Theoretical study of damage accommodation in salt subject to viscous fatigue

Underground salt cavities used for compressed air energy storage undergo cyclic loads and are subject to a fatigue phenomenon that reduces rock strength and stiffness. Understanding such behaviors and developing relevant constitutive models require a micro-mechanical analysis. This study investigates damage and fatigue in salt rock, the extent of which is influenced by its polycrystalline nature, on the basis of self-consistent upscaling approaches for viscous heterogeneous materials. We develop a model that treats monocrystals as spherical inclusions embedded in an infinite homogeneous matrix with purely elastic inclusion-matrix interactions. To predict grain breakage and its subsequent impact, we also introduce a failure criterion. The model provides micro-mechanical interpretations of the common viscoplastic and fatigue behavior of salt such as damage and accelerated creep from grain breakage and the shakedown effect observed in elastoplastic media. Finite Element (FE) simulations confirmed the macrostrain and microstress predictions obtained by homogenization. The FE program will be used in future studies to simulate inter-granular fracture propagation. This study provides new perspectives on research pertaining to the microscopic origin of fatigue in viscous polycrystalline materials.