Study On The Influence Of Local Impact On Coal-Rock Microstructures And Equivalent Theoretical Model

Coal-rock and various types of rock are significant engineering medium and are closely related to the development of various industries(mining,oil,natural gas,transportation,buildings&bridges,water conservancy&hydropower,nuclear waste geological storage,etc.).With the transformation of modern production modes,many engineering fields and the occurrence of natural disasters involve the dynamic mechanical properties and dynamic failure of rock under impact load,such as blasting breaks rock(coal),reservoir permeability modification,rock burst and coal and gas outburst in mine engineering,explosive fracturing and seepage enhancement in oil and gas exploitation,earthquake prevention and disaster reduction in slope and water conservancy projects,rock breaking by ground penetration projectile in military engineering,and so on,all of these involve rock dynamics.This scientific problem has become one of the key,difficult and hot issues in rock mechanics research.Aiming at the actual engineering needs of reservoir permeability,high efficiency rock breaking(coal)and prevention of dynamic disasters such as coal and gas outburst and rock burst induced by dynamic load,etc.This paper mainly explores the evolution law and damage characteristics of coal-rock internal microstructures under local impact load,the intrinsic physical mechanism of coal-rock deformation and failure,and the essential characteristics of the coupling relationship between internal microstructural evolution and macro-fracture and instability.Taking coal and rock samples as the object of study,a self-developed pendulum hammer dynamic impact loading test device of coal-rock was used to carry out local impact loading tests on coal-rock samples.With the help of ultrasonic detector,mesoscopic observation system of coal-rock surface cracks,dynamic and static strain gauge and infrared thermal imager,the evolution laws of microstructures,surface crack propagation,damage and deformation characteristics and temperature field in different regions of coal-rock samples under local impact load were studied respectively.Moreover,the localization effect of microstructures evolution,surface crack propagation,deformation and failure in coal-rock under local impact load were analyzed.Based on fi-actal theory,the fractal characteristics of surface cracks of coal-rock samples under different impact loading conditions were compared and analyzed.The three-dimensional reconstruction of the fractured section morphology of coal-rock under local impact load was carried out by measuring the convexity and concavity of the fractured section.The fractal characteristics of rough fracture section of coal-rock samples under local impact load were analyzed by using a quadratic improved cube covering method for calculating fractal dimension of rough surface.The results show that the fractal characteristics of rough fracture section of coal-rock samples under local cyclic impact load are obvious.An equivalent calculation method of fractal dimension of rough surface based on RGB image was proposed,and the reliability and accuracy of the method were verified by comparing the measured data.The coupling relationship between fractal dimension of fracture surface and impact loading area of coal-rock samples under local impact load was obtained by this method.A software for dynamic monitoring,imaging and analysis of infrared video temperature was developed independently.The software combined with infrared thermal imager can effectively monitor the evolution of infrared thermal radiation temperature field of coal-rock samples under impact load.A quantitative characterization method of infrared thermal radiation temperature field of coal-rock based on infrared thermal imaging was proposed,which includes three quantitative indexes of infrared thermal radiation temperature field based on fractal theory,entropy theory and statistical theory:fractal dimension,entropy and variance of temperature difference.The evolution of infrared thermal radiation temperature field of coal-rock samples under local impact load is quantitatively characterized by these three quantitative indices.It is found that the entropy can better characterize the periodic variation of infrared thermal radiation temperature field of coal-rock samples during cyclic impact,while the fractal dimension and variance of temperature difference are more capable of identifying the precursors of infrared thermal radiation of coal-rock samples rupture.According to the model and scheme of coal-rock impact test studied in laboratory,the corresponding numerical simulation model of coal-rock dynamic was established by using ANSYS/LS-DYNA3D non-linear dynamic numerical analysis software.Based on the HJC dynamic constitutive model of coal-rock,the dynamic response characteristics and damage failure modes of coal and rock under impact load are studied.The dynamic mechanical characteristics and damage failure laws of different regions of coal and rock under local impact load are mainly explored.The effects of impact times,impulse magnitude,impulse loading sequence and impact loading area on dynamic damage and failure of coal-rock were analyzed.On the basis of local impact test and numerical simulation,a dynamic constitutive model describing the comprehensive response characteristics of coal-rock under medium and high strain rates,including non-linear elasticity,plasticity,damage softening and strain rate dependence,was constructed by using statistical damage theory and component model theory,and the determination method of parameters in the model was given.According to the dynamic mechanical characteristics of compact hard coal and soft coal with pore and fissure development under medium and high strain rate loading,the viscoelastic-plastic constitutive model of coal-rock containing damaged body was simplified and improved,and the dynamic constitutive model suitable for hard coal and soft coal was established.Based on the equivalent partition mechanical model of local impact,the damage characteristics of coal-rock samples under local impact load were divided into different zones by introducing the partition equivalent factor in different zones of coal-rock samples.The equivalent model of coal-rock damage zoning under local impact and conventional full impact was established,and the data of static load test and impact test for different loading areas were fitted.The coupling relationship between the equivalent factor and the impact loading area in the equivalent model was obtained.The results of damage partition equivalent model of coal-rock under local impact load are in good agreement with the measured results of local impact test,which verifies the applicability of the equivalent model.It shows that the equivalent model can fully reflect the coupling relationship between damage in different areas of coal-rock under local impact load and impact loading area.In addition,the damage mechanism of coal-rock under local impact load was discussed.