Research On Mechanical Response And Energy Development Characteristics Of Impact-prone Coal

Coal bumps have been a threat to coal mining in China.Impact-prone coal is a kind of special rock material that induces coal bumps disasters.Understanding the deformation process and energy evolution of this material under different loading modes will help to reveal the mechanism of coal bumps and lay the foundation for the prevention of coal bumps.Although many scholars have done a lot of research on the deformation and failure of coal,there are still some shortages in the study of the mechanical response and energy development characteristics of impact-prone coal.In order to fully reveal the mechanical properties of impact-prone coal,this thesis focused on the impact-prone coal of Xinzhouyao Coal Mine and studied the influences of bedding and cleats on the mechanical properties of impact-prone coal,the nonlinear mechanical characteristics of stress-strain curves and lateral strain-axial strain curves of impact-prone coal,the energy development and distribution characteristics under uniaxial cyclic loading process,and tensile mechanical response and energy dissipation characteristics under dynamic and static combination loading.The research results obtained are as follows based on the above research:?1?Influence of bedding and cleats on the mechanical properties of impact-prone coal during the uniaxial compression process?1?According to the orientation relationship of the bedding,face cleats and butt cleats of the coal samples of the Xinzhouyao Coal Mine,the ultrasonic wave velocity test,uniaxial compression and acoustic emission monitoring were conducted considering the bedding and cleat angles.The influence of bedding and cleats on the mechanical properties of impact-prone coal was fully analyzed.When the angle of bedding plane was 0°,and the angle of face cleats and butt cleats were 90°,the coal samples were characterized as the highest strength,severe failure state,obvious brittleness characteristics,the short and intense burst stage of AE hits number,the longest nonlinear stage of the stress-strain curve and the smallest deformation modulus.When both bedding dip and face cleats dip were 45°,and the dip of butt cleats was 90°,the mechanical properties of impact-prone coal were characterized as the lowest peak strength,the obvious main fracture plane and its branch cracks,the step-shaped drop of stress-strain curve,several peaks in the booming stage of AE hits number and the moderate value of deformation modulus.When the dips of bedding and butt cleats are90°and the dip of face cleats is 45°,the coal samples were characterized as the moderate strength,the tensile failure mode the bedding planes,several plate-shaped cracks on the failure surface,step-shaped drop of stress-strain curve,the multiple peaks of AE hits numbers,and the largest deformation modulus.?2?An anisotropic model of impact-prone coal rock considering bedding and cleats was established.The orientation distribution relation of bedding,face cleats,and butt cleats are considered in the model.The model included the elastic stage,the crack growth stage,and the post-peak stage of coal during the compression process.This model was implanted into FLAC3D,and the uniaxial compression experimental curves and failure modes of the coal model under different bedding dips were compared with that of the real experimental results.The model well demonstrated the elastic stage,the crack growth stage,and the post-peak stage of the real experimental results.The peak strengths at the bedding dips of 0°,45°and 90°were within the scope of the experimental results.The failure mode agreed well with the experimental results.?3?The model was used to simulate the failure mode of coal during the uniaxial compression process in FLAC3D.The bedding dip was within the scope from 0°to15°at intervals of 15°.when the bedding dip was 0°,the damage is mainly caused by the failure of the coal matrix;when the bedding dip was 15°or 30°,the failure mode was mainly the shear failure of the face cleats;when the bedding plane dip was 45°,60°or 75°,the failure mode was the shear failure of the bedding plane;when the bedding dip was 90°,the failure mode was the tensile failure of the bedding plane.?4?The strengths of impact-prone coal during the uniaxial compression process were simulated with the bedding dip increasing from 0°to 90°at intervals of 15°.When the bedding dip was 0°,the coal strength was the highest,47.1 MPa;In the process that the bedding dip increased from 0°to 45°,the coal strength gradually decreased.At the bedding dip of 45°,the coal strength was the lowest,21.7 MPa;with the bedding dip increasing from 45°to 90°,the coal strength showed an upward trend.When the bedding angle was 90°,it reached another peak,24.1 MPa.The strength of coal containing only face cleats or bedding were simulated respectively:the influence of bedding on coal strength rock was decisive,but the influence of cleats on coal strength was not negligible.The coal strength was weakened and the bedding dip corresponding to the minimum strength was changed by cleats.Ignoring cleats could overestimate the strength of coal and even misjudge the direction of coal failure.?2?Nonlinear characteristics of impact-prone coal at the compaction stage?1?According to the obvious nonlinear characteristics of impact-prone coal at the compaction stage,a nonlinear model of the compaction stage for impact-prone coal and other porous rock was proposed with the reference to the Duncan-Chang nonlinear model in soil mechanics.Hyperbola model was adopted to simulate the development of the stress-strain curve of the coal at the compaction stage.After the compaction stage was completed,the traditional linear stress-strain relation was used to describe the mechanical behavior of impact-prone coal.Similarly,a hyperbolic model of the lateral strain-axial strain of the impact-prone coal at the compaction stage was established.The two models were called the non-linear model of the compaction stage of the impact-prone coal,which includes the initial elastic modulus a,the reciprocal of the axial strain when the principal stress difference tended infinity b,the linear elastic modulus E_e,the initial Poisson's ratio f,the negative reciprocal of the axial strain when the lateral strain approached infinity D,the Poisson's ratio of the linear elastic phase?_e.The physical meanings of the six parameters were introduced graphically.The parameters of a,b,E_e,f,D,and?_e were obtained by using the least squares method.?2?The model was used to analyze the influence of some factors such as bedding on the nonlinear mechanical behavior of coal at the compaction section,and to reveal the anisotropic behavior of the non-linear behavior of the impact-prone coal.For the nonlinear characteristics of stress-strain,with the bedding dip increasing from0°?45°?90°,the initial elastic modulus increased continuously and the value of E_e/a gradually became smaller,which indicated that the nonlinear stage was getting shorter;1/b decreased first and then increased,and the overall value was near 0.01.?3?For the nonlinear characteristics of lateral strain-axial strain,with the bedding dip increasing from 0°?45°?90°,?_e decreased first and then increased,which indicated that the lateral stain was not obvious at the elastic stage for coal with bedding dip of 45°.The initial elastic modulus increased first and then increased,which indicated that the lateral strain was not obvious at the compaction stage for coal with bedding dip of 0°and 90°,while the lateral strain was obvious at the compaction stage for coal with bedding dip of 45°.?-1/D?decreased first and then increased,and the overall value was less than 0.01.?3?Mechanical properties and energy development characteristics of impact-prone coal under uniaxial cyclic loading process?1?The stress and deformation characteristics,energy development and distribution features of impact-prone coal are analyzed during the cyclic loading process.During the cyclic loading and unloading process,the mechanical properties such as strength and peak strain of the specimens changed little,but the elastic modulus of the coal specimen in loading process was slightly larger than that in the unloading process during the same loading and unloading circle.Several loading and unloading process would reduce the elastic modulus of coal.The elastic modulus degradation rate??35?R_E?was defined to describe the reduction of the elastic modulus during cyclic loading and unloading process.Overall,elastic modulus degradation rate during the cyclic loading and unloading process was not obvious,and the average value of?35?R_E was less than 0.03.?2?The elastic energy and dissipation energy of the coal samples during cyclic loading and unloading process were calculated,and the energy development features in the whole process was analyzed.The input energy density increased nonlinearly with the increase of the load,and most of the input energy was stored in the coal samples in the form of elastic strain energy.From the viewpoint of energy storage rate,the proportion of elastic strain energy of the impact-prone coal was always high and continued to increase;the maximum value of the energy dissipation rate of the coal samples appeared in the initial stage of loading,indicating that the process of energy dissipation was strong and the energy storage effect was weak in the compaction stage.As the load increased,the energy dissipation rate continued to decrease,indicating that the energy storage effect of the sample during the elastic phase was strong and the dissipation effect was weak.In general,the impact-prone coal was characterized the strong capability of storing energy,the unobvious precursors to failure,and the stronger energy release during the failure process.The Acoustic emission monitoring signals indicated that both the energy release and the energy dissipation occurred during sample unloading process.?4?Mechanical response characteristics of impact-prone Brazilian disc specimen under dynamic and static combination loading?1?The dynamic and static combination loading was conducted on the impact-prone Brazilian disc specimen using the modified Hopkinson Pressure Bar?SHPB?device.The whole process was monitored by a high-speed camera.With the static axial pressure increased from 0 to 0.289 MPa to 0.578 MPa,the average dynamic tensile strength of the samples was from 4.35 MPa to 6.28 MPa to 3.65 MPa,showing a trend of increasing first and then decreasing.The dynamic tensile strength of the specimen tended to be more discrete.The coefficient of variation of the dynamic tensile strength of the specimen was from 0.27 to 0.48 to 0.48.The average peak strain of the specimens was from 11.04 e^-3?6.40 e^-3?7.69e^-3,showing a trend of decreasing first and then increasing.?2?When the static load axial pressure is 0,the greater the impact velocity and the higher the dynamic tensile strength,the correlation between the two showed a positive correlation.When the static load axial pressure is 0.289 MPa,the greater the impact velocity,the smaller the dynamic tensile strength.Impact velocity and strength showed a good negative correlation.The greater the strain rate,the smaller the corresponding dynamic tensile strength,and the negative correlation between impact velocity and strength was obvious.When the static axial pressure was 0.578 MPa,the impact velocity and dynamic tensile strength showed a good negative correlation,and with the increase of strain rate,the corresponding dynamic tensile strength showed some fluctuation.In general,when there wass no axial pressure,the dynamic tensile strength increased with the increase of impact velocity.However,with a certain amount of axial compression,the dynamic tensile strength tended to decrease as the impact velocity increased.?3?Static axial pressure and impact velocity affected crack propagation of disc specimens in different aspects.The static axial load mainly affected the timing characteristics of crack propagation in Brazilian disc specimens.After increasing the axial pressure,the cracking and penetration time of the samples had been shortened,and the degree of fragmentation of the samples had been increased.The impact velocity mainly affected the spatial distribution of crack propagation.With the increase of impact speed,the destruction of Brazil's disk surface showed its characteristics from the whole to the local,from the center to the edge.