Research On Dynamic Mechanical Characteristics Of Coal Under Three-dimensional Dynamic And Static Combined Loadin

In this paper,dynamic disasters such as rock burst,roof collapse and roadway collapse are easy to occur in deep coal rock under the coupling effect of three-dimensional static load and dynamic impact.Combined with laboratory test,theoretical analysis,numerical simulation and other research methods,the dynamic mechanical characteristics of coal rock under three-dimensional dynamic and dynamic loading were studied.The improved separated Hopkinson pressure bar(SHPB)test system was used to carry out dynamic impact experiments of different axial prestressing,confining pressure and strain rate on deep coal rock.The dynamic strength,deformation,energy dissipation,fractal dimension,failure mode and other characteristics of deep coal rock under different stress states are discussed.At the same time,the strain rate effect,confining pressure effect,axial pressure effect and residual strength are introduced into the dynamic constitutive model of coal based on the time dependent damage model.A three-dimensional dynamic statistical damage constitutive model considering the strain rate effect,confining pressure effect,axial pressure effect and residual strength characteristics is established,and the accuracy of the constitutive model is verified.Finally,ANSYS/LS-DYNA numerical simulation software is used to simulate the three-dimensional dynamic and static combined impact of coal samples under different strain rates,confining pressures and axial pressures,revealing the stress and failure evolution process of coal and rock in the process of three-dimensional dynamic and static combined impact.The research results of the paper are as follows:(1)Under different strain rates,confining pressures and axial pressures,three-dimensional dynamic and static combined impact tests are carried out on coal samples.The results show that:The stress-strain curves of coal samples under different strain rates,confining pressures and axial compression show the same trend.Meanwhile,the double-peak phenomenon occurs at different strain rates,confining pressures and axial pressures,and the curve shows a certain leap,especially the double-peak phenomenon at different strain rates.The reason for this phenomenon may be related to the role of carbon in crystal micro-cracking.The dynamic peak stress and secant modulus of coal sample increase linearly with the increase of strain rate,increase linearly with the increase of confining pressure,and increase first and then decrease with the increase of axial pressure,basically showing the change relationship of unary quadratic equation,and the critical axial pressure of dynamic mechanical properties of coal is 8MPa(44%σ_c),namely:When the axial prestress is 4～8MPa(22%σ_c～44%σ_c),the cracks of coal sample are continuously compacted and the dynamic compressive strength is enhanced with the increase of axial compression.The axial prestress strengthens the dynamic peak strength.When the axial compression is 8～12MPa(44%σ_c～66%σ_c),the peak dynamic compressive strength decreases with the increase of axial compression,and the axial compression promotes crack propagation and weakens the peak dynamic compressive strength.(2)A dynamic statistical damage constitutive model was constructed considering the strain rate effect,confining pressure effect,axial pressure effect and residual strength characteristics of coal rock.Through verification,the accuracy of the constitutive model is high,and with the decrease of the damage correction coefficient q,the ductility of the fitting curve increases,the peak strength increases,and the residual strength of coal rock increases.Adding the damage correction coefficient q into the constitutive model can reflect the residual strength characteristics of coal and rock under three-dimensional dynamic and static combined loading to a certain extent.(3)The calculation formula of energy absorbed by coal samples under three-dimensional dynamic and static combined loading is derived and the composition of dissipated energy is analyzed.The dissipative characteristics of coal samples under three-dimensional dynamic and static combined loading are obtained,and the failure mode and fractal dimension characteristics of coal samples are analyzed.The results show that the process of energy conversion analysis should be divided into two stages under three-dimensional dynamic and dynamic combined loading of coal and rock.The first stage is that the confining pressure and axial pressure load will make the coal sample obtain part of the strain energy.The second stage is that the coal sample is subjected to axial impact load and the static load coupling work in the first stage.When the strain rate,confining pressure and axial pressure were changed,the energy absorption density of coal samples increased with the increase of incident energy,and the curves showed a good linear relationship.Energy absorption and transmittance decrease with the increase of strain rate,reflectance increases with the increase of strain rate,energy absorption and transmittance of coal sample increase with the increase of confining pressure,on the contrary,the reflectance of coal sample decreases with the increase of confining pressure,energy absorption and transmittance increase first and then decrease with the increase of axial pressure,but the reflectance of coal sample first decreases and then increases with the increase of axial pressure.Under the three-dimensional dynamic and static combined loading,the crushing degree of coal sample increases with the increase of strain rate,decreases with the increase of confining pressure,and first increases and then decreases with the increase of axial pressure.Correspondingly,the crushing degree of coal sample decreases with the increase of strain rate,increases with the increase of confining pressure,and first decreases and then increases with the increase of axial pressure.The failure mode of coal samples under three-dimensional dynamic and static combined loading is basically"X"type and"conical"shear failure mode,and a small amount of tensile failure occurs.The fractal dimension increases with the increase of strain rate,decreases with the increase of confining pressure,and first increases and then decreases with the increase of axial pressure.(4)The effective stress of coal sample in the impact process increases with the increase of strain rate,confining pressure and axial pressure.Due to the existence of axial pressure and confining pressure,the effective stress will accumulate on the two end faces of coal sample,which leads to the failure and deformation of both ends of coal sample more easily.In the three-dimensional dynamic and static combined loading,"X"shear failure usually occurs during the impact of coal sample,while under conventional uniaxial impact,the failure model of coal sample is mainly tensile failure.At the same time,the failure deformation of coal sample is more serious with the increase of strain rate and axial pressure,and the failure is less likely to occur with the increase of confining pressure.The research results of this paper reveal the dynamic mechanical properties of coal and rock under three-dimensional dynamic and static combined loading,which provides engineering value and scientific significance for the prevention of deep coal and rock dynamic disasters and the evaluation of mining engineering stability.