Research on dynamic mechanical properties and transient magnetic field characteristics of composite coal and rock mass

Coal and gas outburst is the comprehensive mechanics result of roof-coal body-floor. It is difficult to fully reveal mechanism of coal and gas outburst by simply studying mechanical characteristics of coal or rock. In order to reveal dynamic mechanical properties and transient magnetic field signal characteristics of composite coal and rock mass, the dynamic mechanical properties of composite coal and rock mass were studied by Hopkinson pressure bar experiment system, and the signal characteristics of transient magnetic field of composite coal and rock mass were analyzed during dynamic failure process. The experiment results show that after composite coal and rock is impacted, the rock fragmentation is large, the coal fragmentation is small, and the coal with low strength has good stress attenuation and wave clipping effect on the composite sample. The stress attenuation decreases to 1/5 of the original after the stress wave passes through the composite sample. The attenuation and weakening effect of the composite coal and rock mass on the stress wave mainly depends on the microstructure of the coal body, and the microstructure of the coal body makes the plastic deformation of coal body increase and elastic modulus decrease gradually after being subjected to impact load. Compared with the composite samples, the single sample shows obvious brittle failure characteristics. The average strain rate, maximum strain rate, fracture stress limit value and failure strain of the composite samples have certain correlation with the amplitude of transient magnetic signals. With increase of the impact speed, average strain rate, maximum strain rate, fracture stress limit value of the composite samples, the amplitude of transient magnetic signals generated by the composite sample increases gradually, and the magnitude of damage strain and the amplitude of transient magnetic signals show a negative correlation, but the correlation between them is not strong and the discrete type is large.