What is metal fatigue and why is it critical in aviation maintenance?

• A. Fatigue is progressive crack growth from cyclic loading; critical because small cracks can grow to cause structural failure.
• B. Fatigue is caused by a single overload leading to immediate fracture.
• C. Fatigue occurs only in composite materials.
• D. Fatigue is wear due to engine oil.

Answer: (A)

Metal fatigue is progressive cracking that starts from repeated loading and unloading. In flight, parts endure countless stress cycles; even when each cycle is well below the material’s strength, tiny cracks can form at stress risers such as holes, sharp corners, or surface imperfections. Each cycle allows these cracks to grow a little, and the growth rate depends on the stress range, material properties, surface condition, temperature, and environment. After many cycles, a crack can reach a critical size and then propagate rapidly to failure, which is why fatigue is so critical in aviation maintenance. Aircraft experience huge numbers of cycles from takeoffs, landings, pressurization, and gusts, so maintenance focuses on detecting and controlling fatigue through inspections and non-destructive testing, and by using damage-tolerance or life-limit strategies to prevent in-service failure. It’s not about a single overload, it isn’t limited to composites, and it isn’t wear from engine oil.