Molecular insights into fatigue damage characteristics of monocrystalline and polycrystalline salt rock under cyclic compression

This study presents a comparative investigation of the mechanical behavior of monocrystalline and polycrystalline salt rock under both constant-rate and variable-rate cyclic compression, revealing their fatigue damage characteristics at the micro-scale and elucidating the underlying dislocation motion mechanisms and fatigue damage evolution. Results demonstrate that under constant-rate cyclic loading, increasing the loading–unloading rate significantly reduces the hysteretic energy dissipation, softening rate, dislocation density in monocrystalline models, and atomic shear strain, thereby enhancing cyclic stress response and stabilizing the face-centered cubic lattice structure. Plastic yielding in monocrystalline salt rock can be delayed by either increasing the loading rate or employing variable-rate loading. In contrast, polycrystalline salt rock enters the plastic stage immediately upon the first cycle, with its lattice structure exhibiting recovery ordering after initial damage—a phenomenon most pronounced at lower loading rates. Regarding microscopic deformation mechanisms, dislocations and strain localization in monocrystalline salt rock are concentrated at the crystal top, forming continuous oblique patterns, while damage in polycrystalline salt rock is primarily confined to grain boundary regions. Under the same average loading rate, simulations reveal that the depressurization loading mode corresponding to the air extraction phase induces more severe structural softening, indicating that the accelerated loading path can more effectively inhibit the propagation of microcracks and the accumulation of intergranular damage. This research provides atomistic insight into cyclic deformation and defect evolution in salt rock, which may help inform future multiscale assessments of salt cavern stability.