Dissemination of IT for the Promotion of Materials Science (DoITPoMS)



When a material is subjected to a stress that reaches the yield stress, it deforms plastically (permanently). Provided the stress is kept below this level, then in principle it should only deform elastically. However, if the homologous temperature is relatively high (above ~0.4) then in practice plastic deformation can occur, even if the applied stress is lower than the yield stress. This deformation is usually progressive with time and is commonly known as creep.  During loading under a constant stress, the strain tends to vary with time approximately as shown below, where the effect of changing the applied stress is also indicated. The graph below is a plot of creep strain.  The elastic strain has been omitted.  In practice, it is quite common to do this (for plasticity, as well as for creep).  It’s worth noting that elastic strains rarely exceed a small fraction of a %, at least for metals, whereas both plastic strains and creep strains commonly reach the range of several tens of %.

Schematic creep curves for 3 different levels of applied stress

 Schematic creep curves, for 3 different levels of applied stress (σ1 > σ2 > σ3)

The terms “Primary”, “Secondary” and “Tertiary” creep are widely used.  At a simple level, they are often associated respectively with the concepts of: (i) setting up some kind of mechanistic balance, (ii) steady state (constant strain rate) deformation occurring once this balance has been set up and (iii) the breakdown of this balance, often with defects starting to appear and failure rapidly following.

In reality, even without going into details of the mechanisms involved, this picture is simplistic and potentially misleading.  For example, the transition between primary and secondary regimes is often poorly-defined and indeed a true steady state may never be set up.  Furthermore, creep tests are commonly carried out with a constant applied load (rather than a constant true stress). Thus, for tensile tests, the tertiary regime may actually be a consequence of the fact that the true stress is rising throughout the test, with the rate of increase becoming greater towards the end of the test.  At least in some cases, the tertiary regime may be associated with the true stress starting to approach or exceed the yield stress of the material.  The situation can be further complicated by the possibility that significant microstructural changes (such as recrystallization), which could strongly affect the mechanical response, may occur during the test.

These issues are examined in this TLP, together with some details concerning creep mechanisms and the ways in which creep testing can be carried out, and the resultant experimental data interpreted.