The steady-state strain rate of creep deformation, at a given temperature, has been found to be directly dependent on the applied stress. The functional dependence of strain rate on stress can be expressed by Norton’s law13

[3.13 ]

where K isa constant and n is the stress exponent. Similarly, for a constant applied stress, the rate of creep deformation increases with increasing tem­perature. The effect of temperature can be understood by including an extra term indicated in the following equation

[3.14]

where Kj is another constant and Qc is the activation energy of creep defor­mation. The activation energy term is included due to the fact that the creep deformation is considered to be a first order reaction rate process. The mag­nitude of the activation energy is dependent upon the physical mechanism governing the deformation process.

The effect of stress and temperature is clearly illustrated by Fig. 3.3. With increasing stress and temperature, the instantaneous strain at the time of stress application increases, the steady-state creep rate is increased and the rupture lifetime is diminished.