Influence of loading and unloading effect on mechanical properties of impact rock under impact load

In order to study the effect of basic roof period pressure on the dynamic characteristics of impact pillar, one-dimensional dynamic and static combined loading SHPB experimental device is used to carry out dynamic impact experiment on the impact sandstone pre-treated by cyclic loading and unloading. In the experiment, three different axial cycle thresholds are pre-set: 6, 12, 18 MPa, and then carry out dynamic compression experiments under different impact pressure conditions. The dynamic mechanical response, energy evolution and fragmentation distribution characteristics of impact sandstone under different cycle thresholds and impact pressure are discussed. The results show that the dynamic stress-strain curve of rock samples can be divided into elastic stage, yield stage and failure stage. The initial stage of dynamic stress-strain curve is almost straight line, and there is no fracture compaction stage in static uniaxial compression. The average dynamic compressive strength, the average dynamic elastic modulus and the average dynamic deformation modulus first increase and then decrease with the increase of the cyclic threshold. The average dynamic peak strain decreases first and then increases with the increase of cycle threshold. The average dynamic compressive strength, average dynamic elastic modulus and average dynamic deformation modulus of rock samples at the critical threshold are the largest, and the average dynamic peak strain is the smallest. The density of reflected energy and dissipated energy decreases first and then increases with the increase of the cycle threshold. The transmission energy increases first and then decreases with the increase of the cycle threshold. At the critical threshold, the density of reflected energy and dissipated energy is the smallest and the transmitted energy is the largest. There are four kinds of failure modes of rock samples under impact load: crushing failure, rock fragmentation, rock side spalling and rock splitting. Under each impact pressure, the fractal dimension of rock samples decreases first and then increases with the increase of cycle threshold, and increases nonlinearly with the increase of dissipated energy density. The densification of impact rock is better, the intensity of impact failure is weaker, the fragmentation degree is larger, the dissipated energy density is smaller, and the fractal dimension is smaller. The research results show that when the basic roof period pressure is less than the damage threshold value of the impact pillar, the periodic pressure can improve the rock density and affect the damage degree of the dynamic impact.