关键词: 砂岩, 峰前峰后, 循环加卸载, 表观弹性模量, 下限应力, 动力特性

Abstract: To comprehensively examine the dynamic characteristics of sandstone at different stages before and after the peak, we conducted uniaxial graded cyclic loading and unloading tests. These tests aimed to analyze the impact of the lower limit stress on various parameters, including the apparent elastic modulus, hysteretic loop area, damping ratio, damping coefficient, and dynamic elastic modulus of sandstone during the loading and unloading stages. The research findings indicate the following: 1) Due to the aggravation and accumulation of internal damage after peak, the hysteretic curve lies on the right side before the peak at the same lower limit stress. With an increase in the lower limit stress, the increments in strain and maximum strain gradually decrease. This suggests that despite limit strength has been reached after peak, cyclic loading and unloading under high-stress conditions can enhance the compactness of post-peak rock samples, as long as the rock remains intact and stable. 2) The changes in sandstone under cyclic loading and unloading before and after peak load can be observed through variations in the sloping (a) of apparent elastic modulus curve. At small lower limit stress, the coefficient of compactness enhancement (a₁) is greater than the coefficient of compactness degradation (a₂); additionally, the slope a is positive, indicating that the compressive effect of axial stress on rock mass is significantly stronger than the effect of deterioration damage. As the lower limit stress rises, the rate of decrease in a₂ is higher than that of a₁, suggesting that the compressive effect of axial stress gradually weakens compared to that of deterioration damage. 3) Under four different lower limit stress conditions, the hysteretic loop area, damping ratio, and damping coefficient are higher after the peak than before, while the dynamic elastic modulus after peak is lower than before. These findings suggest that the degree of internal deterioration damage in rock samples during cyclic loading and unloading after the peak is higher than that before the peak.

Key words: sandstone, before and after peak, cyclic loading and unloading, apparent elastic modulus, minimum stress limit, dynamic characteristics