-air-ground cycles. Fatigue initiates microscopic cracks that grow with each cycle until sudden failure. The Comet disasters in the 1950s highlighted fatigue risks, leading to modern damage tolerance design philosophy requiring inspection and crack detection before failure.