Study On Mechanical Property Of Soft Coal Under Impact Load And Its Effect To Coal And Gas Outburst

With coal mining going to the deep extension, coal and rock dynamic disasters, such as rock burst and coal and gas outburst, are becoming more and more serious. How to prevent and reduce the disasters appears much more urgent to ensure that the coal mining is safe, equitable and efficient. According to the study of materials mechanical properties, the materials often showed different in damage forms and mechanical characteristics in the static load and dynamic load damage. As the literature study of the coal and rock mechanical properties was mostly in quasi static, the dynamic characteristics study of coal and rock was less. Only a few scholars made a few research of the anthracite mechanical characteristics under uniaxial compression.In this paper, the outburst and non-outburst coal of 13 coal seams in pan-xie coal mine were made into two different kinds of coal and rock specimens to conduct quasi static mechanics experiment and uniaxial compression experiment. The methods of experimental research, theoretical analysis and numerical simulation were used to research the dynamic mechanical properties, the constitutive relations and the energy consumption law under the condition of impact load. Innovative results were achieved as followed:Uniaxial compression experiments were carried out in two groups of outburst and non-outburst coal specimens of 13-1 coal seams with split-Hopkinson pressure bar (SHPB) device. After experimental waveforms collected and analysis by three wave processing method, the dynamic stress strain curve and related kinetic parameters under different strain rate were obtained of the two kinds of coal and rock. The experimental results show that the deformation process of two kinds of coal and rock under impact load can be summarized as four stages such as the initial nonlinear stage, yield stage, strain-hardening stage and unloading damage; the dynamic strength, peak strain, dynamic elastic modulus, etc. are all in significant correlation with strain rate, such as the significant dynamic mechanical properties of strain hardening and strain rate hardening. By the dynamic and static experiment contrast, the coal and rock dynamic stress growth factor DIF and the dynamic strain growth factor DEIF were obtained, and the strain rate hardening effect of outburst coal was found to be more apparent than non-outburst.On the basis of analysis of the deformation characteristics of coal and rock under impact load and the references of former research achievements about rock materials and anthracite constitutive relation, the zhu-wang-tang model of constitutive equation was reasonably simplified according to the viscoelastic theory and damage mechanics theory; and by introducing a damage variable which is dependent on strain rate and strain, a dynamic constitutive equation was built for the application of gas coal damage effect and strain rate effect and the dynamic stress-strain relationship of the two kinds of coal before peak stress. Combined with the impact uniaxial compression experiment data, the corresponding regenerated dynamic stress-strain curve was obtained after the unknown parameters fitting of the constitutive equation. The results show that the fitted dynamic stress strain curve can fit the experiment curve well, and the coal and rock material has high sensitivity to strain rate response; and also the gas coal acts brittleness more obvious than anthracite under the condition of high strain rate.Based on coal and rock dynamic constitutive equation, the VUMAT subroutine of coal rock material was developed by using Fortran, and the coal and rock dynamic constitutive equation and the ABAQUS was combined by using the commercial finite element software ABAQUS’material interface. The subroutine was used for finite element simulation of the dynamic mechanical behavior at different strain rate of coal and rock materials. After comparison and analysis, the coal and rock material subroutine developed in this paper can well reflect the dynamic mechanical properties of coal and rock material.On the basis of quasi static experiments and SHPB dynamic impact experiment, the energy density of two kinds of coal and rock were calculated during the quasi-static loading, and the stress wave energy in different strain rate and absorb energy density during coal and rock crushing and energy absorptivity were also got. the results show that the absorb energy density of two kinds of coal and rock increased exponentially with the increase of strain rate; and compared with the quasi static loading, the energy density of outburst coal increased faster and the strain rate effect was much more significant.Besides, the fractal research of experimental fragments was carried out by using the 0.5 mm and 1 mm,10 mm,20 mm and 30 mm coal sample sieve. The results show that with the increase of strain rate, the fractal dimension of the fragments became bigger, and the specimen broke more seriously, and the particle size of the fragments got smaller, and so the fractal dimension and the strain rate are logistic; the average particle size of fragments has a negative exponential relation with strain rate, with the smaller particle size needing larger energy.In order to simulate the damage of coal under peak load of 30MPa,50MPa,70MPa, the coal and rock dynamic constitutive equations which have been constructed on part 3 was embedded into software of finite element analysis. Combined with analysis resualt of energy on and on-site case, the process of coal and gas outburst is divided into two distinct stages, which are critical outburst stage and outburst stage. And the minimum breaking energy of coal under critical outburst stage is proposed, which is about (5～10)×104J/m3. Through numerical analysis, evolutionary process of the splitting crack on the coal under different impact loads was studied effectively. The results show that structure of interval split was appeared on the coal under impact load. And there are two kinds of splitting damages, one is called splitting damages caused by intense tensile stress, the other one is called stepped splitting damages caused by tensile-shear and punch-shear. The result of this research can make us a better understanding of mechanism of coal and gas dynamic phenomenon under deep mining at low gas, high stress, strong disturbance conditions, which can provide thoretical foundation for preventing and curing dynamic disasters on coal mine caused by coal and gas outburst.