Published online by Cambridge University Press: 05 June 2012
Creep is time-dependent plastic deformation that is usually significant only at high temperatures. Figure 8.1 illustrates typical creep behavior. As soon as the load is applied, there is an instantaneous elastic response, followed by period of transient creep (Stage I). Initially the rate is high, but it gradually decreases to a steady state (Stage II). Finally the strain rate may increase again (Stage III), accelerating until failure occurs.
Creep rates increase with higher stresses and temperatures. With lower stresses and temperatures, creep rates decrease but failure usually occurs at lower overall strains (Figure 8.2).
The acceleration of the creep rate in Stage III occurs because the true stress increases during the test. Most creep tests are conducted under constant load (constant engineering stress). As creep proceeds, the cross-sectional area decreases so the true stress increases. Porosity develops in the later stages of creep, further decreasing the load-bearing cross section.
Viscous flow: Several mechanisms may contribute to creep. These include viscous flow, diffusional flow, and dislocation movement. Viscous flow is the dominant mechanism in amorphous materials
In polycrystalline materials, grain-boundary sliding is viscous in nature. The sliding velocity on the boundary is proportional to the stress and inversely proportional to the viscosity, η. The rate of extension depends on the amount of grain boundary area per volume and is therefore inversely proportional to the grain size, d, so. Viscous flow is thermally activated, so η = ηo exp(QV/RT].