Creep is time-dependent permanent deformation under sustained load. It becomes especially important when temperature is high enough that thermally activated deformation mechanisms operate appreciably on the service time scale.

A constant-load creep test commonly shows three regimes:

1. Primary creep: the creep rate decreases as deformation proceeds, often because strain hardening initially outpaces recovery.
2. Secondary creep: the rate becomes approximately constant. This minimum or steady creep rate is widely used for material comparison and life models.
3. Tertiary creep: the rate accelerates because damage, necking, cavities or microstructural degradation reduce the remaining load-bearing capacity.

A common empirical form for secondary creep combines stress sensitivity with Arrhenius temperature dependence:

$$\dot\varepsilon_s=A\sigma^n\exp!\left(-\frac{Q_c}{RT}\right),$$

where $A$ and $n$ depend on material and mechanism and $Q_c$ is an apparent creep activation energy.

The exponential temperature factor makes creep extraordinarily temperature sensitive. A modest increase in absolute temperature can change deformation rate and service life by orders of magnitude.

Creep is therefore not characterized by one ordinary yield stress. Engineering at elevated temperature requires specifying stress, temperature and time together, and often choosing materials whose microstructure remains stable over the intended lifetime.