Creep (deformation)

Creep is the time-dependent deformation of a material under sustained load. It can occur even when the applied stress is below the material’s short-term yield strength.

Primary creep: The initial stage of creep, in which the deformation rate generally decreases over time. Work hardening typically slows deformation during this early stage.
Stress (mechanics): A measure of internal force per unit area within a material. Creep laws relate deformation rate to the sustained stress.
Creep rupture: Failure after a material deforms under sustained stress over time, often at elevated temperature. Rupture tests estimate how long components can withstand specified conditions.
Fatigue (material): Progressive damage and failure caused by repeated or fluctuating loading. Fatigue is driven by load cycles, whereas creep develops under sustained loading.
Secondary creep: The stage of creep in which the deformation rate is approximately constant. This steady rate often dominates a component’s service life.
Strain: A measure of deformation expressed as change in length or shape relative to an original dimension. Creep is tracked as strain accumulated over time.
Creep-resistant alloy: An alloy designed to retain strength and resist time-dependent deformation at elevated temperatures. Turbine and boiler components rely on alloys engineered for long-term hot service.
Stress relaxation: The decline of stress over time when a material is held at fixed deformation. It is the fixed-strain counterpart to creep under fixed stress.
Tertiary creep: The final stage of creep, marked by accelerating deformation and damage before failure. Necking, cavities, or cracks can speed deformation toward rupture.
Yield strength: The stress at which a material begins to deform permanently under a short-term test. Creep can accumulate below this short-term threshold.

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