Creep tests
Typically, a creep test consists of subjecting the material to constant stress at high temperatures (\(T > T_F/2\)) (see following figure). An elastic deformation consecutive to application of the load occurs in the material almost instantly, and this deformation then increases under constant stress in a progressive and non-linear manner during the primary regime. The strain rate stabilises very quickly, and the material now enters the secondary (or stationary) creep regime that corresponds to the longest stage of the deformation. Finally, once the deformation has become too extensive, a third stage appears, which is usually very brief. This is the tertiary regime that leads to fracture.
Depending on the application and the characteristics of the material to examine, different parameters can be measured, for example:
The time t corresponding to a given strain \(\varepsilon\) (\({0,1}{\%}\), \({1}{\%}\), \({10}{\%}\), etc)
The strain \(\varepsilon\) corresponding to a given time t de sollicitation (100, 1000, 10,000 hours, etc)
The time to fracture \(t_R\)
The strain rate in the stationary regime, which is expressed by
\(\dot{\varepsilon}_{st} = A\cdot \sigma^n \cdot \exp \left( \frac{-Q}{RT} \right)\)
with \(Q\) the activation energy for microstructural mechanisms in play (\({ \rm J.mol^{-1}}\)), \(R\) the universal ideal gas constant \((R = {8.32}{ \rm \, J.mol^{-1}.K^{-1}}),\) \(A\) and \(n\) experimentally determined constants, \(\sigma\) the stress applied, T the temperature. Note that the value of \(n\) varies from \(n = 1\) for low stresses applied \(\sigma\) (diffusion-creep) to \(n = 3\) to 8 for higher stresses \(\sigma\) (dislocation-creep). The stationary creep rate is all the more important when temperature and/or stress is high.