Fatigue

Fatigue is progressive material damage caused by repeated or fluctuating loads. It can initiate cracks and lead to sudden failure at stresses below a material’s static strength.

Stress cycle: A sequence of changing stress values that returns to a reference point, often described by its range and mean. Repeated stress cycles are the loading history that accumulates fatigue damage.
Cyclic loading: Loading that repeatedly varies in magnitude or direction over time. Fatigue damage arises from repeated or fluctuating loading rather than a single static load.
Fatigue testing: Experimental testing that subjects specimens or components to repeated loads to assess fatigue behavior. Tests establish fatigue data used to compare materials and estimate service life.
Static failure: Failure caused by a single load or sustained load exceeding a material’s strength. Fatigue can occur below the load that would cause static failure.
S–N curve: A graph relating cyclic stress amplitude to the number of cycles until failure. It summarizes how stress level affects fatigue life for a material and test condition.
Stress concentration: A local increase in stress caused by a geometric discontinuity or material flaw. Notches and defects raise local stresses where fatigue cracks often begin.
Miner's rule: A linear cumulative-damage rule that estimates failure under varying stress levels by summing cycle fractions. It offers a simple way to estimate damage from a variable loading history.
Creep: Time-dependent deformation under sustained stress, often accelerated at elevated temperature. Creep accumulates under sustained loading, unlike fatigue’s defining repeated-load damage.
Fatigue crack initiation: The formation of a crack at a material surface or internal defect under cyclic loading. This is the transition from localized cyclic deformation to a detectable fatigue crack.
Plastic deformation: Permanent change in a material’s shape after stress exceeds its elastic range. Localized cyclic plasticity can accumulate even when nominal stresses remain modest.

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