Creep tests

The deformation of a material during constant temperature and stress is called creep. This deformation is viscoplastic and time-dependent.

Although creep is possible at all temperatures above absolute zero, this deformation is thermally activated, i.e. small changes in temperature lead to significant variations in strain rate. In the case of metal alloys, creep is only significant at relatively high temperatures. Thus, creep is negligible for alloys used in structures such as bridges or ships. This is not the case for nuclear power plant vessels, which have to operate for many years at temperatures in excess of 400°C. Creep can also be significant in the hot parts of gas turbines and aircraft turbo-machines, where the temperature of some parts is currently as high as 1,100°C. The creep behaviour of type 316 L steels and 800 alloys, for nuclear power plant vessels, or of various nickel-based superalloys, for aircraft turbomachinery, is beginning to be well understood.

Creep must be taken into consideration whenever the working temperature is close to or above 0.2 T _f (where T _f is the melting temperature of the alloy in question) and/or if there are significant stresses. In the oil industry, for example, creep may not be negligible in cracking units, where very high temperatures and stresses may be encountered.