A creep constitutive model of salt rock considering hardening and damage effects

Salt rock is an ideal medium for storing fossil energy and highly radioactive nuclear waste, and the study of creep mechanical properties of salt rock holds great significance for the safe operation of underground storage in salt caverns. In this study, a creep constitutive model for salt rock considering damage and hardening effects was developed, based on the component combination model and fractional calculus theory, so as to reasonably reflect the two mechanisms of damage and hardening existing in the creep process of salt rocks. The elastic element considering time-dependent damage was used to describe the damage deformation of salt rock in the initial loading stage in this model, and the fractional Murayama body was used to describe the viscoelastic plasticity creep mechanical behavior of salt rocks in transient creep stage. Meanwhile, a hardening function, which describes the strengthening characteristics with time of the yield strength of salt rocks, was introduced to reflect the hardening mechanism of salt rocks, and a transient plastic element was available to depict unrecoverable transient deformation. A nonlinear dashpot element with strain-triggering was constructed based on Kachanov creep damage law and Lemaitre strain equivalence principle. The dashpot better characterized the nonlinear deformation of salt rock in the accelerating creep stage. One-dimensional and three-dimensional creep equations for salt rock considering hardening and damage effects were derived based on the combination model theory. With the characteristics analysis of the isochronous stress-strain curves of the existing uniaxial and triaxial salt rock creep tests, the start-up stress threshold of Murayama body was determined. The parameters in the model were identified by combining isochronous stress-strain curves and creep tests data. The results show that the established creep constitutive model can describe the creep mechanical properties of salt rock in different stress states using only one set of parameters, which can provide a certain theoretical basis for predicting the creep deformation characteristics of salt rock.