Creep-fatigue characteristics of rock salt under different loading paths

Abstract

The salt behavior subjected to creep-fatigue load is meaningful to ensure the stability of the salt cavities used for underground energy storage. The salt specimens are tested under three paths of creep-fatigue load, including creep followed by fatigue (C + F), fatigue followed by creep (F + C) and creep-fatigue (CF). The stress-strain curves, deformation, loading modulus and acoustic emission (AE) count were systematically analyzed. The axial, lateral and volumetric deformations present a four-phase and two-phase change trend at C + F (F + C) and CF loading, respectively. The deformation curve for an individual creep or fatigue stage at C + F and F + C loading resembles that in pure creep or fatigue tests. The prior creep or fatigue load decreases the strain rate in the subsequent fatigue or creep phase. The prior creep load decreases the loading modulus of rock salt in the subsequent fatigue stage, and the loading modulus presents a downward trend with increasing the cycle numbers for the salt samples at C + F loading. The hold time during the loading-unloading stage at CF loading exerts little effect on the variation trend of loading modulus. The hold time increases the irreversible deformation for each cycle and induces the salt samples to be prone to dilate at CF loading. Continuous AE activities were monitored in the creep stage at C + F loading, while minor AE responses develop in the creep phase at F + C loading and the hold time stage at CF loading. These results suggest that the combined creep and fatigue load in sequence and simultaneously result in a negative and positive interaction between them, respectively. The deformation behavior is explained from the viewpoint of micro-mechanism.