Investigation On The Stress-Wave Induced Micro Friction Of Rock Joint

Earthquake plate motion,rock landslides and local friction strength of rocks are closely related to the friction of rock joint.The study of the dynamic friction characteristics of rock joint has always been a difficult topic because joint surface contains various scales and physical mechanisms.Although the research on the friction performance of the joint surface under quasi-static conditions is sufficient rich,the research under dynamic conditions is still a big challenge.In this paper,the dynamic friction characteristics of granite joint surfaces are discussed in detail based on the selfmade Hopkinson bar bundle device and true triaxial preloading device.In order to systematically explore the initial stage characteristics of the dynamic friction of the rock joint surface,the theoretical and numerical simulation analysis is carried out.Based on the improved Hopkinson beam experimental device,the friction characteristics of joint surfaces with different angles under different impact velocities are studied,and the variation characteristics of friction coefficient with slip displacement and joint angle are obtained.At the same time,the author found that the upper region of the joint surface slips before the lower region by using the concept of displacement gradient.The critical slip angle of the interface is between 10° and 15°.By analysis of the experiment results with different joint angles and different impact speeds,it is found the development of the friction coefficient has two stages,namely the rapid decline stage and the stable stage.The concept of key weakening distance is proposed.The friction coefficient and the critical weakening distance both show an exponential decay relationship with the increase of normal stress.Based on the relationship surface between friction coefficient with normal load and slip velocity,it is found that fretting friction on joint surface has a certain velocity strengthening effect.The author also studies the dynamic friction characteristics from the perspective of energy dissipation.The results show that under smooth joints,the absorption coefficient at 20° joints is the largest.As the impact velocity increases,the energy absorption coefficient also increases.Under rough joints,the difference of energy absorption coefficient between 20° and 30° joint surface is not large.Even if the 10° joint surface does not slip,a large energy loss occurs.The difference of the absorption coefficient is mainly related to the friction loss on the interface.Based on the self-developed true triaxial preloading experimental device,the development law of dynamic friction of granite under complex stress state is studied.Based on the principle of dynamic friction experiment of true triaxial preloading,the calculation formula of dynamic friction force is derived,and the testing method of lateral dynamic friction force of granite specimen is established.The granite specimens were subjected to impact loading under different loading paths and impact speeds.The result shows that the development of normal stress and friction force was divided into stress-rising stage and stress-falling stage,and the maximum stress increased with the increase of the impact speed and the initial confining pressure.The duration of normal stress increases with the increase of impact velocity,and decreases with the increase of restraint conditions.The duration of shear stress is relatively short,which is about 2030us.The friction coefficient decreases obviously with the increase of confining pressure,and increases with the increase of impact velocity,which is explained from two aspects,that is,increasing the pressure will lead to an increase in the area and an increase in the destruction of the interface asperities.Finally,the friction coefficient is fitted by the empirical formula,which is consistent with the theoretical results.The finite subgroup theory is introduced to establish a theoretical method for analyzing the dynamic evolution of the surface topography of rock rough joints,and the fretting friction evolution process of local fracture on the joint surface is revealed.The surface state of the interface before and after slipping was photographed and tracked by a step meter and an optical microscope.The main slip modes are found:shearing of local asperities and local slip.The state of the interface before and after impact was described by four roughness parameters(Z2s、Rs、θmax*/(C+1)and θs).The research shows the roughness of the joint surface at different joint angles decreased under dynamic impact.When the interface slips,the roughness retention rate becomes lower as the joint angle decreases,and the reduction of roughness of the smooth joint is obviously lower than the rough joint.Then the anisotropy of roughness before and after slipping is studied based on the statistical results of parameters.Finally,the concept of the finite group is introduced,and the roughness of the surface is fitted by the oblique subgroup.Combined with the friction-diffusion equation of the interface,the dynamic evolution analysis of the roughness of the interface is carried out with the oblique subgroup as the initial state.The dynamic evolution law of rough joint surface is revealed.Using the finite element numerical analysis method,the local wave characteristics of finite fracture of rock rough joint surface are studied,and the wave system development law in the process of local finite fracture is revealed.The wave effect caused by local fracture of joint surface is studied by numerical analysis.Before the stress wave reaches the interface,there is weak disturbance which is caused by gravity.The local failure of the interface will produce longitudinal wave,shear wave and surface wave by theoretically analysis.Through the wave field analysis of single microstructure and multiple microstructures,longitudinal waves will appear before microstructure destruction,and longitudinal waves,shear waves and surface waves will appear after microstructure destruction.Then,the acoustic emission signal during the sliding process was analyzed.Under low-speed loading,the acoustic emission signal fluctuation is more obvious.At high speed,the duration of the acoustic emission signal is shorter.According to the wavelet energy spectrum analysis,the main frequency band of acoustic emission tends to shift to high frequency with the increase of impact speed.According to above results,the acoustic emission frequencies in two slip modes of concave convex sliding and shear failure were determined.The local damage source locations were determined based on cross-correlation analysis.This research has a good reference significance for the work in many fields,such as on-site microseismic positioning.Based on the analysis methods of experiment,theory and numerical simulation,this paper mainly studies the wave induced friction and inertial friction stages in the friction process.In the wave-induced phase,the evolution law of the friction coefficient is studied.Although the friction coefficient decreases with the slip displacement,it is still relatively high.In the inertia friction stage,the interfacial friction coefficient decreases to a lower position.The local limited fracture of the surface is the mainly reason for the rapid decrease of the friction coefficient in this stage.The paper systematically establishes the research method of micro friction of the rock joint surface,which provides theoretical guidance for the research of the evolution of the microseismic system.