difficulties in modeling elastoplastic damage behavior of rock salt

Dear all,

I want to construct a constitutive relation to model the elastoplastic damage behavior of rock salt which is considered as an excellent host medium for gas storage facilities. Because rock salt is a kind of typical geomaterials, confinement sensitive and inclusion effect should be considered in the constitutive model. However, I have encountered many difficulties in the course of the study. There are some issues I am a little confused.

In most relative literatures, deformation is decomposited into elastic and plastic parts. However, when we discuss the material deformation mechanisms, it will be found that a considerable damage deformation induced by the microvoids and microcracks cannot be ignored, especially in geomaterials. Indeed, some authors think the effect of damage on the plastic deformation can be considered through the strain equivalence theory. However, they only admit that plastic flow will occur in the undamaged material area while disregarding the valuable damage strain.

Fortunately, I find that the strain rate is decomposited into elastic, plastic and damage parts in some papers (eg. Michael Brunig 2003). To determine the onset and the continuation of damage, the damage surface and the damage potential function are employed in analogue to yield surface and plastic potential function concepts of the plasticity theory. I am very appreciated with this approach. However, comparing to traditional methods where the damage evolution equation is directly derived by the damage dissipation potential, there is not a definitive method to determine the damage evolution model in the new proposed approach. Could you give me some advice for constructing damage evolution model for rock salt characteristed by dilatancy?

In additional, I do not know how to consider the damage effect on plastic flow. Do you think there is coupling between plastic and damage? As far as I know, plastic is induced by dislocation and sliding, and only will occur in the undamaged material area. At one hand, fiction sliding will cause the growing of microcracks and induce damage evolution. At the other hand, the damage evolution will disturb local stress state and effect fiction sliding, finally induce plastic strain. So, damage and plastic deformation occur at the same time, their evolution law should be coupled. Is it necessary to consider the damage effect on plastic flow when damage strain induced by microcracks has been assumed?

I would be really glad if someboby could help me to solve them out and give me some advice. Thank you.