Creep of materials is classically associated with time-dependent plasticity under a fixed stress at an elevated temperature, often greater than roughly 0.5 T{sub}m, where T{sub}m is the absolute melting temperature. The plasticity under these conditions is described in Fig. 1 for constant stress (a) and constant strain-rate (b) conditions. Several aspects of the curve in Fig. 1 require explanation. First, three regions are delineated: Stage I, or primary creep, which denotes that portion where in (a) the creep-rate (plastic strain-rate), ε{top}· = dε/dt is changing with increasing plastic strain or time. In Fig. 1(a) the primary-creep-rate decreases with increasing strain, but with some types of creep, such as solute drag with 'three-power creep', an 'inverted' primary occurs where the strain-rate increases with strain. Analogously, in Fig. 1(b), under constant strain-rate conditions, the metal hardens, resulting in increasing flow stresses. Often, in pure metals, the strain rate decreases or the stress increases to a value that is constant over a range of strain. The phenomenon is termed Stage II, secondary, or steady-state creep. Eventually, cavitation and/or cracking increase the apparent strain-rate or decrease the flow stress. This regime is termed Stage III, or tertiary creep, and leads to fracture. Sometimes, Stage I leads directly to Stage III and an 'inflection' is observed. Thus, care must be sometimes exercised in concluding a mechanical steady-state (ss).
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