What does "fatigue" in aircraft structures refer to?

"Fatigue" in aircraft structures specifically refers to the progressive and localized structural damage that occurs due to cyclic loading. This phenomenon is particularly important in aviation because aircraft are subject to repeated stress and strain during flight, which can eventually lead to the development of cracks or failures in structural components.

When an aircraft is subjected to repeated loading and unloading, even if the loads are below the material's ultimate strength, microscopic damage can begin to accumulate over time. This can result in a reduction in the structural integrity of component parts, making it crucial for aircraft maintenance personnel to monitor and inspect components regularly for any signs of fatigue.

In terms of the other choices, while maintenance checks can help identify fatigue issues, they do not define what fatigue is. Environmental factors can cause wear and tear, but they do not relate directly to the mechanism of fatigue. Lastly, design flaws, though critical in the context of overall aircraft safety, do not specifically address the concept of fatigue either, which is primarily concerned with how materials behave under repeated stress. Therefore, option B provides the most accurate description of fatigue in aircraft structures.