Creep-fatigue interaction of rock salt using discrete element simulation

Abstract

The rock surrounding underground salt caverns hosting compressed air energy storage (CAES) systems is subjected to simultaneous creep and fatigue loads. A proper design considering the mechanical response and damage evolution of rock salt under such loading conditions is crucial for ensuring long-term stability. In addition to the conventional laboratory creep-fatigue tests, numerical simulation can serve as a complementary and validating method for unveiling the underlying mechanisms of interaction occurrences. The integration of these methods offers significant value in advancing our understanding of the meso-mechanical properties of rock salt. This study delves deep into the creep-fatigue interactions of rock salt using discrete element method (DEM) simulations. By establishing a DEM model of rock salt, that employs a hybrid contact model of Burger's and linear parallel bond (LPB) to represent creep and damage behavior, respectively, we unveil the S-shaped evolution of the strain–time curve, encompassing initial, steady, stable damage, and acceleration deformation stages. Our findings highlight that stress intervals significantly reduce fatigue and creep life compared to pure creep and fatigue conditions. Furthermore, we demonstrate the effectiveness of a bilinear cumulative damage rule in describing the creep-fatigue life of rock salt under uniaxial conditions, with longer stress intervals leading to increased cumulative crack numbers and earlier crack initiation times. Ultimately, our numerical salt model exhibits an inclined shear crack and numerous micro-cracks surrounding the macro-shear crack, aligning with fracture modes observed in laboratory tests and advancing our understanding of rock salt behavior in CAES system.