Tertiary Creep

Abstract

AbstractIn the tertiary stage, the creep rate is continously increasing eventually leading to rupture. Many mechanisms can contribute to the increasing creep rate such as particle coarsening, substructure coarsening, cavitation, changes in the dislocation density and necking. A large number of empirical models exist for the description of tertiary creep and the development of creep damage not least in the context of continuum damage mechanics (CDM). However, there are also basic models. An equation is presented that can describe the whole creep strain versus time curve. Only parameters that are already defined for secondary creep are needed. During the tertiary stage the true applied stress increases rapidly and faster than the counteracting dislocation strength, which is one main reason for the increase in the creep rate during the tertiary stage. Cavitation is of importance, but the cavitation is often local and therefore gives a modest contribution to the creep rate. According Hart’s criterion, necking starts right at the beginning of the tertiary stage. But the necking is not fully developed until close to rupture. This is demonstrated both by uniaxial and multiaxial models and it is also consistent with available experimental data.