Study On Mechanical Properties And Cumulative Damage Of Cyclic Impact Coal Under Three-dimensional Static Load

Dynamic disasters such as rock bursts,coal and gas explosions are associated with damage accumulation and strength degradation caused by profound mine mining disturbances.Deep coal development requires the solution of a number of fundamental theories and problems in order to ensure safety.The purpose of this study is to investigate coal mining in deep mines,and to explore deformation,failure,and instability in coal under 3D stress.Moreover,the evolution of mechanical properties of coal under repeated loading and unloading under dynamic and static loads as the mainline,comprehensively using laboratory experiments,theoretical analysis and numerical analysis,carries out the mechanical properties and damage monitoring experiments of cyclic impact coal under three-dimensional static load and constructs a 3D numerical model of ANSYS/LS-DYNA.This paper aims to analyze the stress balance of coal under combinations of 3D dynamicand static loading.Based on a meso-geometric model for coal damage degree characterization,the study examined the mechanical properties of coal under 3D static loading conditions.The model was developed to study the cumulative damage evolution of 3D static cyclic impacts on coal.This paper presents the following innovative achievements:（1）Based on the various characteristics of dynamic stress wave and stress balance factor during impact test,a new stress balance analysis index,"stress balance coefficientξ,,is put forward.The stress balance degree of specimens in the SHPB test can be determined using a new method.When the specimen length-diameter ratio is less than0.5,ξis more than 78.7%,but when the specimen length-diameter ratio exceeds 0.5,ξdecreases rapidly.Based on the comprehensive analysis of the stress wave equation considering the transverse deformation,it is possible to determine the optimal impact test sample size under a 3D static load of raw coalΦ50 mm length-diameter ratio 0.5,quantitative description of the pressure balance degree the influence process.It provides a new research method for analyzing the stress balance of different material specimens.（2）Based on the mechanical properties and damage evolution test of cyclic impact coal under 3D static load,the relationship between cyclic impact strength,deformation,energy,CT,ultrasonic and axial pressure,confining pressure,impact time is demonstrated.The evolution characteristics of cyclic impact stress-strain curve under 3D static load are revealed.The stress-strain curve associated with cyclic impact under 3D static load can be classified as closed type and open type as the number of cyclic impact cycles increases.During the initial stages of cyclic impact,the stress-strain curve transitioned from a closed state to an open state,and after the peak,the irrecoverable deformation increased.In situations where the confining pressure and impact load are equal,the greater the axial pressure,the shorter the total cycle impact time.With the same axial pressure,the greater the confining pressure,the longer the total cycle impact time.（3）A variation coefficient of dynamic peak stress in cyclic impact is presented,and it was determined that 93.75%,43.75%,and 100%of the specimens show phenomena of first impact emotional strength hardening,strain hardening,and dissipated energy release.When the confining pressure is greater than the axial pressure,the initial stress state of the specimen changes under the confining pressure,so that the principal stress in the coal body is distributed in the radial direction in the initial state,and then the axial primary crack is closed and the radial crack is extended slightly.The tension of the axial crack is formed under the action of partial stress,and the compaction effect of the first impact on the crack increases the strength of the cyclic impact to form the first impact dynamic strength hardening phenomenon.（4）The evolution law of energy dissipation in cyclic impact under three-dimensional static load is obtained.The results indicate that the release of dissipated energy always lags behind the peak of incident energy,and decreases gradually with the increase of cyclic impact times.When the specimen is destroyed,the dissipative energy release（ΔW_S）is almost zero,and the rate of dissipated energy release(P_WR)reflects well the development and completion of coal-rock fractures under cyclic impact.P_WR can be divided into:a)rising slightly at the initial stage and then gradually oscillating and descending;b)monotonous decreasing type.The crushing energy consumption density（e_d）shows a downward trend as a whole,and there are two kinds of rising and sudden changes:a)the initial stage of cyclic impact,and b)before the sharp decrease.This shows that the crushing energy dissipation density（e_d）is related to the fracture development of impact,and the processes of fracture compaction and expansion may lead to significant mutation of e_d.When the axial pressure is 5MPa and 6MPa,the greater the confining pressure,the greater the total cumulative crushing energy consumption density,and the maximum increment is 6.09J/m³,with an increase of 309.7%.Coal has a higher ability to withstand cyclic impact;however,when the axial pressure exceeds 7MPa,the effect of confining pressure is significantly reduced.（5）Based on the crack evolution of 3D reconstruction of CT,it is revealed that there is a compaction effect of cyclic impact on the fracture of coal under 3D static load.The total crack volume and total area of coal samples decreased from 390.1mm³ and691.3mm² before impact to 191.7mm³ and 290.8mm²,respectively,after impact.The strength hardening effect and strain hardening effect of cyclic impact coal were investigated from the viewpoint of the meso-damage evolution.Overall,the ultrasonic wave velocity,the maximum amplitude in the time domain and the number of cyclic impacts showed a downward trend,however,93.8%and 87.5%of the specimens showed an increase in ultrasonic wave velocity and maximum amplitude in the time domain after the first impact,corresponding to the crack compaction stage in the cyclic impact process.（6）The characteristics of the definition methods of three kinds of damage variables are comprehensively analyzed.It is explained that the mechanical parameters and endogenous energy dissipation are not suitable for defining the damage variables in coal cyclic impact.The cyclic damage variables based on acoustic wave velocity are defined.Two damage evolution types of accelerated cumulative damage and S-shaped curve cumulative damage of coal under 3D static cyclic impact are found.The cumulative damage evolution models of cyclic effect based on ultrasonic velocity are established.It is revealed that axial pressure and confining pressure can promote and inhibit the development of cumulative damage at the initial cycle stage.Axial and confining pressure play a significant and secondary role in controlling the evolution type of cumulative damage,respectively.The constitutive damage equation of coal under 3D static load considering cumulative damage is constructed.The purpose of this paper is to provide a new method for determining the stress balance of specimens based on the stress balance coefficient.The response rules of axial pressure and confining pressure to coal strength,deformation,energy dissipation,CT and ultrasonic testing are revealed.A model of cumulative damage evolution based on ultrasonic velocity is developed.The constitutive damage equation of coal under three-dimensional static load considering cumulative damage is constructed.The research results have laid a theoretical and technical foundation for systematically studying the stability of goaf,roadway,and other engineering rock mass under repeated disturbances such as blasting and mining.It has important theoretical significance and engineering application value for guiding shaft and roadway engineering construction and ensuring engineering stability.Figure[69]Table[22]Reference[201]...