Definition
A form of material failure caused by repeated cycles of heating and cooling. As a part heats up it expands, and as it cools it contracts. When this happens over and over, internal stresses build up and eventually cause cracks or weakening of the material, even though the part was never overloaded mechanically.
Plain English
Damage that builds up in a metal part because it keeps getting hot and then cooling down. The repeated expanding and shrinking slowly weakens the part until it cracks.
Context Anchor
Seen in aircraft maintenance discussions of engine, exhaust, brake, and other parts that repeatedly get hot and then cool down.
Derivation
Thermal comes from the Greek therme, meaning heat. Fatigue comes from the Latin fatigare, meaning to tire out. Together the term describes a part that has been 'tired out' by heat cycles -- a useful image, since the part is not broken by a single event but worn down over time.
Why Pilots Care
Unchecked thermal fatigue can lead to cracks in critical engine parts and sudden failure in flight.
Analogy
It is like a dish that is heated and cooled many times until small cracks begin to form. The damage comes from the repeated temperature change, not from one single use.
Grounding Statement
Each heat cycle makes the part expand, and each cooling cycle makes it contract; repeated many times, that movement can start cracks.
Intuition Check
Thermal fatigue does not mean a pilot is tired from heat. Here, “fatigue” means a material is weakening because heat changes have stressed it again and again.
Example Sentence 1
The mechanic found cracks in the exhaust manifold caused by thermal fatigue after years of heating up in flight and cooling down on the ramp.
Example Sentence 2
Frequent short flights can increase thermal fatigue in engine components because of rapid heat cycles.