Research On Elastoplastic Constitutive Model And Damage And Fracture Mechanism Of Coal Gangue Concrete

Coal gangue is a kind of solid waste in the process of coal mining.Due to the low comprehensive utilization rate of coal gangue,wide area and large amount of storage,it has caused great harm to the ecological environment and life safety.On the other hand,the state continues to strengthen environmental protection and restrict the exploitation of natural aggregate,which leads to the continuous rise of the price of concrete raw materials,but the demand of concrete market remains stable.In the process of coal gangue crushing and reprocessing,there are primary cracks in the coal gangue.Under the action of external force,the micro crack tip will first produce stress concentration phenomenon,and then expand and extend.Therefore,based on the platform construction resources of"coal gangue resource utilization and energy-saving building materials"of Liaoning Provincial Key Laboratory,based on the principles of elastic-plastic mechanics,damage mechanics,irreversible thermodynamics and fracture mechanics,through laboratory tests,theoretical research,mechanism analysis and numerical simulation,this paper deeply studies the elastic-plastic damage constitutive model and damage fracture mechanism of coal gangue concrete The research can provide experimental and theoretical basis for the safety design and practical engineering application of gangue concrete structure.The main research results are as follows:(1)The micro morphology characteristics,chemical composition and physical and mechanical properties of coal gangue coarse aggregate and natural crushed stone coarse aggregate are compared and analyzed;the basic mechanical properties of coal gangue concrete are tested,the relationship between the basic mechanical performance indexes of gangue concrete with different gangue replacement rate and ordinary concrete is expounded,and the theoretical relationship between the basic mechanical performance indexes of gangue concrete with different gangue replacement rate are established.The theoretical relationship between mechanical properties provides model parameters for damage fracture model of gangue concrete.The uniaxial cyclic tension and compression tests of coal gangue concrete are carried out.The failure process and failure mode are described.The stress-strain curve of coal gangue concrete is obtained.The relationship between plastic strain and unloading point strain and stress degradation phenomenon are analyzed.The cyclic tension compression envelop line and unloading reloading curve are determined.The uniaxial cyclic tension unloading reloading curve adopts broken line type.The unloading curve of the cyclic compression is curve type,and the reloading curve is broken line type.The constitutive relation which can accurately describe the one-dimensional cyclic tension and compression of gangue concrete is established.(2)The distribution and morphology characteristics of chemical elements in the interface structure of coal gangue concrete and ordinary concrete under uniaxial and triaxial action are compared and analyzed.The interaction mechanism of coal gangue concrete interface is analyzed,and the interface structure model of coal gangue concrete is established.Aiming at the stress-strain curve of coal gangue concrete under triaxial mechanical behavior,based on D-P plastic potential function,a willam Warnke five parameter yield model with strengthening parameter k is established for coal gangue concrete considering deviatoric stress and hydrostatic pressure at the same time.By using cumulative plastic strain as internal variable,the plastic stress-strain softening rules of subsequent yield surface and loading surface of gangue concrete with different gangue replacement rate are established.According to the principle of classical plastic mechanics,the stress-strain constitutive relationship of gangue concrete based on incremental form is established.(3)According to the uniaxial and triaxial mechanical properties of coal gangue concrete,the mechanical damage mechanism of coal gangue concrete is analyzed,and the definition of damage variable is proposed.Based on the principle of irreversible thermodynamics,damage mechanics and plastic mechanics,the total elastic-plastic Helmholtz free energy of coal gangue concrete including damage and plastic strain is deduced.The elastic-plastic damage constitutive model of gangue concrete with different replacement ratio of gangue and the evolution rule of damage internal variables are established.The relationship between model parameters and the content of coal gangue coarse aggregate is analyzed,which is in good agreement with the triaxial test data.The elastic-plastic damage constitutive model of gangue concrete with different replacement ratio of gangue and the evolution rule of damage internal variables are established:Coal gangue coarse aggregate and confining pressure can delay the initial damage of coal gangue concrete.After damage,the more the content of coal gangue coarse aggregate is,the faster the damage rate is.However,the greater the confining pressure is,the faster the damage rate is.(4)The three-point bending fracture test of coal gangue concrete is carried out,and the load-crack opening displacement curve,load-deflection curve,crack initiation load and ultimate load are obtained.The more the amount of coal gangue coarse aggregate is,the smaller the crack opening displacement and deflection are when the ultimate load is reached.Based on the three-point bending test,it is verified that the digital graph correlation method(DIC method)is feasible to test the crack propagation of coal gangue concrete;based on DIC method,the crack propagation mechanism of coal gangue concrete is analyzed,and the crack propagation width is obtained;the double-K fracture model is used to analyze the variation of fracture parameters such as initiation fracture toughness,instability fracture toughness,fracture energy and ductility index of coal gangue concrete with coal gangue content,which provides experimental parameters for the establishment of damage fracture model of coal gangue concrete.(5)Based on the damage mechanics and the double-K fracture model,the D-R damage fracture model of coal gangue concrete is established by introducing damage scale.The crack initiation damage threshold(D_IG)is used to judge whether the crack of coal gangue concrete expands or not,and the allowable damage scale R_Ic is used to judge whether the critical failure of coal gangue concrete occurs or not.It can accurately reflect the whole damage evolution process of gangue concrete with different replacement rate from the micro damage crack to the beginning crack of macro crack to the ultimate instability failure.Based on the fracture mechanics principle,the stress field change of the crack tip of coal gangue concrete is analyzed,and the threshold value of crack damage is deduced.According to the closing force of both sides of the micro crack expansion area,the fracture zone length of coal gangue concrete is calculated;the numerical simulation of three-point bending of coal gangue concrete is carried out,which verifies the rationality of D-R damage fracture model,and can better reflect the damage evolution law of gangue concrete the crack tip.The paper has 139 figures,23 tables,173 references.