Definition
The progressive weakening and eventual cracking of a metal part caused by repeated cycles of stress, even when each individual stress is well below the metal's breaking strength.
Plain English
When metal is bent, flexed, or loaded over and over again, tiny cracks slowly form inside it. Given enough cycles, the part can fail — even though no single load was ever strong enough to break it on its own.
Context Anchor
Seen when comparing metal aircraft structures with composite structures, and in discussions of aging aircraft, inspections, and structural cracks.
Derivation
Fatigue comes from the Latin fatigare, meaning to tire out or wear down. Just as a person gets tired from repeated effort, metal gets 'tired' from repeated stress cycles and eventually gives way.
Why Pilots Care
Metal fatigue can cause sudden structural failure without any obvious overload, which is why aircraft parts have strict inspection intervals and retirement lives.
Analogy
Think of bending a paperclip back and forth. The first bend doesn't break it, and neither does the tenth. But keep going, and it snaps — not from one big force, but from many small ones adding up.
Grounding Statement
A metal part may handle one load safely, but many repeated loads over time can start a crack.
Intuition Check
Metal fatigue does not mean the metal is tired like a person. It means repeated loading has weakened the metal or started cracks in it.
Example Sentence 1
One advantage of composite materials is that they don't suffer from metal fatigue the way aluminum airframes do.
Example Sentence 2
Designers calculate retirement times for aluminum parts to prevent failure from metal fatigue during normal operations.