Experimental Study On Dynamic Mechanical Properties Of Coal In Hongqinghe Coal Mine

As a typical strong dynamic pressure mine in western China,Hongqinghe Coal Mine has experienced frequent shock incidents in recent years,which has had a great impact on mine safety and economic benefits.In the impact of the Hongqinghe coal mine,the bump–prone coal provides a good energy storage carrier for the rockburst,the high ground stress brought by the large mining depth provides a high static load,and the roof fracture provides a strong dynamic load.Based on Hongqinghe coal mine,this paper uses laboratory tests,theoretical analysis to study the dynamic mechanical characteristics of bump–prone coal.The purpose is to explore the failure mechanism of shock instability and provide theoretical support for rockburst prevention.Based on the combined action of high static load and strong dynamic load on the coal of Hongqinghe coal mine,a method of three-dimensional(3D)coupled static and dynamic loads was designed to study the mechanical characteristics of the bump–prone coal from the three aspects of strength and deformation characteristics,energy dissipation characteristics and failure characteristics.Based on the characteristics of the frequent disturbance of a large number of low-amplitude loads before the impact and the single shock of high-amplitude loads when the impact occurs in Hongqinghe coal mine,the tiered cyclic impact with the loading method of cyclic weak impact and single strong impact is designed to study the dynamic mechanical characteristics of bump–prone coal under the action of multi-round impact of graded loads.In view of the lack of research on the reasonable sample length–to–diameter(L/D)ratio of bump–prone coal in the split Hopkinson pressure bar(SHPB)test,through theoretical analysis and summary of the test data,a reasonable sample length–to–diameter ratio of the bump–prone coal is given.The main results are:(1)The strain rate effect under 3D coupled static and dynamic loads is studied.The dynamic peak stress exhibits an obvious strain rate effect,which increases in a power function relationship with the increase of the strain rate.Both the dynamic peak strain and the dynamic maximum strain show obvious strain rate effect,which increase linearly with the increase of strain rate.Both the dynamic secant modulus and the second type of secant modulus show no strain rate effect.As the incident energy increases,the reflected energy,transmitted energy,and fragmentation energy consumption all increase in a linear relationship.The energy consumption increases in a power function relationship with the increase of strain rate.The energy consumption density increases with the increase of impact pressure.The failure morphology has a significant strain rate effect.As the strain rate increases,the degree of sample fracture increases.(2)The strain rate effect is decoupled from the L/D ratio effect,and the L/D ratio effect under 3D coupled static and dynamic loads is studied.Under the same strain rate,the dynamic strength increases with the increase in the L/D ratio,showing the opposite L/D ratio effect to the static condition.The dynamic peak strain decreases with the increase of the L/D ratio,and the dynamic maximum strain first decreases and then increases with the increase of the L/D ratio.As the L/D ratio increases,the proportion of reflected energy in the incident energy gradually increases,the proportion of transmitted energy gradually decreases,and the proportion of energy consumption is basically about 20%.For the same impact pressure,the energy consumption density decreases as the L/D ratio increases.The failure mode has a significant L/D ratio effect.As the L/D ratio increases,the failure mode evolves from tensile failure to compression–shear failure.(3)Based on the assumption of uniform stress,dynamic stress balance,dispersion of dynamic mechanical parameters,and ease of sample processing,a reasonable L/D ratio of 0.8 for the bump–prone coal in the SHPB test is given.(4)The dynamic mechanical properties under tiered cyclic impact loads are studied.The dynamic peak stress of the cyclic weak shock and single strong shock decrease with the increase of the number of weak shocks.The secant modulus tends to decrease with the increase of the number of shocks.Both the cumulative weak shock absorbed energy and the single strong shock absorbed energy are positively correlated with the number of weak shocks.The energy absorption efficiency is closely related to the wave impedance of the sample.The total energy consumption increases with the increase of the number of weak shocks.The average energy consumption is about 30%.The failure degree of the sample gradually increases with the increase of the number of weak impacts,and the failure mode evolves from tension failure to tension–compression–shear composite failure.