The flow, or plastic deformation, of metals when held for long periods of time at stresses below their normal yield strength. The effect is particularly noticeable when the temperature during stressing approaches the recrystallization temperature of the metal.
Creep may produce effects of consequential magnitude at normal temperatures as well as at elevated temperatures, depending on the material and the degree to which freedom from continuing deformation can be tolerated.
In addition to determination of creep rate at elevated temperatures, timed testing includes measurement of time for fracture, when sufficient load is applied, and measurement of stress relaxation by creep. These additional tests are called creep rupture, stress rupture, notched-bar rupture and relaxation tests. Standard practices for most of these long time tests can be found in ASTM E139, Conducting Creep, Creep Rupture and Stress Rupture Tests of Metallic Materials. These tests are conducted with a relatively slow heating rate.