AN INTERNAL VARIABLE MODEL FOR THE CREEP OF ROCK-SALT

The creep strain rate epsilon of rocksalt, like that of other ductile crystalline materials, can be described by a power law equation of the type epsilon-alpha(sigma(a))n, where the active stress sigma(a) is the difference between the total deviatoric applied stress sigma and an internal stress sigma(i). In this paper, the origin and the nature of this internal stress, which develops during inelastic deformation of the material, are discussed. It is shown that this internal stress can serve as an internal (or state) variable in the constitutive model of rocksalt, which reflects the microstructure evolution of the material under the competitive action of hardening and recovery mechanisms. An analysis of experimental data, both our own and those taken from the literature, demonstrates that such a law is able to correctly reproduce rocksalt creep test results in the steady-state domain. The proposed model is in accordance with the macroscopic and microscopic behaviour of salt, and with direct measurements of the internal stresses made by others on this material.