Fatigue characteristics and energy evolution of red sandstone under freeze-thaw cycling and staged cyclic loading-unloading

LIU Dangdang; XUE Junjun; XU Hongliang; CHEN Mou; SU Xugang; ZHANG Jun; YAN Gongbin; HE Xin

doi:10.11799/ce202602020

Abstract：Rock slopes in open-pit coal mines are subjected to the long-term synergistic effects of freeze-thaw cycles and cyclic loading-unloading.To further understand the impact of freeze-thaw weathering on sandstone damage from an energy analysis perspective,staged cyclic loading-unloading tests were conducted on red sandstone specimens subjected to 0,20,40,and 60 freeze-thaw cycles.The freeze-thaw damage to the specimens was quantified using the wave velocity reduction rate and the porosity growth rate.The energy density per cycle and acoustic emission energy during the cyclic loading-unloading tests were then calculated and analyzed.The results indicate that:(1)The damage to the red sandstone intensified with increasing freeze-thaw cycles.After 60 cycles,the wave velocity decreased by 50.83%and the porosity increased by 34.75%.(2)The fatigue stress-strain curves of the red sandstone specimens exhibited a"sparse-dense-sparse"trend.The fatigue strength decreased from 80 MPa for the unfrozen-thawed state to 40 MPa after 60 freeze-thaw cycles.(3)The total energy density and elastic energy density of the red sandstone specimens showed a"step-like"increase with rising cyclic loading-unloading levels.Prior to fatigue failure,the majority of the input energy was converted into elastic energy,while the dissipation energy fluctuated within a relatively small range.(4)The characteristics of the acoustic emission energy time series can effectively characterize the failure process of red sandstone under the synergistic action of freeze-thaw cycles and cyclic loading-unloading.The overall level of acoustic emission energy increased sharply before the instability and failure of the red sandstone occurred.(5)Pre-existing damage induced by freeze-thaw cycles exacerbated microcrack propagation in the red sandstone specimens during the initial stages of cyclic loading-unloading,making damage accumulation more prone to occur.These findings provide a theoretical foundation for assessing and preventing the long-term stability of red sandstone slopes in open-pit coal mines located in cold regions.