In terms of aircraft structure, how is 'fatigue' defined?

Fatigue in aircraft structures is defined as the progressive and localized structural damage that occurs as a result of cyclic loading. This phenomenon arises when materials are subjected to repeated stress over time, even if those stresses are below the material's ultimate tensile strength. Each loading cycle can cause microscopic cracks to develop, propagate, and eventually lead to failure if not properly managed.

Understanding fatigue is crucial in aircraft design and maintenance because aircraft experience varying loads during flight, such as turbulence, takeoff, and landing. Engineers must account for the effects of these repetitive stresses to ensure the safety and longevity of the aircraft.

The other options depict different scenarios that do not accurately capture the essence of fatigue. The sudden failure of materials under high stress refers more to brittle fracture than fatigue. The temporary loss of strength from thermal effects relates to thermal fatigue, which is a different phenomenon. Lastly, damage occurring only during turbulence does not embody the broader definition of fatigue, which encompasses all forms of cyclic loads, not just those experienced in turbulent conditions.