| To study the mechanical mechanism of the instability and failure of the coal body in the mine,the dynamic problems such as the instability and failure of the coal wall during the mining of the coal body were explored.Take the raw coal from a coal mine in Yulin,Shaanxi as a test example.Using a ?50 mm SHPB test device,impact compression test and impact split test were performed on hard coal samples.Combined with a high-speed camera,the dynamic mechanical properties and crushing morphology of hard coal are analyzed.The results of the impact compression test are as follows.With the increase of impact velocity,the peak stress and peak strain of hard coal samples show an increasing trend.The dynamic elastic modulus of the hard coal sample under passive confining pressure is 1.42 times that of the radial coal when free.The uniaxial compressive strength of the sample is 11.121 to 112.651 MPa,which is 1.2 to 5.4 times that under radial freedom.The peak strain of the sample is 0.012 to 0.035,which is 1.18 to 1.2 times that under radial freedom.The passive confining stress generally increases with the increase of the axial stress,but there are obvious fluctuations in the rising stage of the curve.The maximum passive confining pressure stress value has a power function relationship with the impact velocity.Incident energy,reflection energy,and absorption energy are linearly related to impact velocity.The transmitted energy generally increases first and then decreases.Absorbed energy accounts for only 15%to 20%of incident energy.The energy dissipation rate fluctuates with the increase of impact velocity.The Weibull distribution can well describe the particle size distribution of impact crushing of hard coal samples.The crushing degree index d0 decreases as a power function with increasing impact velocity.The crushing characteristic index m0 fluctuates with the increase of impact velocity.The results of the impact split test are as follows.The impact speed is from 2.4 m/s?3.4 m/s?4.3 m/s?5.4 m/s,the dynamic tensile strength of the hard coal sample is about 0.95 times?1.65 times?2.24 times?2.59 times the static tensile strength.The dynamic tensile strength of hard coal samples increases with increasing impact velocity.The increase rates were approximately 73.7%?35.4%?15.8%,and the increase rates were significantly slower.When the impact velocity is 2.307 to 2.570 m/s and 4.247 to 4.370 m/s,the peak stress and strain rate are relatively concentrated,and the change range is small.When the impact velocity is from 3.106 to 3.772 m/s,the peak stress and strain rate increase linearly with the increase of impact velocity,and the increase is stable.When the impact velocity is greater than 5.0 m/s,the increase in peak stress and strain rate decreases first and then increases,with some dispersion.Observing the crack propagation during the whole process of coal impact loading based on the high-speed camera is as follows.The final fracture shape of the sample is directly related to the bedding surface.When the bedding plane is parallel or perpendicular to the direction of incidence,the failure mode of the specimen is tensile cleavage along the direction of incidence.When the bedding plane is at a certain angle with the incident direction,in addition to the tensile splitting along the incident direction,the shear failure along the bedding crack direction occurs simultaneously.And due to the influence of the bedding plane,the crack propagation path is not smooth.Figure[44]table[9]reference[74]... |
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