Experimental Analysis And Mechanism Study Of Coal Damage And Degradtion Induced By Gas Adsorption

With the high development of the national economy today,coal is still the major energy sources in China,which will amount to 50%of overall energy needs by 2050.How to ensure a safe and stable energy supply is an important guarantee for sustained and healthy economic development.However,with the increasing depth and intensity of mining,geological hazard of coal and gas outburst has become a common safety problem in coal mines,which is the coupling action results of coal break under stress disturbance and the dynamic evolution of adsorption gas migration,therefore,to study and simulate the mechanism of damage and deterioration of coal caused by gas adsorption is the basis and necessary condition for exploring the mechanism of coal and gas outburst,and has very significant practical engineering significance and social significance.Coal in gas adsorption,which can be seemed to be a heterogeneous multiphase composite structure,when it is subjected to external loads the micro-defects randomly distributed in the coal body are constantly changing,then the defects joint together and macroscopic cracks are gradually created and eventually destroyed.At the same time,surface free energy of coal is reduced by gas molecules adsorbed on the surface of coal matrix,the instability failure of coal and rock mass structure is accelerated and promoted at the macro level.The process of deformation and fracture induced by gas adsorption and stress loading is very complicated,considering the coal as a continuous medium,just using the method of classical elastic-plastic mechanics or fracture damage mechanics is not consistent with reality.Moreover,the single laboratory test method,low visualization degree of the test instrument,which is difficult to obtain the physical and mechanical parameters of coal in the multi-phase coupling environment,and definitely hinders the development of experimental research and theoretical research.Therefore,this paper through the independent research and development of supporting test system,combining with theoretical analysis and laboratory tests,Based on laboratory deterioration test results,the deformation fracture process and damage degradation mechanism of coal at the macro and micro level in the process of gas adsorption and stress loading,from the angles of coal meso-damage,macro-degradation and fractal dimension of cracks,are investigated and discussed.The main content of this paper is as follows:1)Based on the concept of visual monitoring in the whole test process and constant volume accurate loading in the gas-solid coupling status,a gas-solid coupling test system in the visual and constant volume loading state was designed and developed adopted the SolidWorks(a 3D mechanical designing software)and ABAQUS(an finite element analysis software).The system combined with servo press,has four main components:visual and constant volume tester,gas pressing module,axial loading module,specimen deformation monitoring module.The system composition,main features,Sealing solution and key technologies were given in detail,and relevant applications are carried out to verify the system function indicators,which provided a feasibility test equipment for the follow-up study.2)In order to accurately obtain and in-depth study the regularity of circumferential deformation and the mechanical characteristics of post-peak dilatancy of the coal standard specimen during the whole stress strain test in the gas adsorption state,a new type of standard specimens circumferential deformation test method based on angle measurement was proposed drawing on the experience of MTS chain circumferential extensometer,its test principle was clarified and put into the instrument development,and the calculating equation of circumferential deformation for the system was derived.Which provided feasible monitoring scheme and hardware support for the deformation monitoring of coal and rock mass in adsorption-loading coupling process.3)In order to quantitatively study the law of degradation and damage of coal by gas adsorption and explore its mechanism,making up the briquette similar material,making the standard briquette specimen with adjustable strength,and a series of laboratory tests for gas adsorption-induced coal mechanical property alterations were carried out by using the research and development test equipment,macroscopic test parameters such as strength,deformation and fracture development of coal were monitored and analyzed under the conditions of gas-solid coupling and dynamic and static loading state.The effects of different adsorption conditions and stress loading conditions on the damage and degradation of coal were studied.The test results showed that the increase of gas adsorption content was the key factor which leaded to the decrease of coal strength,the increase of coal volume and the increase of fracture.4)On a macro level,the mechanical analysis of the strength degradation of coal was carried out based on the surface physicochemical theory and molar-Coulomb strength failure criterion,the relationship of the coal strength and deformation with the adsorption equilibrium pressure and the adsorption capacity was obtained;From the microscopic level,the reduction inducement of strength and damage degradation mechanism were investigated by means of continuum damage mechanics,and the constitutive relationship under the conditions of gas adsorption and stress loading were discussed,the damage evolution equation was established and verified.5)Combined with the laboratory deterioration test results,extracted the test process images based on the fractal theory and MATLAB software programming,the characteristics of macroscopic fracture development and evolution after peak were obtained,the fracture density and fractal dimension at each stress stage are calculated;Based on PFC contact model and calculation principle,combined with degradation test parameters,numerical simulation test was conducted to verify the evolution characteristics of coal fracture in the process of gas adsorption and stress loading,according to the test results,the key parameters of the constitutive equation in the PFC of particle discrete element were inversed,and the degradation of coal body in the process of gas adsorption and loading was simulated by quantitative weakening of each parameter.From the perspective of particle discrete element,the simulation analysis of macroscopic variables such as strength,deformation and crack expansion degree was realized.The research content is of great significance for the deep understanding of the physical and mechanical properties of coal containing gas and engineering problems such as coal and gas outburst mechanism.