Mechanical Characteristics of Red Sandstone Subjected to Freeze-Thaw Cycles and Increasing Amplitude Cyclic Load

It is of great significance to study the influence of freeze-thaw cycles on rock for the prevention and treatment of tunnel diseases in cold regions. Up to 300 freeze-thaw cycles were carried out on red sandstone, and the uniaxial compression and increasing amplitude cyclic loading tests were conducted to investigate its physical and mechanical responses. A damage calculating method is established, and a conceptual model is proposed to illustrate the behaviors of rock subjected to freeze-thaw cycles and increasing amplitude cyclic load. Research results indicate that the porosity of red sandstone increases in a negative exponential manner with the increase of freeze-thaw cycles, while the strength, residual strength, elastic modulus and Poisson's ratio decrease linearly. The strength of red sandstone can reduce by 21.27% when subjected to 300 freeze-thaw cycles. There are three kinds of acoustic emission (AE) patterns before failure, which are early active, medium term active and inactive. The damage considering the coupled effect of freeze-thaw cycles and increasing amplitude cyclic load can be calculated based on the elastic modulus degradation and damage release energy. By using the proposed method, a comparison of damage evolution between uniaxial compression and cyclic loading can be conducted, and the mechanical properties and damage propagation characteristics of rock can be predicted. The more freeze-thaw cycles the rock undergoes, the more microcracks inside it, and the more broken it becomes after failure. As the microcracks inside rock increase gradually, the internal structure of rock progressively becomes loose and brittle, resulting in a decrease in macroscopic mechanical characteristics such as strength and elastic modulus. Rock can be deemed as a system composed of a series of springs with different stiffnesses and lengths. The evolution of strength, elastic modulus and acoustic emission can be expressed by the failure and the uneven deformation between adjacent springs. The effect of freeze-thaw cycles on physical and mechanical parameters are revealed.Rock damage and its growth rate increase as the freeze-thaw cycle increases.Damage is analyzed based on elastic modulus degradation and damage release energy.A spring conceptual model is proposed to illustrate the rock damage mechanism.