ABSTRACT
Creep of materials is classically associated with time-dependent plasticity under a ‰xed stress at an elevated temperature, often greater than roughly 0.5TM, where TM is the absolute melting temperature. The plasticity under these conditions is described in Figure 9.1 for constant stress (1) and constant strain rate (2) conditions. As the characteristic aspect of the curve, three regions are delineated; Stage I, or primary creep, denotes that portion where the creep rate (plastic strain rate), ε ε( / )=d dt is changing with increasing plastic strain or time. In Figure 9.1a 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 Figure 9.1b, under constant strain-rate conditions, the metal hardens, resulting in increasing §ow 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 §ow stress. This regime is termed Stage III, or tertiary creep, and leads to fracture. Sometimes, Stage I leads directly to Stage III and an “in§ection” is observed. Thus, care must sometimes be exercised in concluding a mechanical steady state (ss).
