Microstructural evolution and mechanical behaviors of rock salt in energy storage: A molecular dynamics approach

Abstract

The microstructure of rock salt significantly influences its macroscopic mechanical behaviors and deformation phenomena. Understanding the deformation and failure characteristics of rock salt at multiple scales is crucial for the secure and efficient functioning of energy storage in salt caverns. Although the macroscopic behaviors of rock salt are well understood, the microstructural changes occurring under uniaxial compression have not been thoroughly investigated. This study aims to investigate the evolving laws of rock salt microstructure and mechanical properties during the damage process, thereby providing a fundamental understanding of the underlying mechanism. To achieve this, a series of characterization tests on rock salt were conducted to study its microstructure and defects. Building on this, molecular dynamics simulations were utilized in rock mechanics to elucidate the mechanical behaviors, structural evolutions, and dislocation motions of rock salt during the damage process. The rock salt obtained from Qianjiang, China, is a polycrystalline material with an average subgrain size of 46 nm and an average dislocation density of 4.76 × 10⁻⁶ 1/Ų. Uniaxial compression of single crystal NaCl exhibits three stages: elastic compression, rapid dislocation multiplication, and forest hardening. Scattered dislocations and isolated dislocation tangles trigger further plastic slippage, leading to decreased strength. Later, the formation of a dislocation cell network hinders further slip and reinforces the strength of rock salt. Plastic deformation is found to be a dominant factor in grain refinement and the formation of polycrystalline structures. Intergranular cracking results from cross-patterned dislocation lines on grain boundaries, which become vulnerable areas of the structure. This study describes the evolutionary process of rock salt microstructures and mechanical properties, identifies the micro-destruction mechanisms during the damage process at the ionic level, and provides insights into the micro-mechanical behavior of rock salt and for the design and stability assessment of underground energy storage facilities.