Definition of Creep
Plastic deformation is irreversible and it consists of time-dependent and time-independent components. In general, creep refers to the time-dependent component of plastic deformation. This means that creep is a slow and continuous plastic deformation of materials over extended periods under load. Although creep can take place at all temperatures above absolute zero Kelvin, traditionally creep has been associated with time-dependent plastic deformation at elevated temperatures, often higher than roughly 0.4Tm, where Tm is the absolute melting temperature, because diffusion can assist creep at elevated temperatures.
Creep tests can be conducted either at constant load or at constant stress. For experimental convenience, most frequently the creep tests of engineering steels are conducted at constant tensile load and at constant temperature. The test results can be plotted as creep curves, which represent graphically the time dependence of strain measured over a reference or gauge length.