What does "fatigue" refer to in the context of aircraft metal structures?

In the context of aircraft metal structures, "fatigue" specifically refers to progressive and localized structural damage that occurs due to cyclic loading. When aircraft components undergo repeated stress, even if those stresses are below the material's yield strength, microscopic cracks can initiate and grow over time. This phenomenon is particularly critical in aviation because aircraft are subject to numerous flight cycles, which include takeoff, cruising, and landing stresses.

Unlike sudden failures that occur with overwhelming force (as might be hinted at in another choice), fatigue is a gradual process that can compromise structural integrity without any forewarnings. This makes it especially insidious, as the damage may accumulate over time without visible distress until it becomes severe enough to lead to a potential structural failure.

Additionally, while rusting and moisture absorption are significant concerns in maintenance and structural integrity, they are not what defines fatigue in metal structures. Such corrosion processes can lead to weakness but are distinctly separate from the fatigue process, which is directly related to the repetitive loading and unloading that occurs during aircraft operation.