The Role of Long-Range Internal Back Stresses in Creep of Metals

The use of back stress terms in constitutive equations implies the existence of inhomogeneous stresses within the solid. We suggest that such inhomogeneities arise naturally from dislocation processes and are associated with the development of hard and soft regions within the substructure. We point out that back stresses act in the soft parts of the substructure and that constitutive equations containing back stresses should be based on flow mechanisms in these regions. Constitutive equations based on flow in the hard parts of the substructure should contain forward internal stresses. We envision flow in the hard regions to be controlled by recovery and to dominate at high temperatures, while flow in the soft regions involves other kinds of thermally activated dislocation motion and is most important at low temperatures. Power law breakdown is explained with a composite model as a natural consequence of the transition from high temperature to low temperature deformation.