Study On Dynamic Mechanical Properties And Crack Propagation Law Of Coal Under The Action Of High-pressure Gas Impact

At present,the proportion of coal production,consumption and underground mining in China ranks among the top in the world,and China’s energy structure is still dominated by coal.However,most coal mines in China are characterized by low permeability and high gas content,and the difficulty of gas drainage increases with the increase of mining depth.The high-pressure gas blasting and impact technology,which is an anhydrous fracturing and increasing permeability technology,has gradually become the object of research for many related experts and scholars.In this paper,the dynamic mechanical properties and crack propagation law of coal mass under the action of high-pressure gas impact are investigated by combining theoretical analysis,laboratory test and numerical simulation.Firstly,according to the gas jet theory and fracture mechanics theory,the crack propagation process of coal under high-pressure gas impact and the crack initiation and the mechanism of crack propagation at each stage were studied.Then,the dynamic mechanical properties of coal at different strain rates were obtained based on the SHPB impact compression test.Subsequently,the effects of in-situ stress and different impact gas pressure on coal crack propagation were revealed according to the attenuation law of strain wave,internal damage and failure form within coal under the action of high-pressure gas impact.Finally,the numerical calculation model of the coal mass impacted by highpressure gas is established using ANSYS/LS-DYNA numerical software,and the stress wave attenuation and crack expansion law within the coal mass under the action of high-pressure gas impact are obtained,and the crack expansion under different ground stress conditions are simulated.Based on the above studies,the following main conclusions can be drawn:(1)The dynamic mechanical properties of coal are studied by the SHPB impact device.The results show that the dynamic mechanical parameters of the coal have obvious strain rate effects.Based on the damage morphology of coal under different impact gas pressure,the damage mechanism of coal mass under impact compression is revealed,i.e.,axial splitting damage and transverse tensile damage mainly occur.(2)The crack propagation law of coal is studied by using high-pressure gas impact test system.From the strain wave,internal damage and damage morphology of the coal body under ground stress and different impact air pressure,it can be concluded that the initial stress peak of the coal mass under the action of high-pressure gas impact is very small,and no crushing zone is produced,which is basically dominated by tensile damage.The larger the impact gas pressure,the larger the crack propagation range;and the crack propagation range with in-situ stress is smaller than that without in-situ stress.(3)The strain wave propagation and crack extension process of high-pressure gas impinging coal mass are simulated using ANSYS/LSDYNA software.Combining the high-pressure gas impact cracking theory,high-pressure gas impact tests and numerical simulation results,the two main stages of crack propagation in the coal mass under the action of high-pressure gas impact are derived,namely the initial crack formation stage under the action of stress waves and the crack propagation stage driven by highpressure gas.And the second stage is the main stage of crack propagation.(4)Based on the high-pressure gas test,the crack propagation under the action of different in-situ stresses is studied by a combination of theoretical analysis and numerical simulation.The results show that the effect of in-situ stress on crack propagation in different directions is different,and the magnitude of the effect is positively related to the intensity of the in-situ stress.There are 49 figures,12 tables and 98 references in this dissertation.