Damage Evolution and Deformation of Rock Salt Under Creep-Fatigue Loading

During the operation of a compressed air energy storage (CAES) salt cavern, the surrounding rock experiences creep damage during the stages of constant internal pressure and undergoes fatigue damage due to the periodical injection-production. To describe the damage evolution of salt rock under creep-fatigue loading, a novel damage accumulation model based on the ductility exhaustion concept is proposed by applying a nonlinear summation method to represent the synergistic effect of creep and fatigue damage. Low-cycle fatigue (LCF) and creep-fatigue tests of rock salt were conducted under stress-control mode for various cycle stress amplitudes and hold times. Results show that the deformation of rock salt under creep-fatigue loading consists of initial, steady and accelerated phases. The proposed model matches well with the test data and can accurately describe the damage evolution as the applied stress amplitudes and dwell times change. The introduction of the hold times at the upper limit stress causes a strain increment and life reduction, which become more evident as the duration periods prolong and can be understood by the dislocation theory of crystals.