Underground cavities in salt rock Ibrinations used for Compressed Air Energy Storage (CAES) undergo cyclic loads and are subject to a fatigue phenomenon that induces a decrease of rock's strength and stiffness. A micromechanical analysis of this phenomenon is necessary to understand its mechanisms and elaborate relevant constitutive models. 1'he polycrystallinc nature of rock salt has a crucial ef1ct on crack propagation and rock damage and, hence, on fatigue behavior. This behavior was investigated herein on the basis of self consistent upscaling approaches thr viscous heterogeneous materials. Iiw internal stresses in the polycrystal were modeled based on experimental data available for halite single crystals, and a monotonic compression test was simulated, which allowed tracking the triggering of fatigue damage. Results show (lint tensile stresses are developed in the polyciystal under global compressive load, the amplitude of which depends on the macroscopic load rate or frequency. These tensile stresses can exceed in some conditions the tensile strength of grains or of grains interfaces and cause cracking and damage in the polycrystal.