Analysis of Microscopic Pore Characteristics and Macroscopic Energy Evolution of Rock Materials under Freeze-Thaw Cycle Conditions

The repeated cyclic freeze-thaw effect in low-temperature environments causes irreversible damage and deterioration to the microscopic pore structure and macroscopic mechanical properties of a rock. To study the effects of the freeze-thaw cycle on the porosity and mechanical properties, the indoor freeze-thaw cycle test and mechanical tests of sandstone-like materials were conducted. Based on nuclear magnetic resonance, the influence of the freeze-thaw cycle on microscopic pores was analyzed, and the intrinsic relationship between porosity and mechanical strength was discussed. Meanwhile, the energy change in the uniaxial compression test was recorded using the discrete element software (PFC2D). The influence of freeze-thaw cycles on different types of energy was analyzed, and the internal relationship between different energies and freeze-thaw cycles was discussed. The results showed that the microscopic pore structure is dominated by micropores, followed by mesopores and the smallest macropores. With an increase in the freeze-thaw cycle, both micropores and mesopores showed an increasing trend. The porosity showed an exponentially increasing trend with the increase in freeze-thaw cycles. The peak strength and elastic modulus decreased exponentially with the increase in freeze-thaw times, while the peak strain showed an exponentially increasing trend. The strain energy and bond strain energy showed a trend of increasing and decreasing in the front and back stages of the peak strength, respectively. However, the frictional energy always showed an increasing trend. The total energy, strain energy, bond strain energy, and friction energy all showed exponential increases with the increase in the number of freeze-thaw cycles.