Nonlinear mechanical dynamic response characteristics of high stress sandstone subject to low frequency disturbance

The mechanical response characteristics of rock under low-frequency perturbation loads and high-stress conditions are important factors affecting the stability of interlayer rock in the goaf during upward mining. Taking the coal-bearing sedimentary rock sandstone as the research object, the dynamic uniaxial compression tests were conducted on high-stress sandstone (80%UCS) under low-frequency perturbation loads (5 Hz) using a self-developed dynamic-static coupled electro-hydraulic servo testing machine. The progressive damage behavior and nonlinear mechanical dynamic response characteristics of high-stress rock under low-frequency perturbations were revealed. The results are as follows: ① During the process of small amplitude low-frequency disturbance load, high-stress sandstone does not experience dynamic failure, the threshold value of low-frequency disturbance load amplitude to induce sandstone damage is 15%UCS. Under the action of low-frequency disturbance load above the threshold value, the number of disturbance load cycles before the failure of high-stress sandstone decreases exponentially with the increase of amplitude. ② The progressive initiation and accumulation of randomly distributed microcracks within the sandstone, as well as their gradual extension, significantly affect the nonlinear evolution characteristics of dynamic secant modulus, strain peak, plastic deformation, dissipated energy density, and b-value. ③ During the low-frequency perturbation process, the b-value and dissipated energy density exhibit a three-stage evolution characteristic consistent with the strain. The exponential strain evolution curve has a distinct inflection point before the failure of the sandstone, which can serve as a precursor information of dynamic failure under low-frequency perturbations and be used for corresponding dynamic disaster warnings. ④ During the low-frequency perturbation load process beyond the threshold, the dynamic secant modulus evolution curve approximates an exponential decay. The parameters in the nonlinear fitting relationship can be used to quantify the degradation rate of the elastic modulus. And the degradation rate of secant modulus of sandstone changes nonlinearly with the increase of amplitude. ⑤ The magnitude of the RA value in the initial stage of disturbance load can be used to predict the dynamic failure of sandstone in advance. Correspondingly, the sudden increase of RA during the low-frequency disturbance load process can serve as a precursor warning of dynamic instability of high stress sandstone. During the process of small-amplitude disturbance load below the threshold value, the microcracks mainly undergo tensile or mixed fracture, with shear fracture accounting for only 12.15%. During the low-frequency disturbance load processes with amplitudes of 15%UCS, 20%UCS, and 25%UCS, the shear fracture accounts for 37.17%, 52.75%, and 53.62% respectively. The proportion of shear fractures exhibits a logarithmic increase with the increase of amplitude.