Characterization of the negative Poisson's ratio effect of the thermal-damaged crystalline rock by the grain-based model

A natural rock often has a positive Poisson's ratio during compression. However, the thermal-damaged rock can abnormally present a negative Poisson's ratio. Thus far, the related mechanism has not been fully clarified from a micromechanical viewpoint. Here, we established a grain-based model to characterize the negative Poisson's ratio effect of the thermal-damaged crystalline rock using the universal distinct element method. Thermal-induced deterioration of heterogeneous rock microstructures was mainly treated as the loosening of grain contacts and weakening of their mechanical properties, which was depicted by the compression-hardening contact model. As the original rock successively suffered from moderately-to highly-thermal damage, there existed a transition of the Poisson's ratio from positive to negative and a transition of the pre-peak stress-strain relation from approximate linearity to nonlinearity. These macro mechanical behaviours could be well characterized by modulating grain-scale properties of the numerical model. We shed light on the negative Poisson's ratio mechanism of the thermal-damaged rock, which resulted from a more significant reduction of compressive stiffness than the shear stiffness at grain contacts. That is, the ratio of the shear stiffness to the compressive stiffness (λ) is larger than 1.0. With increasing the compressive stress, the compressive stress increased, and λ gradually decreased to less than 1.0. As a result, the negative Poisson's ratio (lateral contraction) was converted to the positive Poisson's ratio (lateral extension). The negative Poisson's ratio effect prominently influenced the mechanical response of the rock under compression, such as the stress distribution and progressive failure characteristics.