At temperatures above 0.4-0.5Tm (Tm is the melting point of the material), plastic deformation occurs as a function of time at constant load or stress. The phenome­non is known as creep “defined as time dependent plastic strain at constant temper­ature and stress.” Note that lead (Pb) may creep at room temperature, but iron (Fe) does not because for lead room temperature represents a higher homologous tem­perature (i. e., T/Tm = ~0.5) than that of iron (~0.16). Because creep occurs as a function of temperature, it is a thermally activated process. Creep must be taken into account when a load-bearing structure is exposed to elevated temperatures for a long duration of time.

A creep test is generally done under uniaxial tensile stress. There are different variants of creep tests, such as compressive creep, double shear creep, impression creep, and indentation creep. However, here we will deal with tensile creep only. A creep curve is basically plotted as a function of creep strain as a function of time at a constant load (or constant stress depending on the availability of such instrument set up to keep stress constant during creep deformation) and temperature. Figure 5.26a shows a creep equipment with a furnace and strain measuring device, a linear variable differential transformer (LVDT). A typical creep curve as shown in Figure 5.26b has three stages: (i) primary stage (transient creep) — work hardening during plastic deformation is more than recovery (softening) exhibiting decreasing strain rate with time; (ii) secondary or steady-state stage (minimum creep rate) — the rate of work hardening and softening balance each other, and (iii) tertiary stage — characterized by an accelerating creep rate where softening mechanisms predomi­nate. The third stage of tertiary creep is often considered as fracture stage rather

(b) (c)

than deformation. A real example of creep rate versus time plot for a Grade 91 steel is shown in Figure 5.26c where the steady-state creep rate occurs as a minimum rate that is typical for creep under constant load. Increase in temperature and/or stress tends to enhance creep strains and rates, as illustrated in Figure 5.27 where we note that increased creep rates are accompanied by decreased time to rupture (tr).