What primarily causes fatigue damage in aircraft structures?

Fatigue damage in aircraft structures is primarily caused by repeated loading and unloading cycles. This phenomenon occurs when an aircraft is subjected to cyclic stresses during operations, such as during takeoff, flight, and landing. Over time, these repeated stress cycles can lead to the initiation and propagation of cracks within the material, compromising the structural integrity of the aircraft.

Unlike static loads, which may cause an immediate failure or result in straightforward stress distribution, the dynamic nature of cyclic stresses leads to microscopic shifts within the material's structure that gradually accumulate damage. Each cycle can imprint stress reversals that, even if they are below the material's ultimate strength, can contribute to fatigue over the life of the aircraft.

High speeds during flight and exposure to harsh weather conditions may influence the conditions in which fatigue damage occurs, but they do not directly cause the underlying mechanism of fatigue, which is rooted in these repetitive cycles of stress. Similarly, long-lasting static loads can lead to different types of failures, such as buckling or yielding, but they are not the primary cause of fatigue damage.