Mechanical and Acoustic Properties of Sandstone under Freeze-Thaw and Cyclic Loading Alternations

Abstract

Abstract This research investigates the mechanical and acoustic properties of sandstone under alternating freeze-thaw and cyclic loading (FTF) using acoustic emission (AE) and ultrasonic testing techniques. The sandstone specimens are subjected to five cycles of freeze-thaw followed by ten cycles of low stress loading-unloading (stress lower than 40% of the rock's ultimate strength), referred to as the first stage of alternating action (Stage 1). This process is repeated several times until the failure of the rock sample, simulating the alternating effects of multiple freeze-thaw cycles and cyclic loading that may occur in cold region engineering. The research findings are as follows: With the progress of freeze-thaw cycles and alternating action of cyclic loading, the secant modulus and wave velocity of the rock generally show a decreasing trend. The first stress loading-unloading in each stage significantly enhances the secant modulus and longitudinal wave velocity of the freeze-thawed rock. The residual strain generated by the first loading-unloading is significantly higher than that of subsequent loading-unloading. Prior to rock failure, both the secant modulus and wave velocity experience significant decreases. These two phenomena serve as indicators for determining the critical failure of the rock, allowing the classification of rock states into safe, critical, and failure states. Freeze-thawed sandstone exhibits two significant acoustic emission characteristics during the cyclic loading phase: an initial loading phase is accompanied by a surge in the number of acoustic emission events, while after the first loading-unloading, the number of acoustic emission events stabilizes, forming a wave-like pattern of change. Additionally, an AE index is proposed to evaluate the degree of damage to the rock after each freeze-thaw stage, and its effectiveness is verified. The AE index reflects a gradual increase in freeze-thaw damage, followed by a sudden intensification, leading to failure and exhibiting three stages of evolution. An AE index of 2.2 is defined as an early warning indicator, while an AE index of 2.95 is defined as the failure indicator for the rock. Finally, based on experimental data, a damage evolution model is proposed, which describes the entire process of sandstone damage and effectively reflects the three-stage characteristics of rock freeze-thaw damage evolution.