Dynamic mechanical response and failure characteristics of sandstone under dynamic load cycle

For the poor self-stability of rock mass and frequent dynamic disasters in the process of deep mine mining, the uniaxial dynamic load disturbance test of sandstone with different amplitudes and frequencies were carried out using the self-developed ZST−1500 microcomputer controlled electro-hydraulic servo coal and rock dynamic and static combination adaptive coupling test system, and the dynamic mechanical response law and failure characteristics of sandstone under dynamic load were analyzed. The results show that with the increase of dynamic load amplitude, the hysteresis loop of sandstone changes from "sparse-dense" to "sparse-dense-sparse", and the dynamic load cycle is divided into logarithmic rising stage I, uniform damage stage II and exponential acceleration stage III according to the plastic deformation of the sample during the dynamic load cycle. There is a threshold for the weakening effect of dynamic load on the strength of the specimen when the dynamic load level is 70%\sigma _\mathrmc. The amplitude threshold of failure is 17.5%~22.5%\sigma _\mathrmc, and the dynamic load below the threshold value will not cause sandstone instability. The sandstone strength and the dynamic load duration before failure are negatively correlated with the dynamic load amplitude and frequency beyond the threshold value. The effect of dynamic load amplitude and frequency on the weakening mechanism of sandstone is different. Both the dynamic load amplitude and frequency are positively correlated with the average plastic deformation \Delta \bar\varepsilon _(\texti) in every single cycle of dynamic load, but \Delta \bar\varepsilon _(\texti) is more sensitive to the dynamic load amplitude and increases exponentially with the increase of the amplitude in the stages of I and II. The dynamic load amplitude weakens the sandstone mainly by increasing the plastic deformation, while the increase of frequency reduces the cyclic interval of dynamic load, and forms a small amount of continuous damage in sandstone in short time, thus weakening the sandstone. The cumulative energy and ringing count rate of sandstone AE signals increase exponentially with the increase of dynamic load amplitude, and also increases synchronously with the increase of dynamic load frequency. With the increase of dynamic load amplitude, the initiation and propagation degree of sandstone micro-cracks increase, and the proportion of shear cracks increases. Also, the proportion of sandstone tensile cracks increases with the increase of dynamic load frequency.