The time dependence of plastic strain is described by plots of strain against time, also known as creep curves. A typical creep curve is shown in Fig. 3.1,3 and consists of three different regions: the primary, secondary and tertiary creep regions. Usually the primary creep region commences only after the material has experienced an instantaneous strain, e0 which is a result of sudden loading of the material and corresponds to period ‘a’ observed by Andrade.1 The instantaneous strain is composed of elastic (recoverable on release of load), anelastic (recovers with time) and plastic (non-recover­able) components. Though the applied stresses for creep are smaller than

the yield strength of the material, the instantaneous strain is composed of a plastic strain component.

The primary creep (also known as transient creep) region, as the name suggests, describes the first or initial stage of creep deformation and corre­sponds to period ‘b’ in Andrade’s work.1 Such a region is characterized by a strain rate decreasing with time. The decrease in strain rate continues until the secondary stage (also known as steady-state creep) is attained. In the sec­ondary creep region (period ‘c’ in Andrade’s study) the strain rate of defor­mation remains constant. This is evident in Fig. 3.1 with the secondary creep region described by a straight line indicating a constant slope. The secondary stage strain rate is the minimum strain rate of the creep curve. The useful creep life of most engineering materials is generally estimated from second­ary stage creep strain rate values. However such a methodology might not be applicable for materials which have a large primary creep region or where the tertiary creep region completely dominates the primary and secondary creep stages. The tertiary creep region is the last stage of creep deformation and concludes with the failure of the material. In the tertiary creep regime (as identified by Hanson and Wheeler2) the material undergoes deformation at very high strain rates. The tertiary stage of the creep curve usually occurs over significantly smaller time periods in comparison to the primary and the secondary stage, and is often regarded as ‘fracture’ mode.