Characterization Of Mechanical Parameters And Failure Characteristics Of Coal And Rock Mass Under Point Load At Different Indenter Angles

Rock mechanical parameters are one of the fundamental parameters in mining engineering activities,and it is difficult and expensive to obtain them in a well.As an indirect method of obtaining partial parameters of rocks,the point load method is widely used in the field.Indirect testing methods suffer from problems of precision and adaptability to different lithotypes,where indenter angle is an important factor.Therefore,it is necessary to investigate the characteristics of the destruction of coal plutons and their relationships with rock mechanical parameters under the effect of the indenter angle on point load.In this paper,taking the common coal,mudstone,sandstone and limestone in coal mine as the research object,a combination of theoretical analysis,numerical modeling,and laboratory tests are used to investigate the characteristics of the destruction of coal plutons and their relationships with rock mechanical parameters under the effect of different indenter angles on point loads.A theoretical model for the point loading action based on the change in the indenter angle is established,and the formula for the stress distribution of coal plutons reflecting the change in the indenter angle under point loading is obtained.Explored the effect of the indenter angle on the stress distribution and found that the stress in all directions decreased with distance from the loading point,but the stress concentration was more significant under the action of small angle indenters.Studied the law of stress distribution under point load with different indenter angles by numerical simulation,and found that compressive and tensile stresses were generated in coal rock bodies under the action of different angle indenters in both horizontal and vertical directions,and the horizontal and vertical tensile stress regions exhibited a round sphere shape and " inverted bimodal shape ",respectively.And verify the laws of the stress distribution of coal plutons under the influence of the indenter angle in theoretical analysis.Point load tests were carried out with 60°,90° and 120° indenters selected to study the deformation and failure of rocks under point load.By analyzing the load displacement curves of point load tests with different angles of indenters,it is found that the curves present multi peak characteristics and the peaks represent the generation and fragmentation of pressure entities in coal plutons under the action of point loads.As the indenter angle increases,the number of press entities formed is obviously reduced and the magnitude of stress decreases.By analyzing the curves of ultimate intrusion depth of the indenter versus the indenter angle,it was found that the ultimate intrusion depth at the time of reaching failure load decreased as a quadratic function with increasing indenter angle,and the ultimate intrusion depth of different lithogenic specimens tended to be stable at the indenter angle of 121.4°,119.0°,121.0°,109.0°.Analyzing the effect of indenter angle on the failure characteristics,it is found that the specimens under point load of different angles of the indenter all undergo cleavage failure along the loading axis,except that the rock with a larger strength has a larger range of point load indentation,and the indentation shape is more regular and flake collapse will occur.As the indenter angle increases,the indentation diameter increases according to a quadratic function.The indenter angle has a large effect on the indentation diameter of rocks at low intensities,and coal plutons are more stable to destruction by the action of large angle indenters.The data were analyzed,and it was found that the dispersion of the data from the point load test was the smallest at an indenter angle of 120°.The load strength at different angles of the indenter point showed a linear relationship with the uniaxial compressive strength,tensile strength as well as the elastic modulus,with the best fit at an indenter angle of 120°.Analysis of suitability for different lithotypes with indenters at various angles showed that point load intensity increases with increasing indenter angle,fitting obtained a point load intensity of 119.1° to 142.9° from indenter angle tends to a stable value.A testing device for mechanical parameters of coal rocks inside pores is developed based on the optimal indenter angle,and its design idea and design results are presented in detail.